**R&D Plan Update for the**

**Supernova / Acceleration Probe**

**The SNAP Collaboration**

**1 November 2003**

**Table of contents**

[Section 1. SNAP Mission Requirements and R&D Goals
1](#__RefHeading___Toc55464338)

[1.1 Introduction 1](#introduction)

[1.2 High-Level Mission Requirements
2](#high-level-mission-requirements)

[1.3 R&D Goals 3](#rd-goals)

[Section 2. Instrument R&D 5](#__RefHeading___Toc55464342)

[2.1 Instrument Conceptual Design 5](#instrument-conceptual-design)

[2.2 Instrument R&D Plan 6](#instrument-rd-plan)

[2.3 Near Infrared (NIR) Imaging R&D 9](#near-infrared-nir-imaging-rd)

[2.4 CCD R&D 18](#ccd-rd)

[2.5 On-Sensor Filter R&D 38](#on-sensor-filter-rd)

[2.6 Spectrograph 41](#spectrograph)

[2.7 Data Acquisition and Control 57](#data-acquisition-and-control)

[2.8 Front-end Electronics 64](#front-end-electronics)

[2.9 Mechanical systems 70](#mechanical-systems)

[Section 3. Calibration R&D 75](#__RefHeading___Toc55464352)

[3.1 Requirements Overview 76](#requirements-overview)

[3.2 Calibration R&D Trade Studies 76](#calibration-rd-trade-studies)

[Section 4. Ground Segment R&D 79](#__RefHeading___Toc55464355)

[4.1 Segment Definition 79](#segment-definition)

[4.2 Mission Operations R&D Issues 79](#mission-operations-rd-issues)

[4.3 Science Operations R&D Issues 80](#science-operations-rd-issues)

[4.4 Simulation R&D Plan 81](#simulation-rd-plan)

[4.5 CDR Planning 83](#cdr-planning-5)

[Section 5. Telescope R&D 85](#__RefHeading___Toc55464361)

[5.1 Pre-R&D Phase OTA Trade Studies
85](#pre-rd-phase-ota-trade-studies)

[5.2 OTA Specification Package 87](#ota-specification-package)

[5.3 SNAP Technology Mirror 92](#snap-technology-mirror)

[5.4 OTA Risk Assessment 93](#ota-risk-assessment)

[Section 6. Spacecraft R&D 96](#__RefHeading___Toc55464366)

[6.1 Spacecraft Related Analyses and Trade Studies
97](#spacecraft-related-analyses-and-trade-studies)

[Section 7. System Engineering R&D 100](#__RefHeading___Toc55464368)

[7.1 System Engineering R&D Studies 100](#system-engineering-rd-studies)

[Section 8. R&D Phase Management 104](#__RefHeading___Toc55464370)

[Section 8. R&D Phase Management 104](#__RefHeading___Toc55464371)

[8.1 Management Elements 104](#management-elements)

[8.2 Management Processes 107](#management-processes)

**List of Figures**

[Figure 1. SNAP instrument installed in the optical bench.
7](#__RefHeading___Toc55464374)

[Figure 2. Cutaway view of the instrument package.
7](#__RefHeading___Toc55464375)

[Figure 3. Exploded view of the SNAP instrument package.
8](#__RefHeading___Toc55464376)

[Figure 4. Front view of the SNAP focal plane.
8](#__RefHeading___Toc55464377)

[Figure 5. A false color image of the Dumbell Nebula featured on the
cover of the NOAO September 2001 newsletter. The background stars are
only visible because of the 1 μm wavelength response of our CCDs.
29](#__RefHeading___Toc55464378)

[Figure 6. A supernova spectrum acquired by the LBNL Supernova Cosmology
Project on the NOAO RC Spectrograph showing the extended response of our
CCDs to beyond 1 μm. 30](#_Ref503919518)

[Figure 7. Architecture comparison of a conventional back-illuminated
n-channel CCD and an LBNL p-channel CCD.
30](#__RefHeading___Toc55464380)

[Figure 8. Quantum efficiency of the LBNL CCD.
31](#__RefHeading___Toc55464381)

[Figure 9. A 150 mm commercially fabricated wafer containing a variety
of CCDs. The large rectangles are 2k x 4k, 15 μm CCDs and the large
squares are 2880 x 2800, 10.5 μm CCDs. 31](#__RefHeading___Toc55464382)

[Figure 10. A front side illuminated image taken with one of the 2k x
4k, 15 μm CCDs commercially fabricated. 32](#__RefHeading___Toc55464383)

[Figure 11. Fully commercially fabricated CCD. The image on the left is
a 1000 s dark exposure and the image on the left is a 600 nm flat field
32](#__RefHeading___Toc55464384)

[Figure 12. A 150 mm commercially fabricated wafer containing four
3512 x 3512, 10.5 μm SNAP version 1 CCDs.
33](#__RefHeading___Toc55464385)

[Figure 13. Four-side abuttable packaging developed for front side
illuminated CCDs. The CCD is at the bottom of the stack and is glued to
a molybdenum mount. A printed circuit board is connected with wire-bonds
down to the CCD. 33](#__RefHeading___Toc55464386)

[Figure 14. A speckle interferograph at 140 K of the optical surface of
the package shown in Figure 13.. The maximum peak to valley excursion is
6 μm. 34](#__RefHeading___Toc55464387)

[Figure 15. Conceptual design of the 4-side abuttable package for the
SNAP v. 1 CCD 34](#__RefHeading___Toc55464388)

[Figure 16. Comparison of LBNL CCD CTE degradation to two conventional
n-channel CCDs. Since the doses were done at different proton energies,
we convert them to a non-ionizing dose in MeV/g.
35](#__RefHeading___Toc55464389)

[Figure 17. Dark current in electrons per 15 μm pixel per hour as a
function of radiation dose. 35](#_Ref53994849)

[Figure 18. The \"virtual knife-edge\" technique used to measure the
point spread function. 36](#__RefHeading___Toc55464391)

[Figure 19. Normalized charge measured as a function of the scan
position over the virtual knife edge (top), and the derivative giving
the PSF (bottom). 36](#__RefHeading___Toc55464392)

[Figure 20. RMS diffusion in microns as a function of bias voltage for 3
different wavelengths (left) and for both row and column directions
(right). 37](#__RefHeading___Toc55464393)

[Figure 21. Reflected light from a B-band interference filter directly
deposited on an indium-tin-oxide-coated 100 mm silicon wafer.
40](#__RefHeading___Toc55464394)

[Figure 22. Image slicer principle (courtesy J. Allington-Smith, Durham
U.). 42](#__RefHeading___Toc55464395)

[Figure 23. Instrument Block Diagram 43](#__RefHeading___Toc55464396)

[Figure 24. Picture of a block of slices
44](#__RefHeading___Toc55464397)

[Figure 25. The optical design of the instrument
46](#__RefHeading___Toc55464398)

[Figure 26. Spectrograph Implementation.
47](#__RefHeading___Toc55464399)

[Figure 27. Image quality at 1.7 µm. 47](#__RefHeading___Toc55464400)

[Figure 28. Implementation study. 48](#figure-28.-implementation-study.)

[Figure 29. Example of opto-mount clamping and the associated metrology.
49](#__RefHeading___Toc55464402)

[Figure 30. Kinematic mount concept. 49](#__RefHeading___Toc55464403)

[Figure 31. Dual detector definition 50](#__RefHeading___Toc55464404)

[Figure 32. Simulated calibrated spectrum.
52](#__RefHeading___Toc55464405)

[Figure 33. Spectral resolution in the visible (left) and in the NIR
(right). 53](#__RefHeading___Toc55464406)

[Figure 34. Spectrograph development roadmap
54](#__RefHeading___Toc55464407)

[Figure 35. Slicer prototype integrated 55](#__RefHeading___Toc55464408)

[Figure 36. Block diagram of the instrument electronics.
61](#__RefHeading___Toc55464409)

[Figure 37. Imager CCD module. 62](#__RefHeading___Toc55464410)

[Figure 38. MCT focal plane module. 63](#__RefHeading___Toc55464411)

[Figure 39. CCD front end readout module.
69](#__RefHeading___Toc55464412)

[Figure 40. NIR front end readout module.
69](#__RefHeading___Toc55464413)

[Figure 41. Shutter concept. 72](#__RefHeading___Toc55464414)

[Figure 42. The shield is the conical shape between the shutter and the
focal plane 72](#__RefHeading___Toc55464415)

[Figure 43. A front view of the cold plate showing CCD and HgCdTe
sensors mounted 73](#__RefHeading___Toc55464416)

[Figure 44. Concept of bi-pod mounts providing kinematic support of the
cold plate from the telescope structure.
73](#__RefHeading___Toc55464417)

[Figure 45. Candidate concept for the thermal-isolation mounts of the
cold plate to the telescope. 73](#_Ref12857351)

[Figure 46. View of the space thermal radiator relative to the rest of
the instrument. 74](#__RefHeading___Toc55464419)

[Figure 47. Thermal conducting S-links showing their attachment to the
cold plate. The near ends of the links are attached to the radiator.
74](#__RefHeading___Toc55464420)

[Figure 48. Example of commercially available S-links comprised of tens
of thousands of flexible carbon fibers. 74](#__RefHeading___Toc55464421)

[Figure 49. The confidence region in the dark-energy parameters for
different fiducial dark-energy models representing very different
physical origins: a cosmological constant, a supergravity model, vacuum
metamorphosis, axion dark energy, and a leaping kinetic model. The
larger ellipses are the results from SNAP and the smaller ones are for
SNAP plus Planck. Note that *w' ≈ w~a~/2*.
82](#__RefHeading___Toc55464422)

[Figure 50. As can be seen in the R&D phase organization chart, the SNAP
project is divided into its principal systems and subsystems beneath the
Project Directorate. Each major system has a designated System Manager
who is responsible for the successful development and completion within
budget and schedule constraints. 111](#__RefHeading___Toc55464423)

**List of Tables**

Table 1. SNAP NIR Performance Specifications
[10](#__RefHeading___Toc55464424)

Table 2. The specifications for the SNAP NIR focal plane array detectors
[11](#__RefHeading___Toc55464425)

Table 3. Device Delivery Schedule [16](#__RefHeading___Toc55464426)

Table 4. LBNL CCD characteristics. [20](#__RefHeading___Toc55464427)

Table 5. Spectrograph main characteristics
[41](#__RefHeading___Toc55464428)

Table 6. Detector specifications [45](#__RefHeading___Toc55464429)

Table 7. Spectrograph system parameters
[45](#__RefHeading___Toc55464430)

Table 8. Material trade study. [48](#__RefHeading___Toc55464431)

Table 9. Instrument efficiency estimation
[53](#__RefHeading___Toc55464432)

1.  []{#__RefHeading___Toc55464338 .anchor}SNAP Mission Requirements and
    R&D Goals

# Introduction

Our picture of the origin and nature of the universe has been
transformed in the past few years by the first precision measurements of
the fundamental cosmological parameters. Contrary to the expectations of
the theoretical community that the mass density would equal the critical
density, balancing the universe between eternal expansion and ultimate
contraction, measurements reported in 1998 by two different groups
studying supernovae concluded that the universe is expanding at an ever
increasing rate and established a non-zero value for the cosmological
constant. These results have since been confirmed by deeper and more
precise studies of supernovae, and they are also consistent with
independent data from cosmic microwave background measurements and from
galaxy surveys. For example, the 2003 results from the Wilkinson
Microwave Anistropy Probe Satellite reveal a universe that is flat, in
agreement with inflationary models, but finds only \~27% of the total
energy density is in the form of matter (and only 4% is baryonic
matter), while the majority of the energy density is unknown -- the
so-called "dark energy."

These results have caused a great deal of speculation in the theoretical
community and sparked a number of new theories regarding the dark energy
that is driving the accelerating expansion of the universe.
Understanding the nature of the dark energy is one of the key science
goals identified by the Turner panel of the NAS and by the High Energy
Physics Advisory Panel Subcommittee for Long-Range Planning.

In order to carry out more precise measurements of the cosmological
parameters and determine the equation of state of the dark energy, we
have proposed a dedicated satellite-based experiment, the
Supernova/Acceleration Probe (SNAP). SNAP is a \~2-meter satellite
telescope that will obtain a high statistics (\~2000) calibrated data
set of Type Ia supernovae with redshifts z ≤ 1.7 and excellent control
over systematic errors. To obtain high signal-to-noise, calibrated light
curves and spectra for each supernova, SNAP is instrumented with a
0.7-square degree pixel-based imager with sensitivity to wavelengths
ranging from 350--1700 nm, a near-UV to near--IR spectrograph and
precision star guider CCDs. These instruments are integrated on a common
focal plane. From these data we expect to measure the mass density of
the universe to 2%, the density of dark energy driving the acceleration
of the universe to 5%, and its equation of state to 5%. Time variation
in the equation of state, a key attribute distinguishing other vacuum
energies from a cosmological constant, could be detected. Detailed
systematic studies will be carried out to account for potential biases
or sources of error due to interstellar dust, galactic environment, etc.

In addition to the primary scientific goals of measuring the
cosmological parameters, a rich program of auxiliary science can be
carried out with SNAP's comprehensive wide-field data set. This includes
study of weak and strong gravitational lensing that probe the dark
matter content and distribution, time variable objects such as active
galactic nuclei, gamma ray burst afterglows, outer solar system bodies,
and the detailed population and distribution of galaxy types and stellar
evolution.

In this document we summarize the work that has been accomplished to
date and describe the remaining R&D necessary to prepare a Conceptual
Design Report and obtain CDR approval for initiation of the engineering
and design phase. In Section 2, the R&D plan for the SNAP
instrumentation suite is described, including the visible and
near-infrared sensors, filters, spectrograph, data acquisition,
electronics and mechanical systems. Section 3 discusses the development
of the SNAP calibration plan. Sections 4-6 are devoted to computing,
telescope and spacecraft R&D plans. Sections 7 and 8 describe the system
engineering and project management for the R&D phase. Schedules,
manpower estimates and cost estimates are collected in a separate
document.

# High-Level Mission Requirements

The high-level mission requirements can be divided into two categories:
requirements derived from the scientific goals of the project, and
requirements derived from the need to withstand the rigors of launch and
operation in space. These requirements and their derivation from the
SNAP science goals are described in greater detail in the SNAP Mission
Definition and Requirements Document and ongoing studies are further
discussed in Section 2. Here we provide an overview of the most
important requirements.

## Science-driven instrument requirements

The SNAP satellite telescope and instrumentation suite is designed to
provide observational data on a high statistics sample of supernova
consisting of the following elements:

-   Early detection of supernova.

-   B-band rest-frame photometry to follow the supernova light curve.

-   Supernova color at peak and near-peak brightness.

-   Optical and IR spectra at peak brightness to classify the supernova.

-   Photometric redshifts of the host galaxies prior to supernova
    follow-up.

-   Medium resolution spectra/photometry for a limited subset of
    supernovae over the full light-curve.

Each of these observational requirements translates into a set of
explicit requirements on the telescopes and instruments, also detailed
in the SNAP Mission Definition and Requirements Document.

## Requirements for operation in space

Launch and operation in space also impose requirements on the SNAP
instrumentation suite and telescope. Many of these requirements involve
design trade-offs and are still under development.

-   The maximum launch payload limits both size and weight. At this time
    we are contemplating launch aboard a Delta IV-M, which can lift a
    maximum of 2800 kg to HEO. The Delta IV-M offers a 4-meter diameter
    payload fairing, which in principle could accommodate up to a
    2.4-meter telescope.

-   Available power on the spacecraft will be limited to what can be
    provided by on-board solar panels, estimated at approximately
    300-400 W average power load.

-   Thermal management is an important issue affecting all components.
    In the SNAP working concept, the instrument CCDs for optical and IR
    imaging will operate at 140 K for best performance, while the
    telescope will be maintained at room temperature in order to
    simplify pre-flight testing. This approach to thermal management is
    one of the recommendations of a trade study conducted at
    NASA-Goddard.

-   Reliability is a major concern; all instruments must be
    space-qualified, demonstrate a minimum mean time to failure, and
    incorporate redundancy where possible. We are designing for a
    minimum of 4 years of operation.

# R&D Goals

The R&D phase has five main goals:

1.  Develop detailed requirements documents that will drive the later
    design and engineering phase of the project.

2.  Develop and optimize concepts for all components to sufficient
    detail to insure a credible cost and schedule at CDR.

3.  Identify the areas of highest risk and initiate design efforts and
    prototyping to mitigate cost and schedule risk during the
    construction phase.

4.  Identify long-lead procurement items that could pace the final
    completion of the project, and initiate early discussions with
    potential vendors to minimize schedule and cost risk.

5.  Identify and resolve instrumentation issues that affect the
    spacecraft and telescope design so that development can proceed in
    parallel.

The vast majority of R&D issues addressed in this proposal are paper
studies that will document requirements, develop implementation concepts
to meet the requirements, and simulate or otherwise validate these
concepts. We will research existing or planned solutions that meet our
requirements where these can be identified, and develop common solutions
for all of the SNAP instruments wherever feasible. Scientific trade-off
studies between competing designs or options will be carried out to
determine the optimum design to meet SNAP's mission requirements.

Preliminary design work and prototyping will be initiated only where
high risk and/or long lead items are identified with associated cost and
schedule risks to the project. For example, the CCDs and associated
readout electronics represent a new, attractive but unproven technology
that will benefit from early design and prototype effort. The telescope,
on the other hand, requires no real R&D because its optical parameters
fall within the range of previously completed projects; nevertheless, a
long lead time and high cost are associated with its procurement.

2.  []{#__RefHeading___Toc55464342 .anchor}Instrument R&D

This section describes the R&D plan for the SNAP instrument suite. We
start with the conceptual design of the instrument. While this working
concept will be subject to further scrutiny and optimization, it
provides a solid foundation for a feasible and self-consistent
observation strategy that realizes the primary scientific goals of SNAP.
It also forms the basis for a program of limited and focused R&D that
will lead to reliable cost and schedule estimates and a Conceptual
Design Review (CDR) in two years.

The following subsections describe the detailed R&D plans for the
components that make up the SNAP instrument suite: imager, spectrograph,
and electronics and data acquisition. Each section includes a review of
progress in the past year, a discussion of the main R&D issues and
goals, and a summary of CDR planning activities, long-lead procurements,
and risk assessment. Schedules, milestones, manpower, and costs are
available in other documents.

Design and prototyping work for the SNAP instrument suite during the R&D
phase is limited to the following areas:

-   Characterization of commercially available IR detectors with 1.7 μm
    cutoff.

-   Completion of development and commercialization of a new type of
    optical CCD featuring extended red response and enhanced radiation
    tolerance.

-   Design of an integral field unit for the spectrographs.

-   Development of custom integrated circuits for sensor readout.

-   A few mechanical systems.

# Instrument Conceptual Design

Eighteen months ago we concluded a scientific and technical trade study
to determine the optimal configuration of the SNAP instrument suite. The
conclusion was that a wide-field integrated optical to NIR imager with
fixed filters provides the most efficient approach to collecting a high
statistics, comprehensive and precisely calibrated data set of SNe,
while meeting our criteria for technical feasibility, cost and schedule.
A spectrograph is mounted on the backside of the focal plane and
accesses light though a port in the focal plane. The observational
strategy involves visiting the SNAP observation field every four days
and stepping the focal plane across the field to collect a series of
fixed duration exposures. Ground-based analysis of the transmitted data
identifies Type Ia SNe and determines galaxy redshifts; with this
information, SNe are selected for targeted spectrographic measurements
at maximum brightness.

The present working concept of the SNAP instrument package is shown in
Figure 1, Figure 2, Figure 3 and Figure 4 and consists of the following
elements:

-   *Imager.* The imager contains optical and IR sensors integrated on a
    common focal plane operated at 140 K, which is compatible with
    passive cooling. Fixed filters provide continuous overlapping
    photometry in nine bands over the visible and NIR wavelengths for
    B-band restframe measurements of SNe with redshifts up to *z* = 1.7.
    LBNL-developed CCDs cover 350 nm to 1000 nm. They are selected for
    their broad spectral response and high degree of radiation
    tolerance. A total of thirty-six, 3.5k × 3.5k, 10.5 μm CCD detectors
    are required. The NIR sensors are HgCdTe devices. Thirty-six 2k x
    2k, 18 μm detectors will be used to cover the 900 nm to 1700 nm
    range.

```{=html}
<!-- -->
```
-   *Spectrograph.* Spectrographic information for each supernova at
    peak brightness is required. An instrument spanning the wavelength
    range of 350 nm to 1700 nm with a resolution λ/δλ of \~100 can
    measure all the relevant features of Type Ia SNe over this range. A
    two-channel spectrograph utilizes the same focal plane as the
    imager. The sensors for the spectrograph consist of an LBNL CCD for
    optical wavelengths and a HgCdTe device for the near infrared.

-   *Fine star guider.* Four small high speed CCDs are also mounted on
    the focal plane to provide feedback to the spacecraft during
    shutter-open time to help the attitude control system achieve the
    required pointing stability during the several hundred to several
    thousand second exposures.

```{=html}
<!-- -->
```
-   *Electronics systems*. Four systems of components make up the
    electronics suite. The first system includes clock and bias voltage
    generation, analog signal processing, and digitizing for the
    sensors. In the case of the CCDs, we have developed a custom
    integrated circuit (ASIC) for analog signal processing and
    digitization. In the case of the NIR, we continue to follow the
    development at Rockwell of an ASIC that fully manages the readout of
    their HgCdTe devices. The second system funnels the data streams
    from the sensors into the instrument mass memory system performing
    lossless compression on the fly. Third is the system that configures
    the electronics subsystems and monitors the instrument environment.
    Examples of this include setting operating voltages inside the
    front-end electronics and monitoring the sensor temperatures. The
    fourth system oversees the execution of the observation plan that
    performs such activities as opening and closing the shutter, cycling
    the calibration lamps, and transitioning the readout electronics
    from image capture to readout mode.

-   *Mechanical systems*. The mechanical components of the instrument
    comprise a mechanical shutter, a combined function
    particle/thermal/stray-light shield, a 140 K cold plate to which the
    imager sensors and spectrograph are mounted, thermally isolating
    kinematic mounts by which the cold plate is attached to the
    telescope mechanical structure, flexible thermal links between the
    cold plate and the radiator, and the radiator itself that provides
    passive cooling of the cold plate and its sensors.

# Instrument R&D Plan

The SNAP instrument R&D is organized in six working groups studying
visible sensors, NIR sensors, spectrograph, fine star guider system,
mechanical and thermal design, and electronics.

The goal of the R&D period is to develop a conceptual design with an
associated cost and schedule that can serve as the basis for the
Conceptual Design Review (CDR). In addition, we seek to identify and
mitigate project risk due to unproven technologies and long-lead
procurements. The main deliverables of the R&D period are a Conceptual
Design Report including a formal set of requirements linked to the
science goals, a preliminary interface control document that defines
system components and their interactions, a detailed schedule, and a
reliable cost estimate.

![](media/image1.jpeg){width="3.515972222222222in"
height="3.002083333333333in"}

> []{#__RefHeading___Toc55464374 .anchor}Figure 1. SNAP instrument
> installed in the optical bench.

![](media/image2.jpeg){width="4.508333333333334in" height="3.35625in"}

> []{#__RefHeading___Toc55464375 .anchor}Figure 2. Cutaway view of the
> instrument package.

![](media/image3.wmf){width="5.997916666666667in"
height="2.423611111111111in"}

> []{#__RefHeading___Toc55464376 .anchor}Figure 3. Exploded view of the
> SNAP instrument package.

![](media/image4.jpeg){width="4.067361111111111in"
height="2.997916666666667in"}

> []{#__RefHeading___Toc55464377 .anchor}Figure 4. Front view of the
> SNAP focal plane.

# Near Infrared (NIR) Imaging R&D

## Introduction

For supernovae at redshift above z =1, the cosmological redshift moves
the restframe visible emission at \~500 nm beyond the long-wavelength
cutoff of the optical CCD detectors and into the near-infrared (NIR).
Supernova photometry must be done at a common rest-frame (blue)
wavelength if the supernovae are to be used as standard candles; hence,
NIR imaging at wavelengths of 0.9--1.7 μm is a critical aspect of the
mission. NIR photometry is also essential to the diagnosis and
correction of systematic errors due to extinction including those SNe
with z\<1, as all known forms of dust grains give much less extinction
in the NIR than in the visible. Furthermore, the NIR observations
greatly expand the science reach with high statistics, high-z,
wide-field measurements not accessible with any other operational or
proposed instrument.

## NIR system baseline

The NIR system will be an integral component of the focal plane. In the
baseline concept, thirty-six 2k × 2k near-infrared imaging sensors
covering a total of 0.34 square degrees will be placed in four 3 × 3
arrangements symmetric to the CCD placement. The NIR sensors will have a
cell pitch of 18-20 μm, resulting in a total of about 150 Megapixels for
the NIR system. The leading candidate for the NIR sensors are HgCdTe
focal plane arrays (FPA). These devices exhibit low read noise and dark
current while providing excellent quantum efficiency (typically 50%-80%
over the wavelength interval 0.9 -1.7 µm). Two companies are producing
large, astronomical devices: Rockwell Science Center (RSC) who offer a
2048 x 2048 pixel HgCdTe sensor hybridized onto the Hawaii 2-RG (H2RG)
multiplexer, and Raytheon Vision Systems (RVS) who have developed a 2.5
µm 2048 x 2048 device for the VISTA project. RVS is currently developing
a short wave (1.7 µm) version of their VISTA FPA for SNAP. Both RSC and
RVS devices have been successfully used at ground-based telescopes and
it is expected that both companies will be capable of producing a NIR
device that will be suitable for the SNAP mission. A cutoff at a
wavelength of 1.7 µm is a good match to SNAP since the sensors are blind
to the thermal background radiation from the warm telescope. The HgCdTe
technology is also the leading candidate for the NIR sensors on the
spectrograph. Table 1 lists the performance specifications for the SNAP
NIR system. An important element of the R&D phase is the development of
the NIR science requirements. The current performance specifications
represent our best current estimates. In a separate development, Sensors
Unlimited (SU) is developing an InGaAs diode array for SNAP that will be
hybridized to an H2RG multiplexer by RSC. InGaAs has a natural cutoff
near 1.7 µm and shows promise as an alternative technology to HgCdTe.

[]{#__RefHeading___Toc55464424 .anchor}Table 1. SNAP NIR Performance
Specifications

+-----------------+---------------------+-----------------------------+
| **Parameter**   | **Specification**   | **Reasoning**               |
+-----------------+---------------------+-----------------------------+
| Field of View   | \~ 0.3 square       | Match CCD FoV to observe    |
|                 | degrees             | every\                      |
|                 |                     | SN in every color           |
+-----------------+---------------------+-----------------------------+
| Plate Scale     | 0.17 arcsec / pixel | Achieve photometric         |
|                 |                     | accuracy with\              |
|                 |                     | at most 2 x 2 dithering     |
+-----------------+---------------------+-----------------------------+
| Wavelength\     | 0.9 µm - 1.7 µm     | \- overlap with CCD\        |
| Coverage        |                     | - reach into deceleration   |
|                 |                     | phase\                      |
|                 |                     | - observe restframe B-band  |
|                 |                     | out to z = 1.7              |
+-----------------+---------------------+-----------------------------+
| Read Noise      | \< 5 e- (w/         | Assures that photometry is  |
|                 | multiple reads)     | zodiacal\                   |
|                 |                     | light limited               |
+-----------------+---------------------+-----------------------------+
| Dark Current    | \< 0.02 e/pixel/sec |                             |
+-----------------+---------------------+-----------------------------+
| Quantum         | \> 60%              | To achieve adequate S/N to  |
| Efficiency      |                     | study SNe at\               |
|                 |                     | z = 1.7 within time         |
| (Detector)      |                     | constraints                 |
+-----------------+---------------------+-----------------------------+
| Filters         | Three special       | To obtain redshifted        |
|                 | filters             | 'B'-band steps from\        |
|                 |                     | 0.9 to 1. 7 µm              |
+-----------------+---------------------+-----------------------------+

## HgCdTe FPAs

HgCdTe is a ternary semiconductor compound, which exhibits a wavelength
cut-off proportional to the alloy composition. This allows the cut-off
wavelength and thus the operating temperature, to be "tailored" to the
specific application. The SNAP focal plane can be passively cooled to an
operating temperature of 140 K. At this temperature HgCdTe is expected
to have a dark current below the SNAP specification. Experience with 1.7
µm large-format HgCdTe detectors is limited to the Wide Field Camera 3
(WFC3) project on the HST. RSC has produced a large number of devices
for WFC3 (\> 60) resulting in only two suitable for flight. These two
devices fall short of meeting SNAP performance specifications,
particularly in the area of readout noise, and a development program is
required to correct this (see 2.3.3.2). Nevertheless, the SNAP project
will be able to benefit from the extensive WFC3 experience, inheriting
much of the technology development. While InGaAs shows promise as a
short-wave detector, experience with large-format InGaAs detectors is
virtually non-existent. Until we acquire such experience, InGaAs will
remain as an alternative technology for SNAP.

In the current design, the SNAP focal plane will utilize HgCdTe infrared
detectors that have been specifically developed to meet SNAP science
requirements. The actual detector is composed of a thin layer (10 to 20
µm) of HgCdTe with metallized contact pads defining the active area.
Photons with energy greater than the semiconductor band-gap energy
excite electrons into the conduction band, thereby increasing the
conductivity of the material. The FPA is a hybrid consisting of a highly
integrated CMOS multiplexer and an array of infrared sensitive (HgCdTe)
detectors. The two pieces are indium bump-bonded together. The
multiplexer is an array of discrete read-out transistors and, unlike a
conventional CCD, can be read non-destructively.

Rockwell Science Center (RSC) has produced large format HgCdTe infrared
focal plane arrays and has a long history of developing devices for the
astronomical community (NICMOS 256 x 256 FPAs, WFC3/HST 1k x 1k,
University of Hawaii, ESO, Subaru 2k x 2k). Until recently, the standard
process employed by RSC to fabricate arrays has been liquid phase
epitaxy (LPE). In the last several years, RSC has developed a process of
fabricating detector layers in HgCdTe using molecular beam epitaxy
(MBE).

For SNAP, RSC would utilize MBE technology for the NIR diode array,
mated to the H2RG multiplexer. RSC feels that MBE offers them the best
process control needed to tailor the device properties to SNAP
specifications. In particular they believe that MBE will achieve
improved quantum efficiency at short wavelength and will eliminate image
persistence through lattice matching of the HgCdTe to the CdZnTe
substrate, and reduce the intra-pixel variation observed in LPE
technology devices. The H2RG incorporates multiple output modes,
operational modes and data rates.

Building on their long-standing experience with developing large-format
astronomy arrays in InSb (Aladdin, Orion devices) and lately also in
HgCdTe (VIRGO) technology, Raytheon (RVS) is now competing for SNAP NIR
detector development. RVS is taking a different approach for the
detector material growth process. They feel that LPE offers them the
best control of the HgCdTe material properties and will result in the
best match to SNAP specifications. Both RSC and RVS have extensive
experience with both LPE and MBE and have chosen their individual
approach based on that experience. RVS will build on their experience
with their VIRGO 2K x 2K multiplexer that has now been incorporated in
devices produced for the VISTA project.

[]{#__RefHeading___Toc55464425 .anchor}Table 2. The specifications for
the SNAP NIR focal plane array detectors

+-------------------------------+--------------------------------------+
| ###### Parameter              | ###### Specification                 |
+-------------------------------+--------------------------------------+
| Detector technology           | MBE or LPE HgCdTe                    |
+-------------------------------+--------------------------------------+
| Active area format            | 2048 x 2048 pixels                   |
+-------------------------------+--------------------------------------+
| Buttability                   | 4-side                               |
+-------------------------------+--------------------------------------+
| Pixel pitch                   | 18-20 µm square                      |
+-------------------------------+--------------------------------------+
| Fill factor                   | ≥ 0.90                               |
+-------------------------------+--------------------------------------+
| Outputs                       | 16 or 32                             |
+-------------------------------+--------------------------------------+
| Power dissipation             | ≤ 100 mW                             |
+-------------------------------+--------------------------------------+

Measurements that are important to SNAP are read noise, dark current,
quantum efficiency (QE) and intra-pixel variations. Below we list the
status of these measurements. We also will measure other detector
characteristics such as persistence, inter-pixel variations and the
temperature dependence of relevant parameters.

### Dark current

Currently, the performance specification for the NIR detector dark
current is \<0.02 e/pixel/s. For a nominal 300-second exposure this
corresponds to 6 e. For high quantum efficiencies (\> 60%) and a read
noise of 5 e, this corresponds to a S/N of 3 for a z=1.7 type Ia SN, two
magnitudes below peak intensity. This noise level is about half the
zodiacal background light in favorable regions in the north and south
ecliptic poles. These practical considerations make it difficult for
SNAP to reliably detect SN beyond z=1.7. While further studies are
needed to establish the precise dark current requirement for SNAP, the
current specifications strike a balance between integrated dark current
and read noise as the dominant source of noise.

The dark current depends sensitively on the cutoff wavelength of the
HgCdTe detector and the temperature. The Detector Characterization Lab
(DCL) at the Goddard Space Flight Center (GSFC) has measured the dark
current at 150 K for a sequence of 1.7 µm devices produced by RSC for
WFC3. As RSC has gained more experience, the dark current has been
dropping and a number of devices have dark currents below the WFC3
performance specification of 0.1 e/pixel/s. A rule of thumb (not yet
confirmed for these devices) is that the dark current should drop by a
factor 7-10 for every decrease of 10 K in temperature. We are thus
confident that the SNAP dark current specification of 0.02 e-/pixel/s at
140 K is achievable by RSC and we expect that devices produced by RVS
will have comparable dark current performance.

### Read noise

The SNAP specification of 5 e- assumes that the intrinsic read noise for
a single CDS read will be 10 e- and that 4 reads at the beginning and
end of an exposure will reduce this by a factor of √4. This noise
performance has been achieved for all Hawaii multiplexers mated to
HgCdTe material with a cut-off of 2.5 µm or greater. In principle, read
noise should be independent of the wavelength cut-off, but for WFC3
devices this turns out not to be the case.

The Detector DCL at the NASA GSFC has performed extensive measurements
on 1.7 µm devices during the selection of NIR detectors for the WFC3.
The selected flight unit shows a quantum efficiency of 81% at 1.5 µm and
greater than 40% at a wavelength of 1.1 µm, read noise of 24 e- (for a
single CDS read), and a mean dark current of 0.04 e-/pixel/s (at 150K).
While this read noise level is acceptable for WFC3, it is more than a
factor of two larger than the current specifications for the SNAP NIR
sensors. RSC has used a modified process during the final lot production
for the WFC3 devices to reduce the read noise. Through a change in the
order of the processing steps, the read noise was cut in half (to 10-12
e- for a single CDS) but the quantum efficiency for these devices was
low. RSC believes they understand the mechanisms behind this behavior
and that they will be able to control for this in future lots. For the
production of high performance SNAP devices, RSC is budgeting two
process lots in a parallel approach. One approach is to utilize a low
risk evolutionary process to improve read noise and QE while the second
approach will be devoted to improving the quantum efficiency for the low
read noise process production as used for the WFC3 final lot. RSC is
confident that within the R&D period, they will be able to produce
devices that meet the SNAP read noise performance specifications.
Nevertheless this remains an important challenge for the NIR R&D
program.

At present Raytheon has limited manufacturing experience with short
wavelength devices and no dark current or noise measurements of 1.7 µm
detectors are available. However, they have produced devices with a 2.5
µm cutoff for the VISTA project and they are confident that with their
multiplexer technology and the LPE process, they will be able to meet
SNAP specifications.

### Quantum efficiency

The SNAP near-infrared quantum efficiency specifications of \> 60%
results from the need to detect Type Ia SNe out to redshifts of z = 1.7
within time constraints of the SNAP observing program. For WFC3 the QE
has steadily improved as RSC gained more experience with the MBE growth
process and the best devices show a QE that increases roughly linearly
with wavelength from about 40% at 1.1 µm to more than 80 % at 1.6 µm.

Raytheon has claimed they understand the problem with this droop in QE
at low wavelengths and that their LPE growth process combined with their
VISTA multiplexer will not show this behavior.

RSC believes that they understand the MBE parameters that control QE and
have produced devices that seem to validate this claim. The QE for these
new devices is uniformly high and exceeds the SNAP NIR specifications.
At the same time these devices show a large read noise. RSC is
approaching these issues and they believe they can control the
production process and produce devices with high QE and low read noise.
We are thus hopeful that the SNAP QE solution will be inherited from the
WFC3 development.

### Intra-pixel variations

Accurate photometry (\~2% overall statistical, \~2% relative systematic)
is the current NIR performance target until SNAP science driven
requirements have been passed down to the detector level. Sensitivity
variations on the scale of a single pixel can significantly reduce
photometric accuracy of under-sampled images. In the near infrared,
diffraction dominates the point-spread function at all wavelengths. At 1
μm the Airy disk is Nyquist under-sampled by about a factor of three for
a pixel size of 18-20 μm. Intra-pixel variations for PACE detectors have
been measured by Gert Finger (ESO) and found to be quite large.

SNAP simulations have indicated that any reasonable intra-pixel
variations can be corrected with 2x2 dithering, yet this remains to be
verified by measurements. To this end, the Michigan group has built a
spot projection system ('spot-o-matic'), which will allow us to project
spots of order \~ 2 μm and above onto various devices. By degrading the
focus we will be able to project diffraction limited images onto
prototype detectors. By dithering these images we can verify that
correction of inter-pixel variations and diffraction limited
reconstruction of images can be achieved.

RSC believes intra-pixel variation will be reduced through the MBE
process as material deposition - and thus the fields within the HgCdTe
material - can be better controlled, resulting in improved collection of
charge. In contrast, Raytheon focuses on improving intra-pixel
variations through the multiplexer design and they believe that their
development will result in more uniform charge collection right to the
edges of the pixels through larger charge collection area. Our tests
will address this issue.

## InGaAs detectors

At JPL material studies on InGaAs detectors from Sensors Unlimited (SU)
and from Microdevices Laboratories (MDL) were performed. Two types of
InGaAs materials from SU (128 element line arrays with 1.7 µm and 2.2 µm
cutoff) and a 1.7 µm cutoff photodiode from MDL were tested. A downward
shift in the wavelength cutoff was observed at low temperatures for all
devices. This is a characteristic of the arsenites. Sensors Unlimited
will hybridize InGaAs detector material to RSC multiplexers to
facilitate evaluation of the material. In general, the InGaAs array
technology is less mature than HgCdTe and it has to be seen if InGaAs
can become a viable detector option for SNAP. The NIR group will perform
all tests necessary to achieve realistic assessment of the arrays.

## Progress in the past year

Over the last year the SNAP NIR program has crystallized with several
important developments. 1) There are now two competing vendors for
HgCdTe FPAs (Rockwell Science Center (RSC) and Raytheon Vision Systems
(RVS). 2) An alternative sensor technology (InGaAs) is under
investigation. 3) A strong NIR team has been assembled to characterize
devices (Caltech, IU, JPL, UCLA, UM). 4) A detector procurement and
development program has been put in place. 5) An active program of
device characterization is under way. 6) Laboratory facilities are in
place at Caltech, JPL, and the University of Michigan.

In order to gain experience with NIR detector technology and to prepare
for the testing and evaluation of HgCdTe detectors, the Michigan group
has set up an infrared detector laboratory. Two read-out controller
systems from Astronomical Research Cameras (ARC) were purchased and
interfaced with a test dewar. During the past 12 months multiplexers
from RSC (H1RG) and from RVS (VISTA 1k, VISTA 2k) were obtained and
interfaced to the system. Noise and dark current measurements on these
devices demonstrated the ability to test HgCdTe FPAs with the required
sensitivity. Within the SNAP NIR group, Michigan has the unique
capability to project micron size spots at NIR wavelength onto FPAs
inside a test dewar. This capability is essential to measure intra-pixel
variations.

The IU group has developed a QE measurement system, which has been set
up at the Michigan lab. Using a calibrated lamp and a calibrated
photodiode this system can illuminate detectors with an irradiance
calibrated to an absolute accuracy of 5% and a uniformity of 0.5% across
a full FPA. This source will also be used for inter-pixel and conversion
gain measurements.

The Caltech group has recently set up a laboratory to test SNAP devices
and acquired a 16-channel ARC control and readout system, which will be
used with an existing dewar. The group will focus on noise, dark current
and QE measurements with the goal of complete electronic
characterization of the devices to be tested. They also are
investigating designs for automating the large scale testing of SNAP
devices. The UCLA group, under subcontract with Caltech, will
concentrate on the characterization of devices supplied by RVS whereas
Caltech will characterize the RSC sensors. All devices will be exchanged
between the SNAP NIR laboratories for concurrence testing.

The JPL group has begun to evaluate InGaAs detectors as an alternate
sensor technology for SNAP. They will share the 16-channel ARC array
controller used by the Caltech group. They will be responsible for
characterizing the InGaAs FPA developed by Sensors Unlimited /RSC.

## NIR R&D plan

The primary R&D goal is to refine our requirements for the wide-field
NIR imager based on additional mission simulations and evaluation of NIR
detector devices, and to produce a realistic cost and schedule estimate
for the CDR. This will conclude in the NIR systems requirements
specification document at the end of the R&D phase. An important element
of this task is to develop detailed acceptance criteria and
qualification procedures for the NIR devices in coordination with the
vendors. The mechanical and thermal design for the NIR and optical
imagers are closely coupled in the integrated focal plane concept and
will be developed jointly with LBNL.

### Detector characterization and testing

The characterization and evaluation of NIR detectors are a high priority
for the NIR group. The Infrared Detector facilities at the University of
Michigan, Caltech and JPL provide the required environment for the
comparative testing and evaluation of industry supplied infrared sensors
and will produce test data relevant to the success of the SNAP science
program. A contract for the development of SNAP specific NIR detectors
has been put in place with RSC, RVS and SU. Table 3 shows the delivery
schedule. Delivery has already begun for multiplexers that will be used
to test our interfaces and facilities. Over the next 14 months a variety
of devices will be developed, starting with engineering grade FPAs and
ending with science grade FPAs optimized for SNAP requirements. The NIR
group will measure detector properties (read noise, dark current,
intra-pixel variation, persistence, quantum efficiency) as functions of
environmental parameters (thermal conditions, operating modes, radiation
exposure). The results of those measurements will allow us to specify
detector characteristics, which, together with the science requirements
established at the beginning of the R&D program, will define the
acceptance criteria for the SNAP NIR flight devices. Our plan is to
concentrate on the following main issues during the R&D:

-   Characterize the read noise, dark current and QE of NIR FPAs

-   Explore alternatives to HgCdTe detector technology

-   Perform comparative testing of multiplexers and NIR devices from
    different vendors

-   Study intra-pixel variations and establish impact on accurate
    photometry

-   Determine optimum sampling strategy for reducing read noise

-   Establish schemes for vetoing cosmic rays through multiple sampling

-   Establish facilities for receiving and qualifying NIR FPAs

-   Optimize calibration techniques and strategies

-   Characterize spurious read noise due to trapped charges and
    establish optimal reset scheme

-   Develop a mechanical and thermal concept for NIR imager in an
    integrated focal plane

-   Bench test thermal and mechanical design

-   Provide input for simulation group and compare simulation results
    against measurements

-   Establish flight grade requirements for NIR detectors

We note that other projects also plan on extensive NIR imaging
capability, for example the WFC3 on HST and the NGST, and we will
consult extensively with their design groups to take advantage of their
development efforts.

[]{#__RefHeading___Toc55464426 .anchor}Table 3. Device Delivery Schedule

  ------------- ------------ ------ ----------- ---------- -------------- -------------
  **Device**    **Vender**          **Grade**   **size**   **Initial      **Delivery
                                                           Site**         Date**

  H1RG          RSC          mux    engin.      1k × 1k    UM             in hand

  VIRGO-2k      RVS          mux    engin.      2k x 2k    UM             in hand

  VIRGO-1k      RVS          mux    engin.      1k × 1k    UM             in hand

  VIRGO-1k\     RVS          FPA    near\       1k × 1k    UM             in hand
  (2.5 µm                           science                               
  cutoff)                                                                 

  H2RG          RSC          mux    engin.      2k x 2k    UM             in hand

  H2RG          RSC          FPA    engin.      2k x 2k    Caltech        March 2004

  H2RG          RSC          FPA    engin.      2k x 2k    UM             April 2004

  VIRGO-1k      RVS          FPA    engin.      1k × 1k    Caltech/UCLA   April 2004

  InGaAs/H1RG   SU/RSC       FPA    engin       1k x 1k    JPL            April 2004

  VIRGO-1k      RVS          FPA    engin.      1k × 1k    UM             May 2004

  H2RG          RSC          FPA    engin.      2k x 2k    UM             July 2004

  VIRGO-2k      RVS          FPA    science     2k x 2k    Caltech/UCLA   Aug. 2004

  VIRGO-1k      RVS          FPA    engin.      1k × 1k    UM             Aug. 2004

  VIRGO-2k      RVS          FPA    science     2k x 2k    UM             Sep. 2004

  H2RG          RSC          FPA    science     2k x 2k    Caltech        Nov. 2004

  H2RG          RSC          FPA    science     2k x 2k    UM             Dec. 2004
  ------------- ------------ ------ ----------- ---------- -------------- -------------

### Filter design

There are several options for fixed filters that are being evaluated.
The fixed filters may be mounted just above the sensors or deposited on
the sensors as part of the sensor processing. The NIR group will provide
input to the filter design and filter performance specifications.

### Mechanical and thermal design specifications

The NIR array will be integrated along with the CCDs to produce the
complete SNAP focal plane imaging system. This system will be passively
maintained at a temperature of 140 K. As part of the R&D effort, we will
work closely with the CCD and SNAP System Engineering to produce an
overall focal plane mechanical and thermal design which ensures the
proper performance of the NIR camera. Issues to be addressed include:

-   Thermal coupling (both radiative and conductive) of the HgCdTe
    devices to the focal plane substrate and surrounding instrument to
    minimize temperature gradients across individual devices, as well as
    the total array, in order to minimize intra-pixel differences in
    quantum efficiency, dark current, and noise.

-   Mechanical mounting which minimizes residual stress on the devices,
    maintains accurate spatial alignment of the devices within the
    array, adequately isolates the devices from launch vibration, and
    minimizes thermal stress on the devices during initial cool-down on
    orbit.

## CDR planning

The deliverable at the end of the R&D phase is a cost and schedule
estimate for construction of the NIR imaging system. Assembly of a NIR
detector test system will be essential in establishing that requirements
are met and in firming up cost estimates. It is likely that much of the
work for the NIR optical and mechanical systems will be done in parallel
with the primary optical and mechanical assemblies.

## Risk assessment and risk mitigation

The production and availability of the desired 1.7 µm cut-off FPA device
depends on the commercial vendors. Limited space experience with NIR
devices (HgCdTe or InGaAs) exists. The SNAP NIR system will have the
largest number of infrared detectors ever and thus presents unique
challenges. HgCdTe focal plane arrays are a relatively new but rapidly
developing technology. The R&D phase will be used to mitigate risks in
key detector technology areas such as noise, intra-pixel variation and
qualification testing. We intend to mitigate these risks by extensively
testing the HgCdTe devices and by working closely with the JWST and WFC3
IR detector groups.

# CCD R&D

## Introduction

LBNL has developed a new type of large-format CCD based on n-type
high-resistivity silicon with p-channels.[^1] The back-illuminated CCDs
are 200-300 μm thick and the substrate is fully depleted by the
application of an independent voltage through the front side to an
optically transparent backside contact. The extended red response and
the increased radiation tolerance compared to conventional n-channel
CCDs make these devices a suitable choice for the SNAP visible sensors.

LBNL CCDs have already been deployed at ground-based telescopes. The
NOAO September 2001 newsletter cover featured an image of the Dumbbell
Nebula NGC 6853 (see Figure 5) acquired with an LBNL 2k x 2k CCD taken
at the WIYN 3.5 m telescope at Kitt Peak National Observatory (KPNO).
All three false colors made use of the extended red response of the CCD.
A supernova spectrum taken by the LBNL Supernova Cosmology Project on
the RC Spectrograph at KPNO is shown in Figure 6. LBNL CCDs have also
been installed at KPNO on the Multi-Aperture Red Spectrometer (MARS) and
on the Lick 3 m Coude Eschelle Spectrograph.

The research effort on LBNL CCDs is focused on two primary activities:
commercialization of the device fabrication process, and design
enhancements to further enhance radiation tolerance and to improve
spatial resolution. The CCD packaging and testing efforts function in
support of this program, providing packaged devices that are used to
obtain imaging performance measurements as well as radiation testing
data.

As a back-up option to CCDs, we are also exploring the possibility of
using silicon PIN diodes hybridized to a multiplexer such as those used
to read out HgCdTe pixel detectors. Silicon PIN diodes have not been
used in astronomical applications yet, and further development would be
required for SNAP. The R&D program is focused on determining whether PIN
diodes meet the SNAP scientific requirements when operated at cryogenic
temperatures and subject to radiation.

## LBNL CCD technology

Figure 7 compares a conventional back-illuminated CCD with the LBNL
technology. LBNL CCDs are fabricated on float-zone refined
high-resistivity (\~10 kΩ-cm) n-type silicon. Holes, rather than
electrons, are collected in the potential wells. An indium-tin oxide
(ITO) coating on the back surface provides three functions:

1.  A contact for a bias voltage that completely depletes the substrate
    so that photo-generated charge from the whole volume is collected.

2.  A transparent window, permitting back illumination.

3.  Part of the back-surface anti-reflective (AR) coating.

Since the devices are thick, they exhibit excellent quantum efficiency
in the red until just below the silicon bandgap at 1050 nm. Interference
fringing, which is a problem in thin devices at wavelengths where the
absorption length is greater than the device thickness, is completely
absent.

Since blue light is strongly absorbed in the gate structure of any
front-illuminated CCD, the CCDs used in astronomy are usually thinned
and back-illuminated. For most devices (\~20 μm thick 10--50 Ω−cm
epitaxial layer), this means thinning to about 20 μm. The thinning
results in transparency in the red, resulting in a loss of quantum
efficiency (QE) and fringing due to multiple reflections. For the LBNL
devices this aggressive thinning process is both unnecessary and
undesirable.

Advantages of the thick, fully-depleted LBNL CCDs include:

1.  High quantum efficiency (QE) up to wavelengths approaching the
    silicon bandgap at just above 1050 nm, where silicon becomes
    transparent.

2.  Absence of fringing. The interference patterns that are present in
    thinned spectroscopic CCDs are completely absent in these thick
    devices.

3.  Good blue response without special processing and without UV
    flooding. A normal CCD exhibits field inversion near the back
    surface, resulting in the loss of QE for blue light where the
    absorption length is very short. Since holes, rather than electrons,
    are collected in the CCDs, the problem does not exist and the
    fully-depleted substrate has no field-free region.

4.  Radiation tolerance due to the very low concentrations of phosphorus
    and oxygen in the n-type high-resistivity silicon.

5.  Large well-depth of 300,000 e^-^ for 15 μm pixels and 130,000 e^-^
    for 10.5 μm pixels.

6.  Low noise readout.

7.  Control of lateral charge diffusion by over depletion of the
    substate.

We have tested a range of CCD architectures with pixel sizes of 10.5,
12, and 15 μm and pixel counts up to 4800 x 1800. In Table 4 we show
routinely achieved results for a variety of CCD characterization
parameters. A measurement of backside-illuminated quantum efficiency is
shown in Figure 8. The broad spectral sensitivity is readily apparent.

The commercialization of this technology is well underway. CCDs have
been successfully fabricated by DALSA Semiconductor on 100 mm and 150 mm
wafers, but the process requires post-processing at LBNL. We are
exploring several fabrication scenarios that are consistent with the
quantities and timescale required by SNAP.

[]{#__RefHeading___Toc55464427 .anchor}Table 4. LBNL CCD
characteristics.

+---------------------------------+------------------------------------+
| **Parameter**                   | **Typical value**                  |
+---------------------------------+------------------------------------+
| Dark current                    | 0.001 e/s/pixel @ 140 K            |
+---------------------------------+------------------------------------+
| Read noise                      | 2 e @ 50 kHz; 3 e @ 100 kHz        |
+---------------------------------+------------------------------------+
| Sensitivity                     | 3.5 μV/e                           |
+---------------------------------+------------------------------------+
| Well depth                      | 300 ke for 15 μm                   |
|                                 |                                    |
|                                 | 130 ke for 10.5 μm                 |
+---------------------------------+------------------------------------+
| Charge transfer inefficiency    | 2x10^-6^ @ 140K                    |
+---------------------------------+------------------------------------+

## CCD production status

We have fabricated CCDs in a variety of formats up to 2k x 4k on 300
μm thick, 100 mm wafers at the LBNL Micro Systems Lab (MSL), a facility
intended primarily for research and development. In order to produce
thick, fully-depleted CCDs in larger quantities with industrial quality
control, we have been processing waferts at DALSA Semiconductor
Corporation over the last several years using their standard CMOS
process. DALSA processes 675 μm thick, 150 mm wafers, which must be
thinned to 200--300 μm for use in scientific imaging. We have made
unthinned, fully processed CCD wafers at DALSA and they have been of
high quality, with excellent performance for front-illumination,
demonstrating the process flow up to wafer thinning. Figure 9 shows a
150 mm wafer fabricated at DALSA containing a variety of CCDs and Figure
10 shows a front-illuminated image from a 2k x 4k device.

We have attempted to have DALSA fully process thinned wafers, but
breakage on their fully automated equipment is an issue. We did receive
one thinned (300 μm) large format CCD wafer from DALSA that has been
operated successfully in backside illumination tests. shows some
back-illuminated images taken with one device from this wafer. We
believe this to be the world's first commercially fabricated p-channel,
fully-depleted, back-illuminated CCD.

In the last year, much LBNL effort has gone into developing a hybrid
business model in which all of the front-side processing, except
metallization, is completed at DALSA. This includes all the conventional
CMOS process steps. LBNL has the wafers thinned and does the backside
in-situ doped polysilicon (ISDP), which requires a high temperature
furnace (600° C). (Alternately, this step can also be done at DALSA
because minimal handling is required.) The front-side contact etch and
Al metallization must take place after ISDP to avoid the high
temperature, followed by application of the back-side indium-tin oxide
(ITO) and SiO~2~ anti-reflective (AR) coating. Experience has shown that
the MSL is better suited for executing these process steps on thinned,
150 mm wafers.

LBNL has developed careful handling procedures and equipment
modifications to protect the backside of the wafer during the final
fabrication steps. Backside scratches through the ISDP layer are fatal
for fully-depleted operation. To first order, our experience is that the
main culprit is particles and that they can be removed with mechanical
action (scrubbing). Procedures we have adopted include:

-   Particle removal via wafer scrubbing is the most effective technique
    to date.

-   Use of wear resistant materials on vacuum chucks and wafer handlers
    where possible (DuPont VESPEL^®^ is effective but does shed
    particles).

-   Avoid use of silicone parts. Particles deposited by these cannot be
    remove with scrubbing.

-   First wafers through equipment (coater, aligner) tend to have
    significantly higher particle counts.

-   Photoresist aerosol particles are too large to be removed with
    plasma ashing and require the addition of a solvent to the scrubbing
    soap solution.

LBNL has acquired a 150 mm aligner for the etch and metallization steps,
and has also performed some trial runs with CCDs on 150 mm wafers from
DALSA. Two issues have been identified that are currently the focus of
our CCD production R&D effort.

-   The grinding and polishing of 675 μm wafers to 200-300 μm must meet
    stringent requirements on flatness and surface quality. To date we
    have one qualified vendor who meets our flatness requirements. We
    are also looking into chemical polishing.

-   The MSL etcher is an older model that lacks sufficient controls to
    avoid arcing and burning during the etching of large format CCDs. We
    are working with the Berkeley campus to use their etcher and
    considering the purchase of a better etching machine for the MSL.

Resolution of the thinning and etching issues will clear the way to a
demonstration that CCDs meeting SNAP requirements can be produced in
sufficient quantities, and will also allow us to make LBNL CCDs more
widely available for ground-based astronomy.

## CCD production development plan 

The development plan focuses on achieving CCD production of sufficient
quality and quantity to supply the SNAP visible sensors. To this end, we
will pursue several paths towards the successful industrialization of
the process.

For production of LBNL CCDs we are considering four scenarios:

1.  DALSA does front-side processing up to thinning step; LBNL does
    thinning, ISDP, frontside contact-etch, metallization, and backside
    ITO.

2.  A refractory metal such as TiN is used instead of Al, allowing the
    contact etch and metallization to be completed at DALSA prior to
    ISDP. LBNL is responsible for thinning, ISDP and backside AR
    coating.

3.  Working with a traditional CCD vendor to fabricate CCDs using the
    LBNL technology.

4.  LBNL does the entire fabrication on 100 mm wafers.

We now describe the series of activities over the next 24 months we have
planned in order to accomplish the above goals.

### Develop local 150 mm backside finishing processing capability

We have worked with several vendors and qualified one to grind and
polish wafers to thicknesses between 200 and 300 μm. We are also
exploring a final chemical polishing step to eliminate residual
sub-surface damage, if any remains.

The recently installed 150 mm aligner at MSL has been used to finish 150
mm wafers from DALSA (contact etch, metallization, ITO AR coating). In
the process we identified shortcomings with our Tegal etcher,
specifically in the processing of large format devices. This effect is
less noticable on smaller CCDs and may be due to the "antenna effect"
caused by the long clocking traces charging up in the plasma etch
process. A visit to Tegal did not resolve the issues, and we are now
exploring the use of a new etcher recently installed on the Berkeley
campus. Purchase of a more modern etcher for the MSL is being explored.
Solving these problems and finalizing the backside finishing process
steps is presently the most critical area for our CCD R&D program.

### 150 mm SNAP v. 1 CCD 

We have ordered 36 wafers at DALSA with a new mask set containing our
best effort at the SNAP CCD configuration. This layout is dominated by 4
SNAP CCDs with the following configuration:

-   10.5 μm pixel size

-   3.5k x 3.5k pixel count

-   4-tap readout

-   Higher voltage operation mitigation

The layout also contains two versions of a CCD designed for the visible
light sensor for the SNAP spectrograph. The lot will undergo several
processing splits. Some wafers will be completely processed without
thinning and tested under front-side illumination to study HV behavior.
Some wafers will be processed with refractory metal (TiN) through the
contact etch and metallization step. TiN does not melt in the
high-temperature ISDP back-side processing step, unlike Al, allowing
DALSA to fully complete the front-side processing prior to thinning. A
process split to explore the effects of thinner field oxide is also
planned. The layout is compatible with a 150 mm to 100 mm cut down for
final finishing with MSL's established thin CCD process, if required.

Incorporated in the layout of the large format devices are designs to
enhance the radiation tolerance and reduce diffusion. These include a
layout modification of the p+ guard ring to reduce the possibility of a
highly ionizing particle from shorting the substrate voltage power
supply across the oxides, and the use of a thinner field oxide to reduce
the electric fields in the channel-stop-to-channel and
channel-stop-to-bulk regions. The guard ring layout modifications are
also designed to produce devices that can be routinely operated at 80 --
100 V to reduce diffusion.

The first batch of wafers has arrived and one is shown in Figure 12. The
large devices are the SNAP version 1 prototypes.

### New 100 mm CCD

One of the designs of the large format devices in Figure 12 will be
selected after testing to be produces at MSL on 100 mm wafers.

### 150 mm SNAP v. 2 CCD 

There will be a second round of SNAP CCD pre-production. A new layout
and mask set may be required, depending on the results of the previous
version production.

## CCD packaging status and development

Before a CCD can be tested, it must be packaged so that thermal and
electrical contact can be made. Packaging development is taking place at
LBNL and at Yale University, and we are also working with the University
of Arizona Imaging Technology Laboratory to take advantage of their
expertise in bump-bonding.

At LBNL, a class 10000 clean room was constructed for packaging CCDs.
Additional supporting equipment includes a refrigerator for glue
storage, a vacuum oven, a laminar flow bench, and a wire-bonder. There
is a dedicated test and measurement facility adjacent to the cleanroom
for characterization of devices intended for use at ground-based
telescopes and for SNAP pre-production. To ensure high yield and good
quality devices, the staff has recently undergone training for ESD
protection procedures.

Yale University is experienced in packaging CCDs for astronomical
imaging applications, having recently completed the packaging and
testing of 112 CCDs for the Palomar QUEST large-area camera. Yale has a
class 10,000 cleanroom as well as several dedicated laboratories
equipped for sensor packaging and testing, and a staff experienced in
precision instrumentation for astronomical applications. Yale recently
purchased a dewar and a readout controller for use in SNAP R&D.

Several packaging techniques have been developed. For quick and
convenient testing, a window-frame style mount is used in which the CCD
is positioned in the center of a cutout in a printed circuit board and
wirebonds make the electrical connections. For ground-based astronomy a
4-side abuttable mount has been developed, illustrated in Figure 13. A
CCD is glued to a thick AlN substrate that is patterned with electrical
traces and contracts. Wire-bonds provide contact between the substrate
and the CCD. This is then glued to a molybdenum mounting block
containing precision alignment pins. Precise control of the stacked
height of the assembly is provided by glue layers and a precision gluing
fixture. Experience with this packaging design has taught us to avoid
glue lines or vias under the active region of the CCD; these features
"print through" to the CCD images, most likely by introduction of stress
into the silicon. Figure 14 is an interferograph of the CCD optical
surface at 140 K. The peak to valley variation is about 6 μm.

More recently we have been exploring SNAP-specific, 4-side abuttable
packaging designs that eliminate wirebonds to the CCD. Instead, the CCD
is bump-bonded to and fully supported by an AlN substrate, which in turn
is glued to an Invar mounting block. Finite element analysis has
demonstrated that these materials are well matched for the minimization
of thermally-induced stress in the CCD. This concept is illustrated in
Figure 15. The CCD is bump-bonded to the AlN substrate, and the gap is
filled with low-viscosity glue. An Invar foot is attached to the AlN
substrate. Traces carrying the electrical signals come through vias in
the AlN substrate and are routed to a cutout in the center of the Invar
foot. (The vias are located on the edge of the AlN, below the guard ring
on the CCD.) In the Invar cutout region, a daughter board is mounted and
wirebonded to the traces. The daughter board supports connectors, and
eventually will host the front-end ASIC.

To qualify the parts and procedures necessary for the SNAP packaging
concept described above, we have begun a packaging effort based on the
bump-bonding technique for our large format, 2k x 4k devices. These CCDs
are well-suited for ground-based astronomical use and the experience is
of direct benefit to the development of the packaging for SNAP. To
support this packaging effort, we have brought on an experienced
technician. From this packaging effort we will develop procedures and
yields for production of SNAP parts. In parallel with the 2k x 4k
packaging effort we are preceding with design and mechanical prototyping
of the SNAP packaging concept targeted at the 3.5 k x 3.5 k SNAP v. 1
CCDs presently in production at DALSA Semiconductor.

The optical prescription of the telescope is being used to develop
mechanical placement tolerances for the packaged CCD modules. Control of
the SNAP focal plane surface to ±20 μm appears to be achievable since we
have obtained package height control of 5 μm at room temperature.

## CCD testing results and plans

To support the testing of CCDs, we have in use three sets of dewars,
Astronomical Research readout controllers, and SUN workstations at LBNL
and another setup at Yale University. We have an ever growing suite of
software for the readout controllers and the workstations to support new
measurement procedures. One of the LBNL dewars is devoted to testing
packaged large-format devices for use in ground-based astronomy, and it
will also be used to characterize packaged 3.5 k x 3.5k v. 1 SNAP CCDs.
A second is used for diffusion and intrapixel response measurements, and
the third dewar is used for routine testing and characterization of
devices from new CCD layouts and process variations. In this section we
summarize some of the CCD testing results obtained at LBNL and describe
our future testing plans. More extensive reporting can be found in the
2002 version of this document in the areas of:

-   Dark current

-   Read noise

-   Pre-irradiaton CTE

-   Well capacity

-   Linearity

-   Persistence

-   Source follower FET device characteristics pre and post ^60^Co
    irradiation.

-   Packaging material radiology.

### Radiation performance

Traditional astronomical CCDs are sensitive to non-ionizing radiation,
such as low energy protons encountered in space, primarily due to
degradation of the charge transfer efficiency (CTE). We have an ongoing
program to study the degradation of CCDs in a variety of particle beams.

*[Proton irradiation results]{.underline}*

Proton irradiation generates displacement damage in the silicon. Mid-gap
levels in the depletion region will contribute to the dark current.
Traps in the channel region capture charge carriers during readout and
degrade the CTE. We studied two sets of four CCDs that were
characterized and then irradiated with 12 and 55 MeV protons at the LBNL
88\'\' Cyclotron.[^2] We used our commercially fabricated 512 x 1024, 15
μm pitch CCDs in front-illuminated mode. The four CCDs from each set
were irradiated at doses of 5 × 10^9^, 1 × 10^10^, 5 × 10^10^, and
1 × 10^11^ protons/cm^2^. The irradiation took place while the devices
were unpowered and at room temperature. After irradiation, the devices
were again characterized to evaluate the performance degradation due to
radiation damage.

CTE was determined using an Fe^55^ source to deposit a known quantity of
charge and measuring the transferred charge. The measurement was done as
a function of temperature, with a 30 kpixel/sec readout rate and an
x-ray density of roughly 1/70 per pixel. The pre-irradiation CTE of the
devices was 0.999999. In Figure 16 we compare the CTE damage rate of our
CCDs with two commercial thin, p-type CCDs that have been characterized
in a similar way.[^3]^,^[^4] The loss of CTE in the LBNL CCDs is 8 to 10
times lower than in these devices. For reference, the expected SNAP
mission dose for CCDs behind appropriate shielding will be \<10^10^
p/cm^2^ or \<5x10^7^ MeV/cm^2^.

Conventional CCDs with phosphorus-doped n-channels are susceptible to
the generation of phosphorus vacancy (PV) traps during irradiation that
degrade the CTE, whereas in p-channel devices the dominant trap is
believed to be the divacancy (VV). VV trap formation is considered to be
less likely, and the trap energy lies further from the mid-band than PV
traps and so should contribute less to the dark current. These are the
properties that make p-channel CCDs more radiation resistant.

We have used our parallel and serial CTE measurements at different
temperatures, radiation dose and proton energy to identify the
degradation mechanism. In addition to the divacancy trap, carbon and
oxygen impurities in the n-type silicon can result in C-O or C-C traps.
A model has been developed that has only the three trap densities as
free parameters; only the VV and CO traps contribute significantly.

Since our CCDs have a much larger depleted volume than conventional
CCDs, there was a concern that unacceptable dark current levels might
result from radiation damage. Figure 17 shows the measured increase of
dark current with radiation dose at fixed temperature. The increased
dark current is modest.

*[Future irradiation tests]{.underline}*

We plan to carry out radiation tests on cold, powered devices. We
believe that our room temperature study of proton generated traps give
some insight into the cold temperature results, but the measurements
will be performed in any case. This will entail measuring CTE and dark
current at 140 K for a few radiation dose levels. Ionizing radiation at
cold temperatures can integrate trapped charge in oxide layers more
quickly than at room temperature due to different annealing times. We
will also measure cold, powered devices at the LBNL ^60^Co source and
look for CTE changes and operating voltage offsets for a few dose
levels.

Space radiation consists more of species than protons. We plan make use
of the "cocktail" beam at the LBNL 88" Cyclotron and Yale University van
de Graff accelerator to study the impact of more heavily ionizing
particles.

We are working to understand the flux and probable energy deposition of
highly ionizing heavy cosmic rays in shielded CCDs. The GEANT and MARS
simulation packages are being used in studies at FNAL to model the
interactions of particles with the SNAP instrument and cosmic ray
shield, using SPENVIS to model the particle fluences and energy spectra
for the SNAP orbit. In addition to computing the total dose at the
location of the focal plane and in the warm electronics area, this study
will address whether enough localized ionization can occur to cause a
fatal discharge of the substrate voltage across a CCD insulating layer.

### Diffusion

An important parameter in CCD performance is charge diffusion. Diffusion
in thick, fully depleted CCDs is a function of the temperature,
thickness, wavelength, and depletion voltage. Short wavelength photons
convert at the backside of the CCD and the resultant holes drift to the
collection well in an electric field generated by the depletion voltage.
A hole never sees a field free region and thus has a well defined drift
time through the device. During the drift, there will be lateral
diffusion which will increase the point spread function. The diffusion
is linear in the CCD thickness and goes as the inverse square root of
the depletion voltage. For the LBNL CCD technology, a 200 μm thick
device depleted at 60 V should have an rms diffusion constant of 4 μm, a
good match for a 10.5 μm pixel and the diffraction scale of point
objects in SNAP.

We have constructed a pinhole projector with a 2.4 μm FHWM beam that can
measure diffusion at this level. Measurements have been carried out at
wavelengths from 450 to 650 nm, with a wide range of bias voltages.[^5]
We use a "virtual knife-edge" technique to measure the point spread
function (PSF), illustrated in Figure 18. The CCD is divided into a grid
and all of the charge to one side of a given row or column is integrated
as the pinhole is stepped across this boundary. The derivative of
resulting distribution is a Gaussian at high substrate voltages with rms
equal to the diffusion; see Figure 19.

We have performed measurements on a 280 μm thick, 800 x1100 15 μm pixel,
back-illuminated device. The diffusion at the maximum substrate voltage
of 77 V is measured to be 6.4 ± 0.2 μm, and the dependence on depletion
voltage, shown in Figure 20, is in good agreement with theory.

The diffusion measurement program is ongoing, and will be used to
characterize the SNAP v. 1 CCDs, which should be operable at voltages
exceeding 100V and which we plan to thin to 200 μm. In addition, the
pinhole projector will be used to study intrapixel variations, which can
occur if the charge that drifts to the collection region encounters
distortion due to irregularities in the termination of the drift field.
Detailed electrostatic simulations indicate that this should not be a
problem. To test this we will scan our pinhole, which is much smaller
than the pixel size, across the backside in submicron steps and analyze
the distribution of the collected charge.

### High voltage testing

We have operated LBNL CCDs with depletion voltages of up to 80V and do
not observe breakdown. A review of the CCD design indicated that
internal fields can be reduced by a factor of two by using a thinner
field oxide. A repositioning of the ground guard ring can further
isolate the substrate power supply from the channel region. These
changes have been implemented in the new CCD layout currently in
fabrication at DALSA, and we plan to test the long-term stability of
these devices operated between 80 and 100 V.

### ASIC development support

The development of an integrated circuit for processing the CCD analog
signals has progressed well, and we are about to begin the first tests
with the correlated double sample CRIC chip interfaced to an LBNL CCD.
Clock and bias will be provided by our test lab controllers. A second
test is planned for the next iteration of the readout chip, which will
combine the correlated double sampler and with an ADC. Again, a test lab
controller will provide clocking, bias generating, and data collection.
Ultimately, the front-end chip will be packaged together with the CCD
and both will be tested together at cryogenic temperatures.

## Other visible sensor options

Conservative project management should include backup plans for aspects
of the project that are still in an unproven, R&D phase. For this
reason, we have selected silicon p-i-n diodes for a limited R&D program
as a backup option for the SNAP visible imager. Other CCD technologies
were considered, but suffer from several problems including low QE in
the red, sensitivity to ionizing radiation in space, and unacceptably
large diffusion.

Silicon p-i-n (or "pin") diodes are a promising new technology with some
of the same advantages offered by LBNL CCDs. They are typically
constructed on fairly thick (150 - 200 μm) silicon that is fully
depleted; this controls diffusion and provides sufficient absorption
length for good response in the near-infrared. Radiation tolerance
should also be better than conventional CCDs because there is no charge
transfer inefficiency. Silicon pin diodes are hybridized to a readout
multiplexer, like those used for hybrid near-infrared HgCdTe sensors.
This is a potential advantage for SNAP because a common readout could be
used for both the visible and near-infrared imagers.

This technology is untried in astronomical imaging and the available
pixel sizes are at least a factor of two larger than what is required
for the SNAP visible imager. Development of smaller pixels may be
possible, but would require R&D on both the sensor and the multiplexer.

We have received a Rockwell Scientific for a 1k x 1k, 18 μm pixel
engineering device from their HyVisi silicon pin diode product line. The
device is hybridized to their H1RG readout multiplexer. We plan to
measure the dark current, read noise and characterize the diffusion at
140 K. We have commissioned and received a study from Rockwell for small
pixel development. If the device parameters meet the SNAP scientific
requirements, and smaller pixels can be successfully fabricated and
hybridized, silicon pin diodes would provide an attractive alternative
to the LBNL CCD technology.

## CDR planning

A major deliverable at the end of R&D is a cost and schedule for
producing fully tested CCDs. From the prototyping runs we will know the
foundry production costs and turn-around times and our assembly and test
program will measure the yield of usable devices. From these we can
establish a fabrication cost and schedule.

The yield of devices will also determine how large a production test
facility we will have to construct. Our experience with the R&D test
facility will establish the equipment and labor costs required for a
production facility.

## Risk assessment

Preliminary indications are that commercially fabricated LBNL CCDs
function well: read noise is low, CTE is high, dark current is low, and
radiation tolerance is acceptable. The main risk is selecting a CCD
fabrication plan that can deliver SNAP CCDs on time. We have outlined
several alternatives, one or more of which should be successful. As a
backup we have initiated a limited R&D program to study hybridized
silicon pin diodes as an alternative visible imaging technology.

![](media/image5.wmf){width="3.9590277777777776in"
height="3.995833333333333in"}

[]{#__RefHeading___Toc55464378 .anchor}Figure 5. A false color image of
the Dumbell Nebula featured on the cover of the NOAO September 2001
newsletter. The background stars are only visible because of the 1 μm
wavelength response of our CCDs.

> ![](media/image6.png){width="4.519444444444445in"
> height="3.154166666666667in"}[]{#_Ref503919518 .anchor}

Figure 6. A supernova spectrum acquired by the LBNL Supernova Cosmology
Project on the NOAO RC Spectrograph showing the extended response of our
CCDs to beyond 1 μm.

![](media/image7.png){width="4.519444444444445in"
height="3.0076388888888888in"}

[]{#__RefHeading___Toc55464380 .anchor}Figure 7. Architecture comparison
of a conventional back-illuminated n-channel CCD and an LBNL p-channel
CCD.

![](media/image8.wmf){width="4.366666666666666in"
height="2.997916666666667in"}

[]{#__RefHeading___Toc55464381 .anchor}Figure 8. Quantum efficiency of
the LBNL CCD.

![](media/image9.png){width="3.553472222222222in"
height="3.2291666666666665in"}

[]{#__RefHeading___Toc55464382 .anchor}Figure 9. A 150 mm commercially
fabricated wafer containing a variety of CCDs. The large rectangles are
2k x 4k, 15 μm CCDs and the large squares are 2880 x 2800, 10.5 μm CCDs.

![](media/image10.png){width="2.0597222222222222in"
height="4.021527777777778in"}

[]{#__RefHeading___Toc55464383 .anchor}Figure 10. A front side
illuminated image taken with one of the 2k x 4k, 15 μm CCDs commercially
fabricated.

![](media/image11.wmf){width="3.3805555555555555in"
height="3.1458333333333335in"}

[]{#__RefHeading___Toc55464384 .anchor}Figure 11. Fully commercially
fabricated CCD. The image on the left is a 1000 s dark exposure and the
image on the left is a 600 nm flat field

![](media/image12.jpeg){width="3.7618055555555556in" height="3.5in"}

[]{#__RefHeading___Toc55464385 .anchor}Figure 12. A 150 mm commercially
fabricated wafer containing four 3512 x 3512, 10.5 μm SNAP version 1
CCDs.

![](media/image13.jpeg){width="4.7347222222222225in"
height="3.5027777777777778in"}

[]{#__RefHeading___Toc55464386 .anchor}Figure 13. Four-side abuttable
packaging developed for front side illuminated CCDs. The CCD is at the
bottom of the stack and is glued to a molybdenum mount. A printed
circuit board is connected with wire-bonds down to the CCD.

![](media/image14.png){width="4.999305555555556in"
height="2.8465277777777778in"}

[]{#__RefHeading___Toc55464387 .anchor}Figure 14. A speckle
interferograph at 140 K of the optical surface of the package shown in
Figure 13.. The maximum peak to valley excursion is 6 μm.

![](media/image15.wmf){width="5.091666666666667in"
height="3.5034722222222223in"}

[]{#__RefHeading___Toc55464388 .anchor}Figure 15. Conceptual design of
the 4-side abuttable package for the SNAP v. 1 CCD

![](media/image16.wmf){width="4.500694444444444in"
height="3.0708333333333333in"}

[]{#__RefHeading___Toc55464389 .anchor}Figure 16. Comparison of LBNL CCD
CTE degradation to two conventional n-channel CCDs. Since the doses were
done at different proton energies, we convert them to a non-ionizing
dose in MeV/g.

![](media/image17.wmf){width="5.107638888888889in"
height="3.4993055555555554in"}[]{#_Ref53994849 .anchor}

Figure 17. Dark current in electrons per 15 μm pixel per hour as a
function of radiation dose.

![](media/image18.wmf){width="2.748611111111111in"
height="2.5104166666666665in"}

[]{#__RefHeading___Toc55464391 .anchor}Figure 18. The \"virtual
knife-edge\" technique used to measure the point spread function.

![](media/image19.png){width="3.9770833333333333in"
height="3.998611111111111in"}

> []{#__RefHeading___Toc55464392 .anchor}Figure 19. Normalized charge
> measured as a function of the scan position over the virtual knife
> edge (top), and the derivative giving the PSF (bottom).

![](media/image20.jpeg){width="4.981944444444444in"
height="2.4479166666666665in"}

[]{#__RefHeading___Toc55464393 .anchor}Figure 20. RMS diffusion in
microns as a function of bias voltage for 3 different wavelengths (left)
and for both row and column directions (right).

# On-Sensor Filter R&D

We are exploring the direct, on-sensor deposition of interference
filters for SNAP. Potentially this could eliminate three issues: the
mechanical support of discrete filter plates above the sensors, the
(correctable) optical distortion due to the thickness of plate filters
and their displacement from the sensors, and the internal reflection and
scattering of bright sources that might mimic a weak source. The primary
goals of this R&D program include:

1.  Develop specifications for the SNAP interference filters and
    demonstrate that these specifications are being met. These include:

-   Transmittance at pass-band center.

-   Maximum allowable change in transmittance versus angle and
    temperature.

-   Pass-band edge 'slope'

-   Out-of band transmittance.

-   Optical flatness.

-   Stability to environmental effects.

-   Sample-to-sample uniformity.

2.  Demonstrate that the filter deposition process has an acceptable
    impact on device yields, and on the operating characteristics of the
    devices. Requirements include:

-   The filter deposition step should exhibit \> 90% device yield.

-   Devices passing the filter deposition step should have a negligible
    increase in readout noise, dark current, cosmetic problems, etc., as
    a result of filter deposition.

The R&D program will proceed through the following set of steps.

## Demonstrate basic proof of principle

Demonstrate basic proof of principle and capabilities of the vendor by
depositing an interference filter on a polished silicon substrate, and
measuring the reflectance of this filter and the mechanical stresses
induced in the wafer as a result of the deposition process. This
requires one fabrication run at Barr Associates where a representative
filter will be deposited on three of four silicon wafers, with the
fourth kept for control. This filter deposition step is already
complete; one of the wafers is shown in Figure 21.

## Develop provisional filter specifications

Develop provisional filter specifications and perform computer modeling
to develop a filter design to meet these specs. The computer modeling
will be performed by our group at Indiana University. This step was
completed in June 2003 and that design was used in Filter fabrication
run #2 (below).

## Filter fabrication run #2

Deposit a representative prototype SNAP filter on a number of commercial
CCDs. These CCDs will be tested for noise, dark current, etc., before
and after deposition. A sufficient number of devices will be coated to
determine device yield and the uniformity of coating. In addition, a set
of these devices will be subjected to a spacecraft environment to
determine stability under environmental variation and over the long
term. All aspects of the filter performance will be tested against the
specifications provided to the vendor. Prior to this run, trade studies
will be conducted to evaluate the chemical compatibility of device and
filter materials, and adjust the filter composition accordingly.

One fabrication run at Barr Associates using 6 commercial CCDs has been
performed. Pre- and post-deposition measurements have been made,
including the investigation of the space environmental effects of vacuum
and low temperature. This task was completed in October 2003.

## Filter fabrication study

Trade studies will be performed at Barr to determine the optimal coating
method for SNAP. Two representative SNAP prototype filter designs (one
in the IR and one in the visible) will be developed by Barr design
engineers per our specifications. A total of six filter deposition runs
will be performed to investigate the feasibility of successfully
depositing the two designs by three different coating methods: thermal
evaporation, ion assist, and sputtering. Both glass substrates and
commercial CCDs will be coated, the former for detailed transmittance vs
wavelength measurements and the latter for detection of damage/changes
to devices. Barr will conduct stress studies on the filters. Indiana
University will conduct pre- and post-deposition measurements of the
CCDs and detailed transmittance vs wavelength measurements on the
witness samples. Environmental effects of low temperature and vacuum
will also be studied at IU.

## Filter masking study

Barr Associates will perform a trade study to determine the best method
of masking CCDs for depositing four quadrant ("checkerboard") filters.
The goal is to minimize the dead zone between filters without damaging
the devices in the process. Both photolithography and mechanical masking
will be investigated. The filter design will be a SNAP prototype band.
The optimal deposition method(s) discovered in the previous activity
(2.5.4) will be used for this study. Polished silicon wafers will be
coated, along with glass substrates witness samples. Indiana University
will characterize the substrates before, during, and after masking and
deposition. Between two and four filter deposition runs will be
required.

## CCD coating study

The most critical element of this R&D effort is to determine if coating
active devices (CCDs and HgCdTe) will irreparably alter their
performance. Barr Associates will deposit a SNAP visible filter design
on an LBNL CCD to verify feasibility. If multiple coating methods from
the previous two studies are still viable, multiple coating runs will be
executed. Indiana University will study the device(s) before and after
deposition to detect any changes the process will induce. If
intermediate masking and coating steps are required, we will make
measurements after these steps as well.

## HgCdTe filter deposition run

Barr and Associates will deposit a SNAP IR filter design on a HgCdTe
device to verify feasibility. Indiana University will study the device
before and after deposition to detect any changes the process will
induce. Knowledge gained from earlier depositions on CCDs (2.5.3 --
2.5.5) will be applied to maximize the chances of success.

## Risk assessment

The present concept of the SNAP CCD deployment is that each CCD would
have four filters in a 2 × 2 array and a single filter on HgCdTe. As
each sequential step in this R&D program is completed, alternate
plans/studies will be initiated as necessary. This could involve
altering, adding, or even eliminating tasks previously planned. For
example, if the direct deposition of filters is successful but multiple
filter deposition found to be problematic, reconsideration of the CCD
size would be required to take advantage of this technology. Also, it
may be challenging to validate the direct deposition of filters on
HgCdTe devices, since these are expensive parts. If either issue were to
prove daunting, one could fall back on a traditional, albeit
non-trivial, solution consisting of mounting discrete filters over the
sensors.

![](media/image21.png){width="3.222916666666667in"
height="3.0861111111111112in"}

[]{#__RefHeading___Toc55464394 .anchor}Figure 21. Reflected light from a
B-band interference filter directly deposited on an
indium-tin-oxide-coated 100 mm silicon wafer.

# Spectrograph

The SNAP spectrograph is required to identify Type Ia supernovae and to
correct their peak luminosity. The SNAP flight plan budgets time for a
spectrum of each SN candidate at peak to confirm it is a Type Ia. A
spectrum also provides the best tool to understand the details of the SN
composition, allowing for corrections to the magnitude if needed. The
spectrograph is also an important tool for a precise spectrophotometric
calibration.

## Requirements

To identify SNe Ia up to a redshift of 1.7 within the time constraints
of the SNAP flight plan, the spectrograph must have high throughput and
broad wavelength coverage. In addition, the capability to simultaneously
measure the spectra of a SN and its host galaxy would be a significant
benefit: the galaxy subtraction would be simplified and the galaxy
redshift can be determined in most cases. Requirements on the pointing
accuracy and on the calibration will also be critical parameters for a
space application.

Thanks to the broad SN identifying features, a spectrograph with a low
spectral resolution is accurate enough for this application, and will be
optimized for a flat resolution λ/δλ over the full wavelength range. SN
models indicate that parameters such as temperature, velocity or
progenitor metallicity, which are directly correlated to the magnitude,
can be extracted from the shape of the spectrum. The exposure time is
optimized for a specified accuracy on these key parameters at the
highest redshift. This leads to a spectral resolution requirement of
λ/δλ ≈ 100, under sampled at one pixel per resolution element. A
dithering in the spectral direction will be implemented to recover the
optimal sampling if needed. A detector with very low noise in the
visible is required to optimize the metallicity measurement, which
depends on features in the UV. Finally, to measure the galaxy spectra
together with the SN, a field of view of 3" is required with a spatial
resolution of 0.15"; this is small enough to cover the SN, while keeping
the background contribution low.

These requirements lead to the baseline specifications shown in Table 5.

[]{#__RefHeading___Toc55464428 .anchor}Table 5. Spectrograph main
characteristics

  ----------------------------------------- -------------- --------------
  **Item**                                  **Visible**    **IR**

  Wavelength coverage μm                    0.35-0.98      0.98-1.7

  Field of view                             3.0\" x 3.0\"  3.0\" x 3.0\"

  Spatial resolution element (arcsec)       0.15           0.15

  Spectral resolution, δλ/λ                 100            100

  Cumulative throughput                     52 %           45%
  ----------------------------------------- -------------- --------------

## Instrument concept

An in-depth analysis of different spectrograph designs and an
exploration of alternatives have been performed. The outcome of this
trade-off study is the selection of an integral field spectrograph based
on a reflective image slicer as the best possible design for SNAP.

Integral field spectroscopy is now a mature technique and has been used
in many ground based instruments, with the latest generation being
developed for 8-meter telescopes like Gemini and VLT (see e.g., Bacon et
al., 1999; Le Fèvre et al., 2000; Davies et al., 1998). The principle is
simple: take spectra of each resolution element in a contiguous sky
area. In this way it is possible to reconstruct a 3D image on the sky
(x,y,λ). We can measure at the same time the spectrum of the SNe
candidate and that of the host galaxy, without slit losses and without
stringent requirements on the pointing capabilities of the spacecraft.

The proposed technical solution for this integral field spectrograph is
based on a slicer unit (e.g., NGST IFMOS study, Le Fèvre et al., 1999,
ASP conference series, Volume 207, 313). The basic principle is
identified in Figure 22: the 2-D field of interest is \"sliced\" in
several strips, with the slices rearranged to enter the spectrograph as
the equivalent of one single long slit. This concept has several major
advantages compared to a long slit spectrograph. All resolved spatial
elements of a contiguous area on the sky have spectra taken; there are
no slit losses; and the telescope pointing is relaxed to a fraction of
the observed field, rather than being constrained to a fraction of a
slit width, allowing for quick acquisition.

![](media/image22.png){width="3.0076388888888888in"
height="3.504861111111111in"}

[]{#__RefHeading___Toc55464395 .anchor}Figure 22. Image slicer principle
(courtesy J. Allington-Smith, Durham U.).

The instrument functionalities to be developed are summarized in the
instrument block diagram shown in Figure 23. Principal components are
described below.

![](media/image23.wmf){width="4.58125in" height="3.4993055555555554in"}

[]{#__RefHeading___Toc55464396 .anchor}Figure 23. Instrument Block
Diagram

### Relay optics

This unit is the interface between the telescope beam and the
instrument. The optical solution is highly dependent on the
implementation of the instrument. The definition of this optical system
requires the knowledge of the spectrograph position with respect to the
telescope focal plane. The beam can be picked off wherever it is most
convenient for the overall instrument. It will be beneficial to correct
some telescope aberrations (e.g., astigmatism) within this optical
system.

### Slicer unit

The slicer unit acts as a field reformatter. The principle is to slice a
2-D field of view in long strips and optically align all the strips to a
long spectrograph entrance slit. The slicing mirror comprises a stack of
slicers. Each slicer has an optically active spherical surface on the
first edge (see Figure 24). A line of "pupil" mirrors reformats the
image, each mirror sending the beam to a slit mirror for the pupil
adaptation to the entrance of the spectrograph.

The long, thin, active surface of each individual slicer will produce a
large diffraction effect. To minimize flux loses to a few percent, the
spectrograph entrance pupil has to be oversized. A combined theoretical
and experimental approach is underway at LAM to define the optimum
entrance pupil. The baseline requirements on the slicer unit are a
positioning accuracy of λ/10 rms for the optical surfaces and a surface
roughness of 5 nm rms (existing prototypes are fully compliant with
these numbers).

![](media/image24.jpeg){width="3.341666666666667in" height="3.0in"}

[]{#__RefHeading___Toc55464397 .anchor}Figure 24. Picture of a block of
slices

### Optical bench

Thanks to the low beam aperture and field-of-view, the spectrograph
optics will be straightforward. The actual baseline is classical, and
comprises one collimator mirror, one prism with a dichroic and two
cameras. Spherical shapes for all the mirrors will provide sharp enough
image quality, but by using aspherical mirrors it becomes possible to
have a very compact spectrograph. The prism solution is adapted to a
flat resolution on the whole wavelength range. The dichroic allows
coverage of the two channels simultaneously, one for the visible (e.g.,
0.3-0.98 μm) and one for the infrared (0.98-1.7 μm).

### Focal planes

The two focal planes will be designed around the visible and IR
detectors. In the visible, the main goals are high QE and very low
noise: given concerns over degradation due to radiation exposure and the
poor performance of thinned CCDs in the red part of the visible, we will
perform studies looking at the suitability of LBNL CCDs. Thinned,
backside-illuminated low-noise CCDs with 1024x1024 pixels will be the
alternative option. For the IR, some factors limit the choice of
detector technologies: the overall temperature for the SNAP instruments
will be fixed in the range 120 -140 K. The spectrograph has to operate
in this range. The cut-off wavelength of the array needs to be as close
as possible to 1.7 μm, while keeping noise figures low. A 1024x1024,
18.5 μm pixel HgCdTe array from Rockwell is under consideration. The
possibility of working at a slightly lower temperature 100 -120 K will
be investigated if there is a significant benefit in detector
performance, particularly in dark current which will dominate the noise
for long exposures of high redshift SNe.

The choice and evaluation of visible and IR detectors will be performed
in France, maintaining a strong connection with the teams involved in
the characterizing sensors for the imager in order to achieve the
simplest solution for SNAP.

Details of the detector performance specifications are given in Table 6.
To achieve the quoted performance on read noise and dark current, a
multi-sampling technique is required. The impact of the rate of cosmic
rays on the readout noise is under study to optimize exposure time and
readout techniques.

[]{#__RefHeading___Toc55464429 .anchor}Table 6. Detector specifications

  ------------------------------------- ----------------- ----------------
                                        Visible           IR

  Detector size                         1kx1k             1kx1k

  Pixel size                            10-15 μm          18.5 μm

  Detector temperature (K)              140               140

  QE (%)                                \>80              \>70

  Read noise (e)                        2                 5

  Dark current (e/pixel/s)              0.001             0.02
  ------------------------------------- ----------------- ----------------

## Summary of spectrograph parameters

The specifications for each spectrograph element are summarized in Table
7. To avoid single point failure, the detectors will be duplicated for
reliability. Two detectors will be mounted in each focal plane. The
field of view has been enlarged to 3" x 6" and the number of slices
required is 40. This does not imply any change in the spectrograph
optics.

[]{#__RefHeading___Toc55464430 .anchor}Table 7. Spectrograph system
parameters

+---------------------------+----------+--------+--------------------+
|                           | Visible  | NIR    | System description |
+---------------------------+----------+--------+--------------------+
| Spatial resolution        | 0.15     |        | 2x20 Slices        |
| (arcsec)                  |          |        |                    |
|                           |          |        | 0.9 x 18 mm        |
+---------------------------+----------+--------+--------------------+
| Field-of-View (arcsec^2^) | 3 x 3    |        | 3 x 6              |
+---------------------------+----------+--------+--------------------+
| Wavelength (µm)           | 0.35-1.0 | 1      | Dichroic           |
|                           |          | .0-1.7 |                    |
+---------------------------+----------+--------+--------------------+
| Spectral Resolution       | 100-200  | 70-100 | Prism              |
+---------------------------+----------+--------+--------------------+
| Detector Size             | 1kx1k    | 1kx1k  | 800 x 200 useful   |
|                           |          |        |                    |
|                           |          |        | 2 abutted          |
|                           |          |        | detectors          |
+---------------------------+----------+--------+--------------------+
| Pixel size                | 10-20 µm | 18 µm  | Camera F/D=12      |
+---------------------------+----------+--------+--------------------+
| Detector Temp (K)         | 140      | 140    | Passive cooling    |
+---------------------------+----------+--------+--------------------+
| Function                  | D        |        | Dithering unit     |
|                           | ithering |        |                    |
|                           |          |        | Shutter unit       |
|                           | Cal      |        |                    |
|                           | ibration |        | Lamp unit          |
+---------------------------+----------+--------+--------------------+

## Instrument design

During the past year a pre-conceptual spectrograph design has been
developed and preliminary performances studied. A second iteration with
more precise requirements will be done next year.

### Optical design

The optics has been designed to minimize the number of mirrors. This
will reduce the risk but will slightly degrade the image quality. A
solution with 7 mirrors is currently presented which fulfils a first
volume allocation. The optical beam then reaches a BK7 prism with its
back face coated with a dichroic. Visible radiation will be reflected,
while infrared radiation will continue to a second prism, in CaF2 this
time. The BK7 prism is used in double pass in the optical domain to
reach the required dispersion with a smaller prism. The NIR beam is
dispersed by the CAF2 prism in a double pass at the required R\~100.
Each beam enters an appropriate camera that images the spectrum on the
visible detector on one side and the NIR on the other side.

Figure 25 shows the optical layout. Spatial dithering occurs randomly
through small changes due to the pointing accuracy. To implement a
controlled spectral dithering, a simple solution can be realized by
moving the prism between two fixed positions by an actuator. This is
under evaluation and should not be technically difficult.

![](media/image25.wmf){width="3.8090277777777777in"
height="2.803472222222222in"}

[]{#__RefHeading___Toc55464398 .anchor}Figure 25. The optical design of
the instrument

Figure 26 shows the implementation of the spectrograph optics and the
interface with the SNAP telescope. The figure shows that the entrance
point to the spectrograph is at the border of the dark zone (the
interior cone in the imaging camera). The instrument itself stands
behind the large telescope imaging plane at the central place. This
configuration allows the spectrograph to be located away from the region
occupied by the imaging detector and its associated electronics.

![](media/image26.wmf){width="2.240972222222222in"
height="3.0055555555555555in"}

[]{#__RefHeading___Toc55464399 .anchor}Figure 26. Spectrograph
Implementation.

### Optical performance

The current design meets the requirements on image quality. Figure 27
shows that it is diffraction-limited at 1.7 µm. Simulation work is
currently in progress to provide new, refined requirements. The first
results suggest that the design will also satisfy the next iteration in
the requirements.

![](media/image27.wmf){width="4.101388888888889in"
height="2.9993055555555554in"}

> []{#__RefHeading___Toc55464400 .anchor}Figure 27. Image quality at 1.7
> µm.

### Mechanical and structural design

Figure 28 shows a first implementation concept for the mechanical
structure of the spectrograph.

![](media/image28.jpeg){width="2.879861111111111in"
height="3.504861111111111in"}

### Figure 28. Implementation study. {#figure-28.-implementation-study. .unnumbered}

The choice of the material for the support structure will be determined
by its density, rigidity, thermal conductivity, ease of fabrication and
compatibility with other focal plane materials. A trade study will be
carried out; some of the materials under consideration are listed in
Table 8 together with their characteristics. The unquantifiable criteria
are noted from A to D (A = excellent). The selection of the best
material will take place beginning in 2004 with the design of the
structure.

> []{#__RefHeading___Toc55464431 .anchor}Table 8. Material trade study.

  --------------- ------------- ----------- --------- -------- ----------- ---------
                  **Zérodur**   **Invar**   **Sic**   **Mo**   **Al        **Be**
                                                               (alloy)**   

  Young modulus   70 000        210 000     311 000   330 000  70 000      300 000

  Density         2.2           8.1         2.9       10.2     2.7         1.9

  CTE @ 300K      0.03          1.4         2.6       5.1      24          12

  CTE @ 30 K      -0.7          0.3                            1.5         0.1

  Thermal cond\   1.3           13.5        156       138      150         180
  @ 300K                                                                   

  Manufacturing   B             A           C         B        A           B

  Machining       B             A           B         B        A           C

  Cost            B             B           B         B        A           C
  --------------- ------------- ----------- --------- -------- ----------- ---------

The structures that will be used to adjust and clamp all the optical
elements must also be designed, taking into account the material used
for the optical elements and the thermal environment. The experience
acquired in the prototype study for the slicer spectrometer for ESA is
relevant. For this spectrometer we have developed an adjusting system by
using a coordinate measuring machine, and a process of clamping with
"floppy" elements.

![](media/image29.jpeg){width="5.988888888888889in"
height="2.1819444444444445in"}

[]{#__RefHeading___Toc55464402 .anchor}Figure 29. Example of opto-mount
clamping and the associated metrology.

### Mechanical interface with the spacecraft

The mounting of the spectrometer on the main focal plane structure
should utilize a kinematic three-point mount, to avoid problems of
thermal expansion. However, a thermal coupling is also necessary. Figure
30 illustrates the philosophy of the kinematic three-point mount.

The volume and position of the spectrometer is defined in the interface
control document. The optical baffling will be studied. Radiation
protection will be provided by the SNAP focal plane radiation shield,
but to avoid secondary particles, a specific protection can be envisaged
for spectrograph detectors.

![](media/image30.jpeg){width="3.6277777777777778in"
height="2.5027777777777778in"}

[]{#__RefHeading___Toc55464403 .anchor}Figure 30. Kinematic mount
concept.

### Thermal and dynamic analysis 

A thermo-elastic numerical analysis will be performed on the mechanical
elements of the spectrograph. It will be necessary to analyze the
behavior of the structure with changing temperature, and to verify, for
example, the stability of the optical elements.

Analyses (static, modal, and answer dynamics) will be made to guarantee
the behaviour of the structure and the clamping system of the optical
elements. Tests shall be realized (sine low level, sine qualification,
and random) in agreement with the requirement. They will verify the
exactness of analyses.

### Spectrograph Focal Plane Design

A focal plane definition for the spectrograph sensors has to be
developed in the next two years to define the readout electronics and
prepare the thermal and mechanical concept. To follow the requirement on
reliability, a concept realizing two detectors has been defined and is
shown on Figure 31. To allow the readout of the IR and the visible
detector data at the same time, a frame transfer is preferred for the
CCD detector. This also allows for the possibility of an electronic
shutter for calibration purpose.

> ![](media/image31.jpeg){width="4.492361111111111in"
> height="3.501388888888889in"}

[]{#__RefHeading___Toc55464404 .anchor}Figure 31. Dual detector
definition

## Calibration

We have produced a first calibration-oriented performance requirement
document, which sets the requirements on three of the major calibration
procedures of the instrument, namely the flat-field, wavelength and
absolute spectrophotometric calibrations. It is shown that wavelength
calibration must have an accuracy of 20 A and that the relative
spectrophotometric accuracy of the hardware must be 1% between the blue
and red end of the spectra. The baseline calibration procedure is very
similar to the one routinely used in long-slit spectrographs. The main
steps are:

-   Apply detector calibration procedures (may include cosmic-ray
    rejection)

-   Apply flat-field calibration procedure

-   Correct for optical distortion of the spectra

-   Apply wavelength calibration procedure (may be merged with previous
    step)

-   Apply absolute spectrophotometric calibration procedure (note that
    this step also corrects for residual slope introduced during the
    flat-field calibration procedure)

We can already give a preliminary list of what would be needed to
calibrate the spectrograph (a more accurate list will be available once
the detailed calibration error budgets are established):

-   Continuum lamp with uniform illumination over the field of view of
    the spectrograph (could be the lamps used for the flat-fielding of
    the imager).

-   Emission-line lamps with lines distributed over the complete
    spectral range (for the wavelength calibration)

-   Small-step dithering capability at observatory level for spectral
    point-spread function stability (to average out the so-called slit
    effect).

-   At instrument level, it might be necessary to implement spectral
    dithering (by mechanically tilting the prism)

-   Set of spectrophotometric standard stars for the absolute flux
    calibration of the instrument.

-   Possible need for a mechanical or electronic shutter (i.e.,
    employing frame transfer into an unexposed part of the detector) to
    allow short exposure times on bright calibration stars.

## Software development

A full simulation of the optical system has been developed, based on
Fourier optics. It is coupled to the detailed design of the instrument
via Zernike coefficients produced by the Zemax program. The output is a
discrete PSF at the detector level, for a monochromatic point source at
a given position.

It has been used to simulate a complete SN spectrum in the 0.35-1.7 μm
range (see Figure 32), and to verify that the expected performance can
be reached. Spectral resolution and spectrograph plus telescope
throughput have been verified.

![](media/image32.wmf){width="3.7416666666666667in" height="3.0in"}

> []{#__RefHeading___Toc55464405 .anchor}Figure 32. Simulated calibrated
> spectrum.

This approach is not practical for more extensive studies, because of
the large variety of discrete PSF representations needed. To solve this
problem, a parameterization based on a shapelet decomposition of the PSF
is currently developed. First results show that the required accuracy
can be reached with a limited number of eigenfunctions. CPU time
necessary for the reconstruction is manageable and the volume of data is
now very small. We have written a Java implementation of the shapelets
library (A.Refregier, R. Massey).

## Performance

### Spectral resolution

The spectral resolution is shown in Figure 33 and has been optimized to
be as flat as possible. The detailed simulation has been used to confirm
this resolution.

### Throughput

The estimated throughput of the instrument is given in Table 9. Thanks
to the good throughput of the reflective silver-coated optics and to the
slicer performance, the expected total throughput of the instrument is 3
to 4 time higher than that of the HST-STIS. To derive this table, the
full simulation of the slicer and the spectrograph has been used
including the diffraction and aberration effects on each optical plane.

## Risk assessment

Our preliminary analysis indicates that the image slicer and the
detectors are the only components requiring effort during the R&D phase
to mitigate risk later in the project. All other components are well
within the current technology.

![](media/image33.wmf){width="6.003472222222222in" height="2.05625in"}

> []{#__RefHeading___Toc55464406 .anchor}Figure 33. Spectral resolution
> in the visible (left) and in the NIR (right).

[]{#__RefHeading___Toc55464432 .anchor}Table 9. Instrument efficiency
estimation

+------------------------------+------------+-----------+-------------+
|                              | \#         | E         | Cumulative\ |
|                              | elements   | fficiency | efficiency  |
|                              |            |           |             |
|                              |            | /elements |             |
+------------------------------+------------+-----------+-------------+
| Telescope                    | 4          | 0.98      | 0.92        |
+------------------------------+------------+-----------+-------------+
| Relay optic                  | 1          | 0.98      | 0.90        |
+------------------------------+------------+-----------+-------------+
| Slicer                       |            | 0.82      | 0.71        |
|                              |            |           |             |
| (mir                         |            |           |             |
| rors+straylight+diffraction) |            |           |             |
+------------------------------+------------+-----------+-------------+
| Spectro                      | Mirrors 2  | 0.98      | 0.57        |
|                              |            |           |             |
|                              | Prism      | 0.81      |             |
|                              |            |           |             |
|                              | Dichroic   | 0.95      |             |
+------------------------------+------------+-----------+-------------+
| Detector visible             | 1          | 0.9       | 0.52        |
+------------------------------+------------+-----------+-------------+
| Detector NIR                 | 1          | 0.8       | 0.42        |
+------------------------------+------------+-----------+-------------+

## Instrument development road map

> After a trade-off study, a pre-conceptual design has been developed
> which has been used to prove the feasibility and the adaptation of
> this technique to the SNAP mission. The detailed simulation which is
> under development has been used to prove the high level performances
> of the optical system. High level requirements have to be finalized
> based on the science specifications.

### Concept development

Figure 34 shows the instrument development roadmap. The detailed
technical requirements are under study and will be finalized in the next
year, taking into account the simulation results and the calibration
studies. Preliminary interface definitions have been set (ICD
00026-MW02-A2003-09-16) and will be pursued in the next months. A new
instrumental concept will be designed. The main technical change will be
a new definition of the allocated envelope for the spectrograph itself.
A new envelope which is shorter than the previous one is proposed, but
no difficulties are foreseen, as this can be accommodated with the
addition of one mirror in the spectrograph optics.

![](media/image34.png){width="3.2916666666666665in"
height="3.5006944444444446in"}

[]{#__RefHeading___Toc55464407 .anchor}Figure 34. Spectrograph
development roadmap

### Software development

A full implementation of the spectrometer simulation will be pursued. A
parameterization of the shapelet coefficients as a function of spatial
position and wavelength will be established. The combination with the
shapelet decomposition of galaxies will be explored, in order to
simulate realistic data cubes for a SN and its companion. The
development of the data reduction procedure will start. We expect that
it will continue during several years. We will rely on the work of the
EUro3D consortium, which develops general-purpose utilities for integral
field spectrometers. Customization to SNAP will nevertheless require
significant efforts. The precise and detailed simulation of the
instrument and associated physics analyses will validate the final
design at the end of the R&D phase.

### Calibration

> Calibration strategy will be further developed: we will detail each of
> the calibration procedures and build the corresponding error budgets.
> We will then define calibration scenarios (on-the-ground calibration
> prior to launch; initial calibration campaign immediately after
> launch; routine calibration sequences) and derive stability
> requirements for the instrument and observatory (feed-back to the
> opto-mechanical design). These detailed calibration scenarios will be
> used as inputs to the operation of the instrument, in particular to
> estimate the operational efficiency of the instrument.

## R&D activities

Two main activities have been identified as R&D studies for the two next
years: one on the slicer to achieve a TRL 6 ( space environment) level
and to adapt the actual technique to SNAP, the other on the focal plane
development of the detectors to validate the performances with the
chosen technologies.

### Slicer R&D

The proposed image slicer is of the same type as the one studied in the
context of the NGST near-IR spectrograph (Allington-Smith et al., 1999;
Le Fèvre et al., 1999). This technology has been ranked at NASA
readiness level 5 by a panel of NASA experts in the context of the
concept appraisal of pre-phase A NGST studies. The readiness level 6 is
required to be \"space qualified.\" Prototyping activities are on-going
at Laboratoire d'Astrophysique de Marseille (LAM) and in collaboration
with other European institutes to validate this technology both for
large ground-based telescopes and for space applications, under funding
by various agencies including ESA, CNRS and CNES. The R&D effort
necessary to adapt this concept to the SNAP requirements therefore
meshes nicely with on-going activities and will be in time with the R&D
phase.

Specifically, we have an on-going program to qualify image slicers for
space instrumentation. We are now in the process of developing a
realistic prototype for a space-qualified unit, based on Zerodur-glass
slices. By the end of 2003 we anticipate that the image slicer
technology will be ranked at TRL 6. We are currently integrating the
prototype and will conduct a set of tests for its qualification. Visible
test at room temperature will permit us to completely understand the
optical behavior of this technology. Afterward a 30 K test will be run
in order to check the performance at cryogenic temperature. A vibration
test with Ariane 5 specifications will prove the survival of launch. All
principal technologies are identical for SNAP, except for a few
parameters such as temperature. Some prototype work will be done during
the two next years for complete validation for the SNAP case. Figure 35
shows the prototype integrated and ready for the test run.

![](media/image35.jpeg){width="3.327777777777778in"
height="2.495833333333333in"}

[]{#__RefHeading___Toc55464408 .anchor}Figure 35. Slicer prototype
integrated

### Detector R&D

As the impact of various sources of noise is critical for the good
performance of the spectrograph, a large fraction of the efforts in 2004
will be dedicated to the electronic noise minimization. A dedicated R&D
program in France, supported by IN2P3/CNRS, will allow us to develop an
expertise in electronics for IR detectors (E. Barrelet/G. Smadja et al.)
and are charged to define the CCD technology and the optimal design for
the spectrograph. Afterward, the chosen solution will be tested and
characterized for the end of the R&D phase. Output documents will be a
development plan for phase B/C/D. Planned developments include a first
study of detector and electronics:

-   For the detector, test benches are under development and have been
    ordered for optical and IR. An H1RG Rockwell detector has been
    ordered and is expected to be delivered January 2004. A dewar
    (80K-140K) will be fabricated which includes accurate temperature
    monitoring and monitoring photodiode allowing to test the Rockwell
    detector in the desired temperature range (leakage currents, readout
    noise, homogeneity, intrapixel variations, clock frequency,
    optimisation of multi-read scheme). Particular attention will be
    given to understanding the 1/f noise. Concerning the CCD evaluation,
    the radiation tolerance of small spectrographic devices will be
    considered. A Deep Depleted EEV CCD will be evaluated, including
    fringing tests.

-   Actual activities on the electronic development are reported
    elsewhere and concern ASIC development in collaboration with the
    LBNL group with a contribution to a rad-hard CCD front end and the
    development of an ASIC modelled on the MEGACAM front end
    architecture. The MEGACAM-like ASIC for CCD readout will be tested
    and a readout demonstrator (FPGA + microprocessor) will be
    constructed.

# Data Acquisition and Control 

SNAP generates prodigious amounts of data and determining how to handle
it in the satellite is an important activity during the R&D phase. Data
generation time profiles, different processing options, telemetry rates,
and ground station visibility hours all need to be modeled to arrive at
an optimal conceptual design. An important R&D task is to document all
the control and command functions required by the instruments and their
interface to the on-board data management system. This research will
generate critical input into the satellite design: weight, power, and
volume of processors and memories; required telemetry bandwidth; and the
number and availability of ground stations required.

The R&D tasks described below are

-   Concept development.

```{=html}
<!-- -->
```
-   Implementation research.

-   Requirements development.

-   CDR planning.

The readout electronics transmits data for 30 seconds approximately
every 300 seconds during photometric observations, outputting a total of
12 Gbits/image, uncompressed. The data are stored and over three Tb are
transmitted every three days using a 300 Mbs downlink for approximately
three hours. Collecting, processing, and making this data available to
the telemetry system are challenging. As part of the challenge, we need
to consider such items as redundancy, non-propagating failure modes,
power, and packaging.

The SNAP instruments will have both common and unique needs to operate
them. Under the research part of this activity, we will document the
configuration and operational requirements of the instruments. These
will eventually map into the low level command set to be executed under
the direction of the observation program script periodically uploaded
from the ground.

Conventional CCD controllers are quite bulky and, more important, power
hungry. In a later section we describe development work on application
specific integrated circuits for analog signal processing and
digitization. This is motivated by reduced power and volume, higher
integration and reliability, and radiation hardiness.

## Progress in the past year

In the last year we have developed the block diagram for the instrument
electronics, Figure 36. All imaging sensors on the focal plane with
their readout electronics are on the left hand side. The central part of
the block diagram shows the Observation Control Unit (OCU) and mass
memory. The right side of the block diagram shows the power, telemetry
and ACS systems. The current concept of the focal plane is that it
operates at 140K and that all the detector control and readout
electronics is located on the focal plane. The progress of module "CRIC
ASIC" is described elsewhere. The block diagram shows redundancy
implemented for the OCU, thermal control, focal plane control, power
subsystem telemetry and ACS system. The redundancy for the control and
readout of the spectrograph is implemented by dual set of detectors with
corresponding control and readout electronics. The imager and
star-guider sensors only have redundant connections to the OCU, but do
not have redundant detectors.

## Concept development 

In this set of activities we identify the internal needs of the data
management system and the impacts of external requirements and
limitations, and establish the criteria for selecting between
alternative implementations.

### Focal plane electronics

The focal plane has two kinds of sensor modules, CCD for visible light
and HgCdTe for NIR. In Figure 37 and Figure 38 we show the CCD and
HgCdTe control and readout electronics. Operating these modules at 140K
will required detail analysis and tests of chosen components. The final
choice, if commercially available components don't meet the
requirements, will be to implement this function in one or two custom
circuits.

### Model data volume and rate

SNAP can generate 12 Gbits of exposure data in 30 seconds every 300
seconds when it is operating in optical photometry mode. This
instantaneous data rate must be memory buffered to match any reasonable
telemetry bandwidth. Lossless compression can be applied to the data
before storage or algorithmic processing of multiple images can be
performed to reduce the size of memory buffer required.

### Study memory/CPU/telemetry impacts

SNAP images are formed by adding four or more exposures. The exposures
are cross-correlated to remove contamination by cosmic rays. There are
two approaches on where to combine the exposures into cosmic-cleansed
images, on the spacecraft or on the ground. Co-adding exposures on the
ground would require very little satellite CPU resources and software,
assuming data compression is done in hardware. But large memories and
high telemetry rates are certainly required. Co-adding images in the
satellite can reduce the telemetry rate requirements by a factor of four
but requires sophisticated on-board software operating in several CPUs.
For example, calibration codes would have to be executed in space since
up-link rates are likely limited to a few tens of kilobits per second
and transferring multi-Gbyte files is impractical. Another restriction
is that only one cosmic ray cleanup algorithm can be used and this must
be perfectly tuned prior to launch. Such code would also be complicated
by the need to dither image placement on the imager for each exposure.

We currently favor transmission of all exposures without processing
other than compression. We also believe that without complex on-board
processing it will be easier to design the electronics and software.

### Develop data processing concepts

Here we need to compare the different data processing paths that we
might execute in the satellite and understand their impact on the above.
The metric to be used includes memory size, number of CPUs, quantity of
software, telemetry rate, doing no harm to the data, inter alia.

### Document SNAP instrument control

On the ground, an observation command set will be assembled to direct
the instruments' autonomous activity for a period of time, say three
days. The details of what needs to be accommodated in this command set
needs to be negotiated with the ground station concept team. Since the
instruments will be developed in parallel at more than one site, weight
should be given to a concept that allows multiple, local development
efforts that are easily integrated into an instrument package very late
in the construction phase.

To execute an observation program, the instruments are cycled through
various modes of operation, for example, electronics configuration,
sequencing shutters and filters, reading out detectors, flashing
calibration lamps, erasing persistent images, and more. There will be
health and environmental monitors in the instruments to insure data
quality. Under this task we need to document instrument control and
monitor needs. An early determination of these needs can allow us time
to develop a common physical and logical interface to both the data
collection system and the control system for all the instruments.

The information transferred across the interface between the instruments
and the spacecraft control and telemetry system needs to be defined.
Clearly the observation command set must pass from the telemetry system
to the spacecraft control and then to the instrument control unit. If an
instrument control CPU is executing the observation commands, it needs
to communicate back to the spacecraft control with information such as
the desired pointing direction. An area requiring special attention is
the how the fine star guider data are handled to select stars and
provide high rate updates to the satellite attitude control system.

## Implementation research

In this set of activities we explore potential physical implementation
of the data management system that match with a still fluid set of
requirements.

### Survey available hardware/interfaces

It is undesirable to develop a new untried data acquisition system for
SNAP unless absolutely required. A survey of existing space memory,
CPUs, and architectures needs to be performed, including consultation
with spacecraft vendors and memory system vendors.

For instrument control and monitoring, we will look for existing
protocols and physical implementations and study their suitability for
SNAP.

### Survey of existing/proposed systems

SNAP is not alone in learning how to deal with large satellite data
sets. We will spend time to research what has been done in other
satellites and what is being considered for the future. There is a
wealth of information being generated by the NGST on this issue.

For compression studies, a variety of algorithms are available. One that
is much discussed for space imaging is the Rice algorithm; it is
available in both software and hardware forms. For space images,
compression efficiency is often determined by read noise. We can use
dark images from our CCD test stand to measure how well they compress.
Access to 16-bit unprocessed Hubble images will also be useful.

## Requirements Development

At the end of the R&D phase we will have made the decision on the extent
of on-board data processing we require based on tradeoffs of CPU versus
memory versus telemetry. Implicit in this is the choice of a data
compression scheme.

A requirements document capturing all the interfaces to instrument data
and control, spacecraft control and telemetry, and ground station
instrument control messages will be generated. A block diagram of the
architecture will be developed showing these interdependencies and data
rates.

## CDR planning

The deliverable at the end of R&D is a cost and schedule for the
engineering and construction of the on-board data management
electronics. This will be supported with a requirements document and
block conceptual design.

## Planning for long lead procurements

At this time, no long lead procurements are anticipated.

## Risk assessment

One of the purposes of this research is to minimize the probability that
custom solutions will need to be designed and qualified during the
construction phase. Our architecture will be strongly influenced by
available commercial or other space-proven hardware.

![](media/image36.wmf){width="5.297916666666667in"
height="8.010416666666666in"}

[]{#__RefHeading___Toc55464409 .anchor}Figure 36. Block diagram of the
instrument electronics.

![](media/image37.png){width="4.0680555555555555in" height="8.0in"}

[]{#__RefHeading___Toc55464410 .anchor}Figure 37. Imager CCD module.

![](media/image38.jpeg){width="4.996527777777778in"
height="1.7493055555555554in"}

[]{#__RefHeading___Toc55464411 .anchor}Figure 38. MCT focal plane
module.

# Front-end Electronics

The SNAP instrument electronics will perform analog signal processing,
digitization, clock, control, bias generation, monitoring, and data
acquisition for the wide-field camera, spectrograph and star-guiders. A
concept for the electronics chain has been developed to read out the
SNAP CCD and the HgCdTe sensors.

The block diagram of the readout electronics for the CCD is shown in
Figure 39. The major blocks per CCD sensor are:

1.  DC voltage generation

2.  Clock generation

3.  Correlated double sampler and analog to digital converter (CRIC
    ASIC)

4.  Temperature monitoring

5.  Interface to instrument configuration bus

6.  Interface to image data buffer

The block diagram of the readout electronics for the HgCdTe is shown in
Figure 40. The major blocks are:

1.  DC voltage generation

2.  Clock and bias generation in the Rockwell SIDECAR ASIC

Due to the large number of sensors and the limited power, weight and
volume that can be accommodated by the spacecraft, the conceptual design
has evolved towards miniaturization of the front-end electronics by
means of application-specific integrated circuits (ASICs). While the
requirements for the front-end circuits are within the realm of similar
custom ICs that have been designed for high-energy physics experiments,
any ASIC development effort that combines analog and digital processing
invokes some schedule and technical risk. The electronics R&D program
emphasizes an early effort on front-end ASICs in order to mitigate
these. At the end of development, the parts will be brought together to
build a proof-of-principle front-end system to read out both HgCdTe and
CCD sensors.

The remaining electronics needed for the bias, control and clock
generation, power regulation and data compression are likely to be based
on commercially available parts or simple VHDL digital circuits that
involve much less risk. R&D in this area described in the DAQ section
will focus mainly on the requirements and conceptual design. Power
consumption, thermal requirements, and physical volume are needed inputs
for the spacecraft design, and will be described in an interface control
document.

The SNAP instrument front-end electronics must address several
challenges, including very low noise, large dynamic range, low power
consumption and radiation tolerance. In addition it must be robust and
reliable enough for operation in space. Conventional implementations
relying solely on discrete parts have been surveyed and will exceed any
reasonable satellite power budget. For CCDs, commercially available
integrated circuits have also been considered; there are none currently
available that meet SNAP requirements. For their NIR sensors, Rockwell
is developing an ASIC (SIDECAR) that may meet SNAP requirements for
HgCdTe readout. For the CCD readout we have developed a custom
integrated circuit, the CRIC chip. With minor modifications it can meet
the requirements for HgCdTe readout as well and functions as a backup in
case the Rockwell IC does not meet our requirements or schedule. Both
the LBNL and French groups in SNAP have extensive experience with custom
integrated circuit (IC) design, including the design of
radiation-tolerant circuits in commercially available deep submicron
CMOS. Operation of the readout electronics at cold temperatures enables
locating it very close to the cold focal plane, eliminating long cable
runs for small analog signals, reducing the cable plant and reducing the
complexity of the overall thermal, mechanical, and electrical design

## Detector signal characteristics

The SNAP satellite instrumentation suite will use science grade silicon
CCDs and HgCdTe sensors. An important difference between traditional CCD
devices and the near-infrared sensitive HgCdTe sensors is that HgCdTe
can be non-destructively read out and multiple samples can be used to
reduce the read noise.

CCDs require several clocks to move the pixel charge for readout. A set
of clocks performs a parallel shift of pixel columns into a serial shift
register. The voltage swings of the clocks are approximately 10 V, the
parallel clock capacitances are \~20 nF, and the serial clock
capacitances are \~10 pF. The drain voltages for the output FETs are
20--25 V and the depletion voltage is 30--100 V. Removal of reset noise
can be performed pixel-by-pixel with an analog correlated double sampler
technique.

For HgCdTe devices, the reset and signal sample times are temporally
disjoint and the correlated double sample operation is performed
digitally. The pixels can be non-destructively read. For example, the
pixels are reset, the reset levels are digitized and stored, and after
an exposure period the pixels are digitized again. A digital subtraction
of the two arrays accomplishes the CDS function. Other noise reduction
techniques, multiple Fowler sampling and up-the-ramp sampling are also
possible in the digital domain. Multiple Fowler sampling does many
consecutive reads after reset and after exposure to statistically reduce
the read noise. Up-the-ramp sampling makes periodic, continuous reads of
the pixels during the exposure. This also reduces the impact of read
noise and can detect cosmic ray charge deposition. The readout clocks
for these devices are CMOS logic levels and the maximum output signal is
500 mV.

## Finalize requirements

The requirements document for the SNAP instrument electronics must
consider both CCDs and HgCdTe sensors and their use in both the
wide-field imager and the spectrograph. While there will be some small
differences in requirements among these use cases, the overall goal to
maintain compatibility with all sensor types in SNAP is a primary
requirement for the electronics. Our work so far indicates that a modest
flexibility is achievable to provide this compatibility with a small
extra effort. The requirements document for the SNAP instrument
electronics will discuss requirements for both CCD and HgCdTe operation,
including:

-   On-detector electronics.

-   Analog signal processing.

-   DC voltages and their cycling.

-   Clock parameters such as levels and shapes.

-   Clock sequencer and modes of operation.

-   ADC interface.

-   Data processor/memory interface.

-   Local resource configuration and control.

-   Overall power and control.

Once assembled, the draft requirements document must be formally
reviewed and finalized.

## Development of conceptual design

After agreement to the requirements document, the system architecture
will be developed and detailed in a block diagram. A good deal of
engineering time will be spent on this high level design to make trades
between ASIC and conventional parts implementations. The use of ASICs is
motivated by at least three important concerns: 1) power consumption, 2)
size and weight of the whole system, and 3) reliability. Experience in
high energy physics has shown that the use of ASICs for readout of a
large number of detector channels can reduce the size of the front-end
electronics and the needed power. Large standing currents to load and
unload highly capacitive interconnect media such as copper tracks or
wire bonds on hybrids or printed circuit boards can be reduced and power
saved. As an order of magnitude, the stray capacitance to ground of an
interconnect between two chips a distance one centimeter apart on a
multi-layer PCB is of the order of 1 pF. This is to be compared to 10 fF
for a one-millimeter interconnect on an ASIC.

To date for CCD readout, we have developed and tested an ASIC that
implements a dual-slope correlated double sampler. This is a 12-bit
resolution, 16-bit dynamic range circuit implemented with three
auto-switching gain ranges.[^6] A second iteration of the CDS chip is
planned for submission early 2004. This iteration will include an
integrated pipeline ADC.

The CCD controller is another candidate for ASIC implementation, though
commercial solutions or FPGAs may also provide a solution. The
generation of clock waveforms and control voltages for the CCD and
HgCdTe sensors requires high voltage, limiting the choices in integrated
circuit technology and posing more risk. We will consider both discrete
and ASIC solutions for this aspect.

Rockwell is developing an ASIC for HgCdTe readout for NASA. We are in
contact with Rockwell and will follow this development effort closely
and are scheduled to receive prototypes for detailed characterization.
This work would be done by Cal Tech. If the Rockwell ASIC does not meet
SNAP requirements, it is also possible that we could contract with
Rockwell to submit a modified version that would be suitable for SNAP.
They have provided us with an estimate for this type of work and we
carry this as a contingency.

In the process of developing a conceptual design it is important to
maintain fallback options that can be exercised in the event that R&D
does not succeed in meeting the target goals. We have a number of
fallback options for the SNAP electronics. For example, if we fail to
demonstrate successful cold operation of the control electronics, the
fallback option is to remove it to the warm area. Similarly if the power
consumption of the control electronics cannot be reduced to a level
consistent with the thermal constraints on the focal plane, we will
either remove some functionality to the warm area, or explore more
sophisticated cooling techniques that can handle increased power. It is
also important to have a fallback in case the Rockwell ASIC for the
HgCdTe readout is unsuccessful or fails to meet our requirements. In the
former case we would adapt our CCD ASIC development for readout of the
HgCdTe sensors; in the latter case it we would negotiate with Rockwell
to see if they could develop a SNAP-specific ASIC that did meet all of
our requirements.

## Design prototype system

To be able to test the ASICs several parts of the readout system that
will probably use conventional technologies need to be designed and
built. For example, front-end test modules are being built to test CCDs
and CRIC chip together operating at 140K.

The completion dates we have scheduled for the fabrication of ASICs
follow from submission dates for multi-project prototyping services. A
first iteration of the CDS ASIC was submitted in January 2003. Printed
circuit boards and computer interfacing and test software have been
developed. Stand-alone testing is completed in 2003 has showed that the
CRIC chip meets the performance requirements for SNAP. The second
iteration of the CDS test chip, incorporating an ADC, is scheduled to be
tested and available for system tests summer 2004.

A goal of the R&D period for HgCdTe readout is to demonstrate a complete
readout chain from the sensor to the digitization of the data that meets
all of SNAP's requirements, whether it is based on the Rockwell ASIC or
our own CRIC chip. This includes testing at low temperature and
irradiation.

## CDR planning

A major deliverable at the end of R&D is a cost and schedule for the
construction of the front-end electronics. The development of a
comprehensive requirements document early in the process will help
insure that all the necessary items are identified. The program of ASIC
development and conventional parts selection will establish the
production costs for these items. We note that ASIC development may not
be entirely complete on the time scale of CDR but this does not preclude
costing production parts on the basis of one or two prototype runs. The
cost and schedule for construction will be developed during the last six
months of R&D using as input the actual material costs and labor efforts
required during prototyping of the components described above.

## Planning for long lead procurements

It is likely that at the time of CDR we would seek approval to move
ahead with the procurement of the ASICs as fast as possible, subject to
the state of the prototype development. There are several reasons for
this. One, ASIC fabrication runs have an associated risk of failure
where months of delay can be incurred while parts are re-fabricated.
Two, a chosen semiconductor technology may not have a long lifetime and
therefore may no longer be available if there is a large gap between
development and production. Three, an engineering team will have been
assembled during the design and prototyping stage and continuity of
personnel and familiarity can be lost and additional engineering costs
incurred if parts are not ordered and tested in a timely manner.

## Risk assessment

There are risks associated with developing custom ICs. Design iterations
may be required. Fabrications may fail or be delayed. But the potential
costs are time delays. Success can eventually be achieved. ASIC
development for SNAP is actually itself a risk mitigation. We are also
trying to dramatically reduce the number of parts that need to be tested
for space environment by using ASICs.

![](media/image39.png){width="4.997916666666667in"
height="1.6659722222222222in"}

[]{#__RefHeading___Toc55464412 .anchor}Figure 39. CCD front end readout
module.

![](media/image40.png){width="5.001388888888889in" height="1.65625in"}

[]{#__RefHeading___Toc55464413 .anchor}Figure 40. NIR front end readout
module.

# Mechanical systems

In this section we briefly describe the mechanical components and the
activities surrounding them during the R&D phase. The needed R&D effort
is considered modest. Mainly, the promising design concepts and
interfaces with the telescope and spacecraft will be developed. This is
reflected by the planned cooperative nature of the engineering efforts,
between the satellite systems engineering team and the instrument group.

## Shutter

SNAP will have a mechanical shutter located behind the telescope folding
mirror. A concept is show in Figure 41. The shutter serves multiple
functions: normal exposures with precise timing, fast exposures for
calibration on bright objects, and a flat for calibration illumination.
The R&D tasks are mainly the conceptual development, and follow on
identification of available solutions, which is needed for costing and
scheduling. Redundant and fault tolerant designs plus low noise
operation are a focus.

During the R&D phase, the shutter concept team will interact with the
calibration team to understand the minimum exposure times desired and
practically achievable. Joint activities with the System Engineering
group will be budgeting space and mass, plus insuring that any net
momentum disturbance during shutter motion can largely be negated, or be
accommodated by the attitude control system,

## Shield

Following the shutter, the optical beam expands to the focal plane
inside a housing we will simply refer to as the shield. This shield must
accomplish several things:

-   Isolate the cold focal plane from the warm satellites thermal
    radiation.

-   Block stray light from impinging on the focal plane.

-   Attenuate the cosmic ray flux as much as is practical.

-   Absorb backscattered light from the surface of the sensors and their
    mounts.

The role of the shield as a cosmic ray attenuator makes this object
potentially quite massive. Its placement and stability are not critical,
when compared to the focal plane. A simpler attachment scheme, albeit
still with thermal isolation, can be used. To refine the mass estimate,
the R&D plan includes simulations of the charged particle attenuation
using standard computer transport codes. The environment is the measured
fluencies of solar and galactic particles. A subtlety is that CCD
images, in particular, are polluted more by the shear number of charged
particles rather than the charge or energy of any one particle. A
balance between attenuation of the primary incoming flux and their
resulting secondary cascades needs to be considered in this shielding
study.

## Cold plate

A robust annular molybdenum plate is the primary precision mount for the
imager sensors, CCDs and HgCdTe, and the spectrograph (Figure 43). The
plate is maintained at 140 K by connections to a large passive radiator.
Small heaters will provide the desired temperature regulation. The plate
is supported by kinematic mounts to the telescope support structure
(Figure 44). The focal plane will not have mechanical adjusters and must
be stable to a couple of tens of microns in focal depth. These mounts
are envisioned to also provide the thermal isolation between the warm
telescope structure and the cold plate. A concept for the mounts is
shown in Figure 45 that is cluster of metal balls in point contact to
provide the desired thermal isolation. An R&D activity is to design,
build, and test a larger unit that is adequate for the SNAP focal plane.
These designs will be a joint effort on concept development among the
instrument mechanical engineer and mechanical and thermal engineers in
the System Engineering group.

## Cooling

The cooling source for the 140 K focal plane cold plate and its sensors
is an approximately three square meter plate that radiates to space.
Figure 46 shows the radiator and its attachments to the instrument. If
this radiator operates at 120 K, there will be 35 W of 140 K gross
cooling available at the cold plate. There is no development required
during the R&D phase for this device. The main effort will be
interaction between the instrument and the system engineering teams to
keep the electrical power dissipated by the sensors and their
electronics plus other thermal loads and the parasitic losses within the
cooling budget.

The thermal paths between the radiator and the cold plate are termed
heat straps. There approximate position is shown in Figure 47. At the
moment these are thought to be commercially available parts using
thousands of high thermal conductivity carbon fiber strands. Such a unit
is shown in Figure 48. The flexibility of these links decouples forces
and motions of the radiator from the focal plane. No R&D effort is
required for these items.

## CDR planning

A major deliverable at the end of R&D is a cost and schedule for the
construction of the mechanical components of the focal plane array.

## Planning for long lead procurements

No long lead procurements are anticipated

## Risk assessment

We do not identify any high risk issues associated with developing the
mechanical components described above.

![](media/image41.jpeg){width="3.1638888888888888in" height="3.45in"}

[]{#__RefHeading___Toc55464414 .anchor}Figure 41. Shutter concept.

![](media/image42.png){width="3.6131944444444444in"
height="2.4895833333333335in"}

[]{#__RefHeading___Toc55464415 .anchor}Figure 42. The shield is the
conical shape between the shutter and the focal plane

![](media/image43.png){width="2.0756944444444443in"
height="1.9784722222222222in"}

[]{#__RefHeading___Toc55464416 .anchor}Figure 43. A front view of the
cold plate showing CCD and HgCdTe sensors mounted

.

![](media/image44.png){width="2.375in" height="2.140972222222222in"}.

[]{#__RefHeading___Toc55464417 .anchor}Figure 44. Concept of bi-pod
mounts providing kinematic support of the cold plate from the telescope
structure.

![](media/image45.png){width="2.15in"
height="2.3131944444444446in"}[]{#_Ref12857351 .anchor}

> Figure 45. Candidate concept for the thermal-isolation mounts of the
> cold plate to the telescope.

![](media/image46.jpeg){width="1.679861111111111in"
height="2.4972222222222222in"}

[]{#__RefHeading___Toc55464419 .anchor}Figure 46. View of the space
thermal radiator relative to the rest of the instrument.

![](media/image47.jpeg){width="2.259027777777778in"
height="2.504166666666667in"}

[]{#__RefHeading___Toc55464420 .anchor}Figure 47. Thermal conducting
S-links showing their attachment to the cold plate. The near ends of the
links are attached to the radiator.

![](media/image48.jpeg){width="4.09375in" height="1.71875in"}

[]{#__RefHeading___Toc55464421 .anchor}Figure 48. Example of
commercially available S-links comprised of tens of thousands of
flexible carbon fibers.

3.  []{#__RefHeading___Toc55464352 .anchor}Calibration R&D

The R&D tasks associated with SNAP Calibration are driven by the need to
prepare a thorough specification of the spectrophotometric calibration
(absolute color calibration), the data processing pipeline (calibration
pipeline), the instrument calibration requirements, and the interfaces
between the calibration team and the instrument and ground systems
teams. This specification will in turn permit the development of a sound
plan for achieving the required color calibration precision.

1.  Our requirements will be written with careful consultation with
    scientists (astrophysicists, solar physicists, space scientists)
    having established credentials in the various aspects of calibration
    at research institutions and at the National Institute of Standards
    and Technology (NIST). Toward this end we will continue to form a
    calibration team whose cumulative experience is appropriate for a
    project of the scale of SNAP.

2.  Absolute Color Calibration: Our requirements will be carefully
    prioritized with respect to potential tradeoffs that control the
    calibration error budget. We have identified several possible routes
    and we anticipate that as further constraints are better understood,
    our trade studies will allow us to properly assess the different
    methods and select the one best suited for the SNAP science.

3.  Calibration Pipeline: Our specifications for a calibration database
    to deliver the required science will be refined. The pipeline
    produces the calibration products used to process each data
    image: a) dark and bias frames to remove dark current and pedestal
    effects; b) flat fields to control for the high-frequency, pixel to
    pixel variations in response; and c) photometric zero-point and
    spectrophotometric references to determine the flux and colors.
    These calibration products must be monitored during operation.
    During the R&D phase we will develop, in concert with the
    instrumentation groups, diagnostic metrics of system behaviors.

4.  Instrument Calibration: By the end of the R&D phase we will have
    developed a detailed calibration plan tracing all steps in the
    calibration process for each instrument. Each step will include the
    procedures and the means to validate those procedures.

Since the methodology for each aspect of calibration is already
established, we do not anticipate having to engage in basic prototyping.
Both instrument calibration and calibration pipelines have been
established by past and present, space missions, e.g., International
Ultraviolet Explorer (IUE), Hubble Space Telescope (HST), Chandra X-ray
Observatory, and the Space InfraRed Telescope Facility (SIRTF). However,
the R&D phase provides the needed time span in which to conduct concept
development and to refine our requirements for SNAP's more demanding
calibration component, the absolute color calibration.

# Requirements Overview

The main result of our R&D study is a detailed plan for calibration
which will be carefully reviewed by all members of the calibration
working group prior to the scheduled CDR project milestones. Here we
present a preliminary list of the requirements that will be specified
during the R&D phase.

1.  Instrument calibration:

-   In flight and ground methods

-   Performance vs environment

2.  Absolute Color Calibration:

-   Minimum error budget

-   Minimum transfer errors from primary source to target object

-   Maximum allowed systematic errors in the optical

-   Maximum allowed systematic errors in the near infrared

3.  Standard Star Network

-   Base source selection: sun, NIST blackbody

-   Fundamental and primary star selection: different brightness and
    spectral types

4.  Calibration Pipeline:

-   Quality of flat-fields

-   Frequency of calibration observations

-   Diagnostics to monitor temporal changes

-   Diagnostics to monitor frequency shifts in response

-   Impact of calibration hardware (e.g., lamps, shutter) on instrument
    design

# Calibration R&D Trade Studies

## Instrument calibration

The basic concern of calibration is to understand the instrument
response to incoming light as a function of wavelength, intensity,
environment and time. Options for establishing ground calibration of the
instruments are end-to-end (the entire telescope) and piece-wise. In
concert with the instrument teams we will study how the science data can
be used to measure and diagnose changes in in-flight performance. For
example, the SNAP primary science mission will observe the same fields
every four days through each filter, and thus, each of the thousands of
stars in each field. This data time series may be used to monitor
changes in response.

## Calibration pipeline

A software pipeline produces the calibration products used to process
each data image: dark and bias frames to remove dark current and
pedestal effects, flat fields used to divide out the high-frequency,
pixel to pixel variations in response, "super flats" to divide out the
low-frequency variations across each detector. Wavelength solutions
determined from lamp emission spectra are applied to spectral data. The
final stage is the application of the zero-point and spectrophotometric
references. It is well known that accurate flat fields are important for
obtaining well calibrated data. Broad spectrum lamps are typically used,
though uniformly illuminating a large field of view over both the
optical and near infrared can impact the number and spectral range of
the lamps, their location within the optical bench, shutter precision
and possible diffusing surface. Alternatives to relying solely on lamps
for the flat fields are to use zodiacal light, moonlight, earthshine,
and/or median filtering deep sky images.

## Absolute color calibration

A number of trade studies have been identified. Achieving high precision
in the color calibration demands precise control of systematic errors
throughout the spectrophotometric calibration chain. The overall
calibration of the SNAP optical and near infrared imagers and the
spectrograph will be conducted through several routes. We envision
employing a variety of well-studied stars, including the sun, and also
performing indirect transfer calibrations that permit comparison with
NIST irradiance standards to close the loop with fundamental MKS
quantities. We expect that other trade studies may become necessary as
our design space is explored in further detail. Here we list the known
major trade alternatives along with brief explanations of their system
impacts:

## Artificial point sources

Precise blackbodies for irradiance measurements have been used for over
thirty years to calibrate optical systems on the ground and in space.
These integrating spheres are calibrated by NIST. The sun itself can
also be used as an irradiance standard. Precise calibration of the sun
is currently under way with SORCE, a space-based experiment. Since our
target objects are much fainter than these standards, we need to
determine an optimal attenuation method suitable for the mission.

## Standard star network trade studies

Absolute spectrophotometric calibration in the optical has been carried
out on the ground for several stars, most notably Vega. HST
spectrophotometric calibrations are based on models of four pure
hydrogen white dwarfs from the UV to the IR and one solar analog in the
IR, fixed on Landolt V band photometry.

The Sloan Digital Sky Survey (SDSS) developed its own calibration
program based on three primary standard stars whose absolute
spectrophotometry traces back to the absolute calibration of Vega. The
SDSS network of secondary standards was developed with three telescopes
of aperture 0.5 m, 1.0 m and 2.5 m (the Sloan telescope itself) in order
to span the required dynamic range in brightness from the bright primary
standards to the faintest secondaries in the optical. The achieved
precision ranges between 1.5% v′, g′, and r′  and 3% u′ and i′.

SNAP is a space observatory, requiring a similar network of standard
stars for the SNAP filters but with a longer wavelength reach -- from
350 nm to 1700 nm. Several choices are available for establishing SNAP's
own network. The first is to carry out a program from the ground.
However, the atmosphere's many absorption bands in the near infrared may
vitiate the reliability of the planned NIR bandpasses. The second option
is a space program, using the Hubble Space Telescope and perhaps also
SNAP itself (after launch), which bypasses atmospheric effects. Other
options include a mix of ground, space and balloon platforms. The total
cost and schedule of the calibration program depend on the choice of
platform(s).

## Selection of standard stars

Bright fundamental standard stars must be chosen and calibrated against
NIST traceable irradiance standards. The sun, for our purposes, is
considered as an MKS irradiance standard. We have selected our
fundamental flux standards to be the HST calibration set of white dwarf
and GV (solar analog) stars, as well as Vega. Primary standard stars are
calibrated spectrophotometrically and photometrically with reference to
these fundamental standards, and must be selected such that a few lie in
the SNAP fields. Primary standard stars will include not only white
dwarf and solar analogs, but also stars covering a range of spectral
types, suitable for use by both the optical and the near infrared
detectors. Spectrophotometric, or color calibration, secondary standard
stars covering a range of spectral types and brightness in and near the
SNAP fields need to be identified and monitored to exclude variable
objects. The secondary standards are calibrated photometrically against
the primaries.

## Error budget

Other trade studies going hand in hand with the above concern the number
of steps needed to transfer the primary calibration to the target
objects, the identification of systematic errors, and the consequent
carry over of systematic effects to each step. This will affect the
achievable precision. We note that this depends on the platform(s)
chosen as do the relative contributions of each systematic.

The goal of these studies is a calibration error budget, in which the
magnitude and distribution of the various sources of error are correctly
identified. Maintaining an overall requirement that meets the needs of
the science program may require some flexibility on the individual
items. This error budget will be sent for review by experienced
astrophysicists.

4.  []{#__RefHeading___Toc55464355 .anchor}Ground Segment R&D

# Segment Definition

The SNAP Ground Segment consists of all ground systems required to
communicate with the spacecraft to send command and control data,
receive scientific and engineering data, and process all data in a
timely fashion as required to operate the spacecraft and carry out the
SNAP scientific mission. To accomplish these tasks, the Ground Segment
is divided into Mission Operations and Science Operations.

Mission Operations will:

-   Receive science observation requests from Science Operations and
    translate these into Command and Control sequences to be transmitted
    to the spacecraft.

-   Up-link Command and Control data to the spacecraft.

-   Track the spacecraft and determine ground station availability.

-   Monitor spacecraft environmental and engineering parameters to
    ensure proper operation.

-   Receive and buffer scientific and engineering data during down-link
    at orbit perigee.

-   Transmit the data to Science Operations for processing and analysis.

Science Operations will:

-   Receive down-linked data from Mission Operations.

-   Verify data validity and reformat for science processing.

-   Apply all current calibrations as required for converting raw images
    into science-quality images.

-   Analyze the images to identify supernovae and other astronomical
    objects of interest.

-   Update the scheduler to communicate additional observation requests
    to Mission Ops.

-   Catalogue and archive the astronomical objects for further analysis.

-   Provide computing and software resources for simulation studies to
    optimize the mission plan and instrument design.

-   Provide computing and software resources for supporting calibration
    functions.

# Mission Operations R&D Issues

The Berkeley Ground Station (BGS) located at UCB Space Sciences
Laboratory currently hosts Mission Operations for the High Energy Solar
Spectrographic Imager (HESSI) mission. In support of this and other
missions, the Berkeley Ground Station has built up much of the
infrastructure and expertise necessary for carrying out the SNAP
mission. Thus for most of the Mission Ops functions for SNAP, there are
no R&D issues. The few exceptions are:

-   Upgrade of the antenna dish for Ka- band telemetry.

-   The current BGS antenna is an 11 meter diameter dish used for S-band
    telemetry. To work for Ka-band, the dish surface would have to be
    re-worked to sub-millimeter tolerance. This will be investigated but
    it is likely that a new dish will be required.

-   High speed data buffer and link to Science Ops at LBNL.

-   The data rates for SNAP are higher and will require a new buffer and
    link. This will be specified during the R&D phase.

-   Contingency Plan for back-up ground station(s) when BGS is down.

-   Several alternate stations with Ka-band capability are possible
    options so a cost/availability trade study will be undertaken in the
    R&D phase. Data storage and delivery to LBNL will have to be
    understood and specified.

# Science Operations R&D Issues

## Software architectural and framework issues

The high-level workflow management functions of Pipeline Manager for
processing data, Scheduler for managing the observation request queue,
and Exception Handler for dealing with failures must work together in a
"closed loop" manner, a challenging architectural design. In addition,
the framework that defines the overall software environment must be in
place before developers can begin to write consistent code.

The Simulation Architecture is nearly complete. As the simulation
implementation begins, the architecture design will move on to the
Science Ops code.

## Software product trade studies

The basic data processing steps are similar for many space-based and
even some high-volume ground-based projects. Many software products and
packages have been developed for performing these basic functions. Much
of the Science Ops R&D effort will be devoted to evaluating these
products for direct use or adaptation in SNAP. Re-use of debugged,
battle-tested code is extremely cost effective where applicable. To this
end, we have initiated a joint study with a team from the Space
Telescope Science Institute (STScI) who developed data processing
packages for the Hubble Space Telescope (HST). We are particularly
interested in the following products:

1.  OPUS, the HST Pipeline Manager

> This package is the "backbone" that manages the various modules that
> carry out the pipeline analysis functions. OPUS can manage several
> simultaneous pipelines and contains a graphical monitor of progress
> and problems.

2.  SPIKE, the HST Scheduler

> This product assembles observation requests from approved science
> programs, examines the various constraints on observation requirements
> and spacecraft operational requirements, and applies an optimization
> algorithm to determine the "best" observational sequence.

3.  DADS, the HST Data archiving and distribution product

> This product uses a catalogue archive to make data available to
> scientific queries. In includes "on-the-fly" recalibration capability.

This study has now been completed and the report documents the trade
studies, operations concepts, ground system architecture and estimated
development costs.

The SNAP team is also working with the Supernova Factory (SNFactory)
group to test some of the software products in a "real world"
environment that has many similarities to the SNAP mission. The
SNFactory uses the Near Earth Asteroid Telescopes to search for nearby
supernovae. When the analysis pipeline identifies a candidate, a
scheduler is used to queue spectroscopic follow-up on a University of
Hawaii telescope at Mauna Kea. As part of the SNAP R&D program, we have
ported OPUS to LBNL and started to evaluate its applicability as the
SNFactory pipeline manager. In addition, we are helping the SNFactory
team identify a scheduling package. We expect that this symbiosis will
facilitate migration of SNFactory experience and even testbed code into
SNAP and we consider it an important element of the R&D program.

# Simulation R&D Plan

Simulation is the link between the SNAP science goals on the one side
and the instrument requirements and mission operational profile on the
other. In the past, many of these requirements and optimization studies
have come from analytical techniques such as the Fisher Matrix formalism
for error propagation. Specific questions were answered by very focused
studies and calculations. While this has served us well, some of the
issues are so complex and inter-dependent that a full Monte Carlo
simulation of the astrophysical and instrumental effects is required. We
now have an end-to-end Monte Carlo that generates supernovae based on a
few model parameters, propagates the light according to a given choice
of cosmology, includes astrophysical effects such as dust extinction and
weak lensing, and incorporates instrumental effects such as
K-corrections.

Instrumental effects are parameterized in terms of read noise, dark
currents, point spread function, and calibration errors. For comparison
with ground-based telescopes, the Monte Carlo also includes an
atmospheric model with air mass and seeing effects. The result of the
Monte Carlo is a Hubble diagram of corrected magnitude versus redshift.
A cosmological fitter then extracts cosmological parameters from the
simulated Hubble diagram for comparison with the generated cosmology.
The fit cosmology tells us how well we can measure the science
parameters of interest. An example of the output is shown in Figure 49,
extracted from a publication on SNAP science.[^7]

![](media/image49.wmf){width="5.008333333333334in"
height="3.6333333333333333in"}

[]{#__RefHeading___Toc55464422 .anchor}Figure 49. The confidence region
in the dark-energy parameters for different fiducial dark-energy models
representing very different physical origins: a cosmological constant, a
supergravity model, vacuum metamorphosis, axion dark energy, and a
leaping kinetic model. The larger ellipses are the results from SNAP and
the smaller ones are for SNAP plus Planck. Note that *w' ≈ w~a~/2*.

The next step in the simulation R&D program will be to add the actual
instrument emulation to produce pixel-level synthetic data. This
synthetic data can then be fed to the analysis programs to extract the
results. By contrast, the current version short-circuits the analysis by
going directly to the parameterized result. To distinguish the two
levels of simulation, we have dubbed the current parameterized version
SNAP*fast* and we use the name SNAP*sim* for the in-development version
with full instrument emulation. SNAP*sim* will be maintained throughout
the mission, since it will be helpful in understanding the SNAP data as
well as an aid in designing the mission.

The specific simulation R&D plan then consists of:

-   Define the scope of the simulation by identifying the requirement
    studies that must be implemented. System parameters and deadlines
    are compiled from subsystem and project managers into a Science
    Requirements Studies Document. This requires minimal resources from
    the simulation group.

-   Design and implement an architecture to support simulation
    activities. This subset of the full SNAP architecture satisfies the
    pressing needs of the simulation group. The design is developed by
    computer scientists after multiple joint meetings where the
    requirements of the scientist "clients" are established. The
    deliverables are first an Architecture Design Document and then the
    working architecture. Since SNAP*sim* implementation cannot begin
    until the architecture is complete, these two items are
    critical-path deliverables.

-   Design and implement the SNAP build and programming environments.
    The deliverables are a repository, programming standards document,
    programming documentation standards document, nightly build and
    testing mechanism, issue tracking, interactive development
    environment with plug-ins and hardware to house SNAP computing. Very
    little in the environment is SNAP specific so we leverage off the
    work of other experiments, e.g., Atlas, IceCube, SDSS.

-   SNAP*sim* Release 1: Designing and implementing the SNAP*fast*
    functionality (parametrized simulation) in the new software
    architecture. This release will provide the framework, classes, and
    tools to perform mission requirements and optimizations for
    long-lead procurement items and for the ZDR and CDR. Release 1 must
    be available to initiate requirements studies and is thus
    critical-path. The delivery of the Simulation Design Document is the
    responsibility of a small team whose members have expertise in
    computing, supernova cosmology, astronomy, calibration, and HEP
    scientific computing. All programming resources will be assigned to
    its implementation.

-   Performing the various requirement studies using SNAP*sim*
    Release 1. Based on their interests and expertise, simulation group
    scientists are given responsibility for individual studies.
    Priorities and resources are assigned based on the delivery
    milestones. The deliverables are defined in the Science Requirements
    Studies Document.

-   SNAP*sim* Release 2: This extension of SNAP*sim* will provide
    pixel-level simulation tools for performing mission requirement
    studies that require image analysis for the ZDR and CDR. Image
    simulation is particularly for weak lensing science and the Weak
    Lensing Working Group is responsible for providing many of these
    packages. Our expertise in galaxy evolution and gravitational
    lensing combined with studies of instrumental behavior will enhance
    existing image-simulation software written by SNAP collaboratiors.
    This code will then be integrated into SNAP*sim*.

# CDR Planning

The ultimate deliverable from the R&D phase is the CDR. The R&D plan
presented here is designed to lead to a credible scope, cost, and
schedule for the Ground Segment part of the CDR. Preliminary results are
expected for the ZDR approximately one year in advance of the CDR.

It is important to note that, with the exception of the simulation, we
do not plan to implement the Science Ops code during the R&D period. We
will, however, develop preliminary designs for this code and pursue some
implementation for the parts of the code identified as carrying risk in
order to understand the scope, cost, and schedule as necessary for
making a reliable estimate for the CDR.

5.  []{#__RefHeading___Toc55464361 .anchor}Telescope R&D

The design, fabrication, and test of a two-meter class space telescope
is a project that can be expected to require four to five years, a time
scale that places the Optical Telescope Assembly (OTA) work on the
critical path for the planned SNAP mission. The R&D tasks associated
with the OTA are therefore driven by two pressing objectives, both
targeted at minimizing the risk of an OTA schedule slip: First,
described in section 5.2, is the need to prepare a thorough
specification of the OTA performance requirements, the OTA interfaces,
and the OTA acceptance test plan, so that a comprehensive cost/benefit
trade of alternative vendors and fabrication/test flows can be conducted
prior to CDR. Second, described in section 5.3, because the primary
mirror schedule drives the entire OTA schedule, we are pursuing an early
decision on mirror blank material, design, and fabricator and intend to
begin vendor evaluation studies, cost/benefit trades early enough to
establish that all significant technology issues are solved well in
advance of CDR. Moreover, our R&D schedule may allow the initial steps
to be taken towards demonstration of a lightweight flightworthy
technology demonstrator mirror blank. Third, described in section 5.4,
is an overview of the continuing OTA risk assessment.

# Pre-R&D Phase OTA Trade Studies

## Trade 1: Optical configuration

Wide-field astronomical telescopes are not new. Beginning with the
Schmidt survey telescopes deployed at mountaintop observatories during
the mid-20th century, a variety of optical configurations have been
proposed and many have been implemented. The usual goal is to achieve
atmospheric-seeing-limited performance in visible wavelengths. For such
uses, refractive correctors are suitable, and therefore many of these
wide-field cameras utilize refractive elements at the entrance pupil
(e.g., the Schmidt) while others use refractive field flatteners (e.g.,
the Sloan Digital Sky Survey instrument). SNAP, however, requires
diffraction limited image quality, and operates over a much wider
wavelength range, 0.35 to 1.7 μm. Refractive correctors cannot do this
far more demanding job. Instead, to obtain a larger field than can be
obtained with two-mirror Ritchey-Chretien optics and to achieve higher
resolution than can be obtained with two-mirror Schwarzschild optics,
one must explore telescopes that have three powered mirrors.
Three-mirror wide-field telescopes have been discussed by many authors
including Paul, Baker, Cook, Williams, Korsch, Angel, McGraw, Willstrop,
etc. Most of these optical trains suffer from a large central
obstruction in the pupil and from placement of the focal plane camera
deep within the optical path. The obstruction leads to a significant
loss of light gathering power and spatial resolution, and in space it is
difficult to provide passive radiant cooling for the buried camera.
Moreover, the Paul/Baker family of wide-field telescopes are unsuitable
for long focal ratios. One configuration, originally proposed by Korsch
in 1972, eliminates both these limitations through the use of a small
secondary mirror, a tertiary mirror used as an inverting correcting
relay, and of a focal plane located to one side of the primary optical
axis where passive detector cooling becomes easy. For SNAP we have
adopted this annular-field three-mirror anastigmat (TMA) configuration
owing to these manifest advantages. In detailed studies during the
pre-R&D phase we have found this TMA to be adaptable to a variety of
lengths and focal ratios, allowing the diffraction pattern to be
tailored in size for best science recovery.

## Trade 2: Warm optics vs cold optics

It is well known that if the principal optical elements are provided
with heaters and thermostats to keep them close to room temperature in
their flight environment, the testing can proceed within an ordinary lab
environment with no need to establish a variety of thermal test
environments. For example, HST and FUSE were designed in this way, and
mirror temperatures are maintained within a few degrees of 290 K on
orbit. However, owing to nonzero thermal emissivity, the thermal
background seen by an IR detector will depend on the temperature of the
optics. For this reason, a near-IR instrument will have a lower noise
level if its telescope optics are allowed to cool to an equilibrium
temperature that is somewhat lower, below 270 K. This will occur
naturally if no heat is supplied to a well-insulated optic viewing deep
space. However, cold optics would require cryofiguring of the three
powered mirrors, which has direct cost and schedule impact. During the
pre-R&D phase, we explored emissivities of various mirror coating
materials and found that protected silver offers the best reflectance
throughout our critical NIR bands, and furthermore has sufficiently low
emissivity that our predicted thermal mirror background will be below
the zodiacal flux level provided that the mirrors are run below 290 K.
Adopting a baseline temperature near 290 K will ease the mirror
manufacture and test, yet not significantly impact the mission science.

## Trade 3: Integrated focal plane vs separate focal plane instruments

It is well known that achieving a simple optical train (and a simple
optical procurement specification) benefits from having a single input
field and a single output field. Additional intermediate foci can be
provided, but a cost is that the imaging performance of these fields may
be strongly interdependent, and optical adjustments that furnish a good
image at one focus may not furnish a good image at another focus. A
particularly promising instrument concept, the Fully Integrated Detector
Option (FIDO), places all SNAP sensors at a common focal plane, where
they are tightly integrated and share mechanical structure, thermal
environment, EMI environment, etc. The alternative is to have the
various sensors located at distinct focal locations, with starlight
delivered by pickoff mirrors and auxiliary optics. The total costs of
the optical train, its testing, the instrument integration, and the
flight operations scenarios all depend on this choice. During the
pre-R&D period we explored these alternatives and adopted the FIDO
option because it appears that all detector components can successfully
operate at a common temperature and steps taken to simplify the optical
system are certain to minimize the time spent on this critical path.

## Trade 4: Low-CTE structure vs constant-T structure

During ground alignment and flight operations, it is vital to maintain
precise relative positioning of our principal optical components in
spite of environmental thermal variations. Two technical solutions to
this problem are (a) the use of structural materials having nearly zero
coefficient of thermal expansion, and (b) structural elements with a
large CTE but maintained at a precise operating temperature by means of
heaters, insulation, and high thermal conductivity links. During the
pre-R&D phase we explored the use of various materials for the precision
metering structure, and concluded that the strength, stiffness,
manufacturability, and CTE of the best contemporary carbon fiber
composites meet all our requirements. This decision allows a
much-simplified active thermal control system. It is our present
baseline choice for the metering structure.

# OTA Specification Package

The R&D-phase endpoint of the OTA specification task is the completion
of our Conceptual Design Report (CDR). This package will include
engineering trade studies, cost/benefit analyses of the alternative
acquisition, fabrication, and test plans generated and reviewed. Its
focus, however, is the performance specification to which prospective
vendors would be held if under contract. We are aware that over
specifying or under specifying the component-level and system-level
details will incur unnecessary costs and delays, and it is our intent to
have our specification reviewed by an independent committee comprising
experienced personnel from government and industry, as a condition for
its release as part of a formal Request For Proposal. Reviewers will be
asked to comment on the document's thoroughness, depth, clarity, and
effectiveness.

We believe that four policies will materially streamline the OTA
specification package and its use in communicating our needs to
prospective suppliers of a complete OTA system or its components:

1.  Our requirements will constrain the end-to-end optical performance,
    but allow a limited range of vendor-specific freedoms in choice of
    materials, technologies, processing, and optical test facilities. In
    this way we will capitalize on the unique strengths and experience
    of the successful vendor(s). For example, some telescope
    manufacturers routinely use vertical beam optical test facilities,
    while others prefer horizontal beam setups. Similarly there exists a
    variety of optical surface and figure tests, ranging from simple
    Hartmann masks through the most sensitive interferometric null
    tests, with variations that have been developed by each prospective
    vendor.

2.  Our requirements will be finalized after careful consultation with
    optical design and fabrication engineers whose credentials have been
    established through broad industry recognition. Toward this end we
    have support contracts in place with individuals whose cumulative
    experience in large space optics is appropriate for a project of the
    scale of SNAP, and we will continue to strengthen our connections
    with industry by continuing contract work with prospective industry
    partners and suppliers.

3.  Our requirements will be carefully prioritized with respect to
    potential tradeoffs that control the ease of manufacture, test, and
    integration. At present we are pursuing several trade studies
    discussed below.

4.  Most telescope manufacturers are specialists. Some have particular
    expertise in mirror manufacturing while others excel in structure or
    integration. In addition, some NASA facilities have particular
    strengths in optical test and flight qualification work. Our
    procurement planning will therefore explore alternative routes:
    single prime vendor (with subcontracts) and combinations of
    suppliers and facilities. Our goal is to identify a route wherein
    the design, manufacturing, and test teams have the requisite
    experience to assure project success, while affording the customer
    sufficient visibility into the project's progress.

## Performance requirements and draft interface definitions

Here we present an outline of the performance requirements and interface
definitions that will be specified during the R&D phase. These
specifications will be carefully reviewed by all members of the optics
working group, and by a review board, prior to the scheduled ZDR and CDR
project milestones. The finished requirements document will constitute
our primary vehicle for communicating the SNAP OTA description to
prospective vendors. Topics addressed will include:

1.  Optical performance

-   Minimum effective aperture and throughput

-   Maximum permissible obscuration

-   Instantaneous field of view requirement

-   Strehl ratio at 0.63 μm wavelength averaged over field

-   Minimum ensquared energy for 10 μm pixels at 0.63 μm wavelength

-   Maximum allowed vignetting

-   Min and max effective focal length at final focus

-   Minimum throughput

2.  Optical interfaces

-   Entrance aperture definition

-   Exit surface definition

-   Built-in test equipment illuminators & reflectors

-   Optical alignment features

-   Outer baffle intrusion limits

-   Inner baffle intrusion limits

-   Secondary baffle envelope

-   Cold stop intrusion limits

3.  Mechanical interfaces

-   Minimum and maximum mass, Center-of-Gravity, and Momemt-of-Inertia
    as appropriate

-   Allowed mechanical envelope compatible with launcher and S/C

-   Anticipated static and dynamic loads imposed by launch

-   Allowed frequency ranges & Q-factors for response to vibration

-   Location and orientation and tolerance of final focal surface

-   Locations and orientations of OTA focal-equipment interfaces

-   Locations and orientations of payload attach fittings

-   Locations and orientations of electrical connectors

-   Locations and orientations of ground support equipment (GSE)
    hoisting hooks

-   Locations and orientations of shipping container attachments

-   Locations and orientations of optical alignment GSE devices

-   GSE wagon

-   Specifications for in-flight adjustors for secondary and tertiary
    mirrors: range, stability, resolution

-   Structural stability requirements imposed by optical performance
    specifications

-   Specifications for ground alignment adjustors and 1-G support
    fixtures

4.  Electrical interfaces

-   Voltages, currents, heater resistances, signal levels for thermal
    control system

-   Connector locations for OTA-supplied cabling & harnesses

-   Drive electronics plan for on-orbit mechanical adjustors

-   GSE connectors harnessing and sensors for ground thermal & vibration
    tests

5.  Thermal interfaces:

-   thermal heat loads during ambient ground testing, if any

-   identify cryocooling requirements during ground processing & testing

-   thermal environment expected on orbit

-   static & dynamic thermal loads imposed by spacecraft, detector
    system, other loads

-   thermal profiles anticipated during optional detector annealing
    cycles

-   transient thermal/structural response imposed by on orbit attitude
    changes

As part of the OTA Requirements Document we have prepared a baseline
optical system study that serves as an existence proof that our planned
goals can be met. The present baseline optical system comprises a
three-mirror anastigmat having a 2.0 meter aperture and an effective
focal length equal to 21.66 meters, that decision being one of the trade
studies identified below. The TMA layout is patterned after a family of
analytic three mirror designs first explored in the 1972-1990 time
frame. Its particular attractiveness for the SNAP mission is its ability
to deliver a high quality image whose geometrical aberrations are 0.03
arc seconds rms blur, averaged over a large field of view amounting to
about 1.37 square degrees. This baseline system, TMA63, is an all mirror
system that requires no refractive correctors. Further details can be
found at the SNAP website http://snap.lbl.gov where the current draft
specifications and performance estimates are presented, and in two
publications (Lampton et al., Proc. SPIE v.4849 pp.215-226 2002; Lampton
et al., Proc. SPIE v.5166 #12, 2003).

## R&D phase OTA trade studies

### Choice of primary mirror material

A number of mirror materials have been chosen for the manufacture of
large aperture space optics. The two leading examples for our
application are:

-   Corning ULE ™ (HST primary mirror and many others)

-   Schott Glass Zerodur ™ (ORFEUS and SOFIA)

Each prospective mirror manufacturer and telescope vendor has particular
experiences with these materials, and we anticipate that the right
material for SNAP will depend on our vendor\'s favorable experience
working his material efficiently. Studies already completed in the
2001-2003 time frame show that the ULE alternative offers less total
mass and greater stiffness for a given aperture owing to the efficient
closed-back geometry that it makes possible, and ULE has a far more
extensive space flight history. On the other hand, the Zerodur
alternative offers potentially quicker blank fabrication. Our choice
will be decided on the basis of best value to SNAP, considering the
entire tolerance budget, test cost, and overall schedule.

### Telescope manufacturer capabilities

Determining the best route for procurement and test of the SNAP
telescope is the single largest responsibility of the SNAP OTA team.
During the pre-R&D period we visited a number of manufacturers of
telescope optics and systems, and through a broad Request for
Information we have received statements of capabilities from prospective
vendors. During the R&D phase we are extending this work and exploring
alternative routes for design, manufacturing, integration, and testing.

### Total cost & schedule vs telescope wavefront error

Generally, cost and schedule requirements rise as allowable wavefront
error is reduced, because additional cycles of measurement and figuring
impose added labor. For this reason it is important to work closely with
prospective bidders to better understand this tradeoff quantitatively
and to identify the optimum specification for wavefront error.

### Total cost & schedule vs primary mirror thickness

As a general rule, a thicker mirror is heavier but stiffer than a
thinner mirror. Consequently a thicker mirror may take longer to
fabricate and involve processing a larger amount of material, yet may
provide considerable savings during the figuring, testing, and alignment
phases of OTA production. These savings result from reduction or
simplification of auxiliary 1 G support structures, air bags, etc. A
further complication is that a heavier primary mirror creates added
stresses in the OTA structural elements, influencing their size and
strength requirements. These factors can best be understood by reviewing
published analyses and performing finite element modeling for the
primary mirror for a variety of potential lightweight structures, and
assessing the degree of complication that thin and super-thin mirrors
introduce. Preliminary efforts along these lines have already begun, and
will be enlarged and extended under our 2004-2005 R&D plan.

### Prototype vs proto-flight OTA metering structure

The metering structure that locates the principal optical elements is
obviously critical in establishing the strength and stability margins
for the entire OTA. It may be advantageous to build one or more
prototype structural items to permit early proof testing, thermal
expansion testing, vibration-table modal testing, and gravity load
response determination. The alternative is to build an early flight
model of the OTA structure, perform qualification tests using dummy mass
components, and (when qualified) upgrade this structure to flight status
and flow it into the integration process.

### Preliminary test requirements and concept

A major consideration in scheduling and pricing a large space optic is
planning the sequence of accept/reject tests that each element of the
optic will undergo, and devising a fair policy to accommodate
manufacturing deviations that may occur in finishing one element by the
appropriate modification of the successive downstream elements. A clear
test plan, carefully explained in the OTA requirements document, and
supported by established industry manufacturing policies, is a high
priority for SNAP. We intend to create a test plan for the SNAP OTA that
offers flexibility to a prospective vendor in those areas that are
non-critical, but that safeguard the key science driven requirements.
Such planning will result from careful Q&A with established industry
procurement consultants. This process has already begun. At present we
are working towards a draft acceptance test plan that assures the
performance of each telescope mirror separately, and then establishes
the performance of the complete system through the use of both
interferometric and direct-imaging techniques.

This plan will be elaborated during the R&D study to include specific
requirements for performance, vertical/horizontal gravity de-loading,
interferometer vs zone testing, full aperture vs partial aperture
testing, accommodation of nonconforming optical elements, and the
inclusion of one or more compensators into the design to allow for
better control of focal plane position, image surface tilt, and other
concerns.

### Draft integration/test flow sequence

Much of the ongoing test planning is built around a nominal overall
sequence for the integration of the entire mission. This sequence is
divided into levels, distinguished largely on the basis of facility
requirements since each level has an associated suite of tests to be
performed:

**Level 6**: The tested/coated telescope mirrors and the flight metering
structure are integrated with the built-in optical test equipment (BITE)
and the optical GSE. The end-to-end telescope performance tests are
conducted.

**Level 5**: The telescope is integrated with the instrumentation suite:
array imager and spectrometer cryostat, auxiliary guide star trackers.

**Level 4**: The telescope is integrated with the outer baffle and aft
closure.

**Level 3**: The payload is integrated with the spacecraft bus.

**Level 2**: The complete spacecraft undergoes qualification testing:
electro-magnetic compatibility, thermal vacuum, and acoustic/vibration.

**Level 1**: The spacecraft is integrated with the booster and launch
shroud.

### Prospective OTA vendors

Because the success of the OTA procurement for SNAP hinges on prompt
effective participation by an industry partner, it is reasonable to ask
which potential industry leaders are known to have the necessary
experience and facilities to allow this project to precede efficiently.
In consultations with other space astronomy groups we have identified a
small number of potential vendors who can manage all or part of the OTA
project as we have envisioned it.

# SNAP Technology Mirror

The R&D phase of the SNAP Technology Mirror task will accomplish as much
risk reduction as possible given the resources available during R&D.
These subtasks include:

a.  A comprehensive specification for the primary mirror, including
    performance, thermal environment, cost/benefit trades, mirror
    materials trade;

b.  Assessment of industry readiness and ability to accommodate our
    resource allocation limitations during the R&D phase;

c.  Sufficient visibility into our optical acceptance test plan that
    mirror related issues can be safely decided.

Towards these ends we have been aggressively pursuing suppliers within
industry to obtain current cost and schedule estimates for the most
attractive alternative primary mirror (PM) configurations. Estimates
have been obtained during 2002-03 and an additional study in 2004 will
provide further guidance. We have also pursued a series of refined FEMs,
coordinated with mirror blank manufacturers, to ascertain the margins of
safety and the gravity-induced surface deflections that will be dealt
with during mirror and system acceptance testing. We are confident that
either Corning ULE or Schott Zerodur material can be used to produce a
satisfactory PM for SNAP, but the better alternative will be chosen on
the basis of the best overall value to the program.

# OTA Risk Assessment

We have identified several potential risk areas in the procurement of
the OTA. Briefly, the ones we have examined so far are the following.

## Mirror fabrication/test risks

Any large glass item poses a potential risk of damage or fracture during
ground handling, grinding, polishing, and testing, particularly where
there may be extreme environmental tests performed using thermal stress,
vibration, and mechanical shock. Fortunately, large astronomical
telescope mirrors have been safely prepared and handled for projects far
more demanding than the SNAP telescope optics. We are assured that a
two-meter class telescope does not pose unusual risks, particularly when
experienced optical manufacturers are brought on board to manage the
production.

One risk area is the possibility of serious figuring errors being
introduced into one or more of the SNAP telescope optics. We plan to
guard against this by implementing (as a minimum) a four-stage test plan
in which each mirror element is tested individually, and then in concert
as the OTA is assembled. We understand that this progressive test plan,
finishing with full aperture end-to-end test performance while mounted
in the flight OTA structure and supported appropriately against 1 G
deformations, provides the needed assurance that a manufacturing fault
will be discovered before the SNAP mission is launched.

Does the launch stress environment pose a threat to the OTA? A number of
two meter class telescopes have been safely launched using
industry-proven three point support systems. These mirror support
structures are fully qualified for optical elements in our mass class
and have proven themselves on a variety of missions. We intend to
capitalize on this experience and adopt proven structural mirror support
methods for the SNAP mission. Our draft specification currently
specifies kinematic flexure bipods affixed to the PM rear surface at the
70% radius location since our FEM analyses show that this mount
configuration offers the lowest peak launch stress.

Can the thermal environment pose a threat to the on-orbit OTA
performance? We believe that by implementing a comprehensive
thermal-vacuum test plan we can quantify the thermal performance of the
OTA prior to flight. In addition, we anticipate that judicious use of
*in situ* electrical heaters and thermostats will allow us to
understand, and overcome, any reasonable thermal off-nominal situations.
A preliminary thermal budget for the PM, for example, indicates that
approximately 20 W steady radiation to the outer baffle and to deep
space will be balanced by a similar wattage of electrical heaters
carried on the rear (convex) surface of the PM.

## Mechanical structural risks

Any large mechanical structure loaded with a ton (or more) of equipment
may fail in a variety of ways when presented with the acceleration and
vibration environment of a space launch. We plan to mitigate these risks
by adopting a comprehensive plan for piece part testing and for
qualification of materials, processes, and finished structures. In the
aerospace industry it is common to perform static and dynamic load
testing to establish measured safety factors for the mechanical elements
of the OTA. In addition, TMAs have 13 adjustments for focus and
collimation, a subset of which are to be motor-driven on orbit. The
motorized actuators for these adjustments will require their own
qualification and test program during the fabrication phase of the SNAP
project. We anticipate preparing a finite element analysis of the
principal flexural modes of our adopted OTA structure, and computing the
expected loads and margins for a variety of launch environments, to
better understand the tradeoffs associated with the alternative
structural concepts.

## End-to-end performance risks

A key issue with regard to acceptance testing is the fact that all
acceptance tests are necessarily performed in a 1 G environment, while
the only performance that affects the science is the performance that is
manifested in a 0 G environment. For this reason we anticipate the need
for careful planning with regard to the de-loading of the principal
optical and structural elements. In particular, a lightweight primary
mirror is certain to deform significantly unless it is designed with
extremely high stiffness, particularly in the azimuthal deflection mode.
An obvious trade is to compare the expected deflections for various
mirror thicknesses, stiffness, and cost, while factoring the test plan
complications that arise from the 1 G test environment. It is
anticipated that full-aperture end-to-end testing will be required,
performed interferometrically, by null test, or by zonal test. We plan
to also perform imaging tests on the complete OTA using an
autocollimation setup and a test flat; gravity unloading will be
mandatory for this test. With a full understanding of these important
cost and schedule drivers, the SNAP team has begun to assess various
gravity unloading strategies and associated test plans using FEM
techniques.

## Schedule delays owing to requirement revisions

The OTA delivers its image to a science instrument package, and the
details of the final focal surface (plate scale, field, focus position)
drive the telescope design. By the time that bids are sent to
prospective vendors we expect that our exact image requirements will be
fixed. During the R&D phase, however, changes in this instrument package
may occur. We intend to provide the manpower necessary to track
instrument revisions and maintain the OTA requirements document in an
appropriately updated manner.

## Manufacturing error budgets

The manufacturability of the OTA will depend on an appropriate
manufacturing error budget, in which a variety of potential error
magnitudes from figuring, fixturing, assembly, and test are correctly
distributed. These decisions will hinge partly on the experience of the
optical fabrication partner chosen and partly on the judgment of our OTA
team members. We intend to create a preliminary manufacturing error
budget as part of our OTA Requirements Document, and invite prospective
vendors to comment on our assessment and/or provide alternative error
budgets based on their experience. In this way we intend to be flexible
with regard to individual error terms yet remain relatively inflexible
with regard to the overall performance requirements.

## Contamination control

During manufacture, test, shipment, integration, and launch the OTA will
be subject to a number of potential contamination hazards. We intend to
develop a contamination control plan in concert with our OTA supplier
that will safeguard the cleanliness of the optical surfaces during all
phases of the program.

6.  []{#__RefHeading___Toc55464366 .anchor}Spacecraft R&D

Major science instrument driven requirements on the spacecraft include
the provision of:

1.  Structural support between the instrument and the launch vehicle
    attach ring,

2.  Power to operate the instrument as well as spacecraft functions,

3.  A communications link for commands to the instrument an for the
    downlink of engineering and science data,

4.  A system to point and hold the instrument steady for viewing of
    science targets,

5.  A propulsion system for orbit insertion, station keeping, angular
    momentum control, and observatory end of life disposal.

The thermal environment of the SNAP telescope and instruments will be
controlled through a combination of passive and active thermal
management included in the instrument, e.g., radiators, baffles, shields
and heaters. The spacecraft will have an adiabatic interface to the
instrument and will control its temperature through radiation from its
outside surface. Two spacecraft attitude control system components, the
star trackers and the inertial reference units will be mounted on the
Instrument optics bench and will be thermally managed by the Instrument.
Similarly, two instrument components, the observatory control units and
the instrument memory units will be mounted on the spacecraft and will
be thermally managed by the spacecraft.

The SNAP Instrument team has completed a strawman conceptual design of a
possible SNAP spacecraft. We believe that a spacecraft comprised of
industry standard components using industry standard interfaces between
the spacecraft components and between the spacecraft and the instrument
will meet all instrument derived requirements. The most difficult
requirement, that of meeting the need to point and hold the telescope
steady for the science observations, is defined in SNAP drawing
00007-PP02-A. A study was performed by an aerospace systems vendor using
a mechanical mathematical model of the observatory developed by the SNAP
project along with a modified version of the ACS system developed for
the Kepler spacecraft. Their conclusion was that a system using standard
mid-price range components taking data from the star guider system built
into the telescope would easily meet the SNAP requirements.

The intent of the SNAP project is to contract with a single aerospace
industrial contractor who would supply the spacecraft portion of the
observatory system as an integrated package. This vendor may perform all
or a portion of the task of Integration and Testing between the
spacecraft and the instrument. The primary spacecraft-related task
during the R&D period is to clearly define the science driven
requirements on the spacecraft system, and to delineate exactly what is
included in the spacecraft portion of the task.

We intend to utilize the Goddard Space Flight Center's Rapid Spacecraft
Development Office (RSDO) or the standard RFP process to select an
industrial teaming partner for SNAP during the time between CD0 and CD1
or just after CD1. While the spacecraft in the RSDO catalogue are fixed
in price, we are aware that SNAP requirements will require significant
tailoring of spacecraft systems, particularly the mechanical structure
which must be a custom design. Thus, we intend to work with the RSDO
spacecraft contractors to develop a more detailed strawman design
together with a detailed cost and schedule to support the CDR prior to
CD1.

During the R&D phase we will continue with trades and analyses intended
to aid in the development of a clearly defined set of requirements on
the spacecraft system. Thereafter we will be in a position to select a
teaming partner to work on the spacecraft strawman design and develop a
detailed cost in support of the implementation phase. Below we describe
the spacecraft-related analyses and trade studies which will be
continued during the R&D period. Several aspects of the spacecraft, such
as structural and thermal design, are also discussed in Section 7, R&D
System Engineering.

# Spacecraft Related Analyses and Trade Studies

## Observatory structural design 

The design of the observatory primary structure is strongly constrained
by payload requirements including maintenance of the OTA alignment,
payload thermal control, and a requirement for modularity to allow the
instrument to be assembled and disassembled for testing and
trouble-shooting. This leads to the use of a custom observatory
structure design driven by payload requirements rather than the use of a
\"standard spacecraft bus\" which undergoes minor modifications to
accommodate the payload. The SNAP spacecraft structure will include a
load bearing structure which connects to the launch vehicle attach ring
and a tie point to the payload which transfers the entire payload
reaction to the launcher. The structure must also include mounting area
for the spacecraft avionics and propulsion components. A relatively
detailed observatory design is required to settle the question of what
form the spacecraft structure will take, and is also needed before the
thermal design can be completed.

A less obvious but very important requirement is that the mechanical
layout be closely iterated with the integration and test plan so that
sub-assemblies can be removed for testing and debugging without
disturbing portions of the system that already have been tested and
installed.

These issues are discussed in more detail in Section 7, R&D System
Engineering.

## Attitude control system (ACS) preliminary system design and error analysis 

Attainment of 0.02 arc-second rms pointing stability of the telescope is
essential for the full completion of the SNAP mission science goals.
While pointing accuracy at this level and better has been accomplished
on a number of previous spacecraft, (e.g., Hubble at 0.005 arc-seconds
with a significantly less rigid structure and the Remote Mirror
Experiment) this requirement is at the high end of standard spacecraft
practice. We have recently completed an industry study which shows that
a viable ACS solution using standard components exists for the
particular mechanical properties and disturbance sources of the SNAP
mission.

We have also created a computer model of the SNAP ACS system. In work
completed we have concentrated on static performance, creating a dynamic
model including ACS feedback with Kalman filtering. We have integrated
wheel dynamic noise disturbance spectra, and included FEM structural
resonances. We will continue and refine this model during the FY04 R&D
program. We will concentrate on dynamic performance, upgrading the
computer model to include telescope flexural behavior, installing a
simplified fuel slosh model, exploring large angle settling behavior,
and estimating maneuver times required for model mission profile. The
inputs to the model will be continuously updated throughout the program
as the designs of the various sub-systems are refined.

The model will determine the attitude determination capability with and
without attitude information from the Optical Telescope Array (OTA). In
this way we will develop an understanding of the relation between the
requirements on the OTA star guider system (e.g., update rate) and the
ultimate pointing capability of the integrated observatory system.

## Propulsion system specification and analysis 

The SNAP spacecraft will use a monopropellant hydrazine system to unload
the reaction wheels used by the ACS system and to meet the NASA debris
requirements. Since the requirements on this system are well defined and
easily met, this system is not expected to be a risk area or one
requiring any technology development. However, portions of this system
interact with other systems in the observatory in ways that have a
bearing on the successful operation of those systems. In particular,
slosh in the propellant tanks could have a significant effect on the
operation of the ACS system, and mechanical properties of the thrusters
and their associated plumbing may significantly impact the modularity
and testability of the observatory.

For this reason we will create a propulsion system concept and generate
a set of specific tanks, thrusters, valves, and plumbing to be used on
the SNAP observatory. From this we will generate mechanical models of
the tanks to be used in the ACS model.

## Power system components specification 

Since power system technology is very well developed, the system is a
low risk area and will use components with a long history of space
flight use. What is usually the most difficult part of the power system,
the solar array panels, is relatively straightforward for SNAP because
the payload thermal system will provide a large area that is always
pointed within 45 degrees of the sun. The thermal design task will
include the provision of flat areas for mounting solar cells with
suitable capability for dumping waste heat. During the R&D phase we will
refine the power estimates and service requirements and then refine the
strawman power system design to develop detailed requirements on the
control unit.

## High gain antenna and gimbal specification and analysis 

The RF performance requirements for the High Gain Antenna (HGA) have
been analyzed as part of trade studies with the ground systems and orbit
analyses. These trades will continue to refine the size of the steerable
antenna needed to communicate SNAP data to the ground at high rates.

The detailed mechanical properties of the HGA and its associated gimbal
assembly are a required input for the ACS model. During the R&D phase we
will identify manufacturers for these units and determine their
mechanical properties for use in the ACS model.

7.  []{#__RefHeading___Toc55464368 .anchor}System Engineering R&D

SNAP System Engineering is responsible for the coordination and
management of all design and engineering activities involved in the
development of the SNAP Observatory. The task is to insure that the
spacecraft, telescope, instrumentation suite and ground data system form
an integrated and consistent design that will successfully meet the
requirements placed on the project.

During the R&D phase, the primary engineering activity is to select and
refine a conceptual design approach for the observatory and mission that
will meet the SNAP high-level mission requirements, and to perform
design and systems trade studies to optimize the baseline approach. Our
intent is to concentrate effort in those areas requiring the greatest
innovation or posing the greatest potential risk, and to postpone the
development of detailed plans for activities that routinely have been
accomplished on previous missions.

# System Engineering R&D Studies

## Structural design studies

The design for the observatory structure requires the establishment of
an initial set of requirements imposed by the telescope and
instrumentation suite. This design is in progress, and will be continue
throughout the R&D phase. The design approach is to start with the
primary metering structure that supports the 3 mirrors and flat of the
TMA plus the instrument's focal plane assembly. This will then be
integrated with the needed supports for light baffles and thermal
shrouds, including the load paths to the launch vehicle interface ring
via the spacecraft bus. Finally, the generic accommodations will be
included for the instrument electronics modules and the many needed
spacecraft components. The solar array will be mounted on the warmer
side of the large stray light baffle that is always oriented roughly
perpendicular to the sun.

An additional task of the observatory structure design study will be to
refine the extent of the \"spacecraft\" portion of the observatory. A
focus of this task is to refine the spacecraft's primary structural role
in supporting the observatory, while also retaining reasonable
implementation flexibility for the aerospace industry teaming partner,
as part of the spacecraft procurement.

Mathematical models of the structures will be developed in parallel with
the structural design concepts, and will also be used to support the
thermal design, and ACS analyses, as well as verifying the adequacy of
the structure.

The layout, design, and analysis will be done by a team of SNAP
engineers.

## Thermal design studies

The thermal control task is closely related to that of the structural
design, so activities in both areas will proceed in parallel with
continuing interaction reviews. Thermal control is of extreme importance
for the proper operation of the observatory, particularly in three
areas:

-   The maintenance of the figure and focus of the OTA;

-   The maintenance of the desired operating temperature of the focal
    plane sensors and control of stray radiation into the IR sensors by
    nearby structure;

-   The reduction of potential thermal disturbances that might
    compromise the pointing capability.

To address these critical issues, we will develop the detailed thermal
models, including their needed control systems. This task will include
development of the control strategies, design of structural element
coatings, viable thermal isolation concepts, heat strapping, etc.,
followed by modeling of this complete system. Given the sometimes less
rigid nature of baffles, shields, and thermal isolators, the mechanical
properties of those design choices will be a needed input for the
structural, and perhaps ACS models. We will also develop a layout for
the solar panels that maintains the cells in their acceptable operating
temperature range.

By the end of the R&D phase, we expect to have a design concept with
sufficient detail to develop the mathematical models of the system that
will demonstrate the routine observatory operation meeting the SNAP
mission requirements.

## Attitude Control System (ACS) design studies

The key ACS feature, needed to achieve SNAP's precise pointing
requirements, is the baselined "through-the-telescope" pointing
determination that is achieved with high speed video CCDs mounted on the
focal plane. The complete attitude determination calls for supplementary
information from star trackers and a gyro. The Kalman filtering
techniques smoothly blends the sensor inputs in an adaptive manner that
will minimize image jitter.

A detailed ACS modeling effort is being directed by Prof. David
Auslander, a pioneer of modern state space control theory. Both ground
up physical models and also state variable models (using NASA's Treetops
software) are being developed as an ongoing cross check. Sensor (e.g.,
photon counts, IRU drift) and actuator (momentum wheel rumble) are
included with significant fidelity. Spacecraft disturbances, in the form
of flexible body dynamics, fuel slosh, and shutter motions will also be
included in these studies. Strategies for the Kalman filtering processes
are also an important part of these studies.

## Payload and observatory integration and test plan

Because of the stringent pointing requirements on the SNAP telescope and
the fact that the system will be designed to work in a zero gravity
environment but must be tested in a 1 G environment, the I&T task will
be at least as challenging as that of the design and fabrication of the
observatory. The I&T plan is driven first by the requirements of the OTA
manufacturing and testing, and second by the assembly and test of the
integrated science payload including the focal plane sensors. Because
the OTA components provide the major testing challenges, the OTA team
will develop the I&T plan outline.

During the R&D phase, manufacturing and test plans for each candidate
mirror fabrication process will be developed. Methods of performing each
required test will be identified, and a plan will be developed that
includes the identification or design of the facilities required to
accomplish the tests. We anticipate that dedicated custom equipment will
have to be developed which may include modification of existing
facilities. An important part of the process will be an evaluation of
what tests can practically be done, and in what areas we must rely on
analysis (e.g., how can we fully test the attitude control system?).

By the end of the R&D phase we will have a definition of each of the
optics related tests, the order in which they will be done, a definition
of all manufacturing and assembly steps, and a preliminary definition of
the facilities requirements and an evaluation of possible ways to
acquire them.

## Mission analysis and orbit-debris strategy

A key element in the SNAP mission strategy is to provide a suitable
platform from which the science measurements can be made using a readily
available launch vehicle. We currently plan to use a highly elliptic,
3-day orbit which will be synchronized so that the perigee over earth is
centered about the primary ground station. This is achievable by a
several available launch vehicles.

Since this orbit crosses the orbit of geosynchronous satellites, the
mission may be required to de-orbit in less than 25 years. During the
R&D phase, we will determine the best approach to satisfying the orbital
debris requirements.

At the end of the R&D phase we will have a baseline orbit definition,
and the related set of requirements for ACS, propulsion, data and
command systems needed to support the orbit insertion. Any impacts on
the ACS system will be iterated into the parallel ACS modeling efforts.

## Launch vehicle requirements

The refined definition of the observatory structure, mass, pointing
requirements, and orbit analysis developed by the System Engineering
activities performed during the R&D phase will result in a further
understanding of the requirements on the launch vehicle. We will use
this information to make a preliminary selection and to initiate
preliminary discussions with the vendor regarding availability and cost.

## Reliability analysis

During the R&D phase the System Engineering group will develop the first
order failure modes and effects analyses for the observatory, with
emphasis on development of a plan for the use of redundancy and
elimination of single point failures. This information is essential to
properly specify requirements on the spacecraft and to guide the layout
of the payload electronics and data system. This activity will be done
primarily by SNAP engineers with assistance from outside consultants.

## System engineering tracking documents

During the R&D phase we will set up a Configuration Control system for
the project and initiate a formal system for the tracking of mechanical,
electrical and thermal properties of each component and subsystem. We
will also set up systems for tracking computing, data and memory
requirements.

8.  []{#__RefHeading___Toc55464370 .anchor}R&D Phase Management

Proper management and systems engineering activities are crucial to the
development of a good project concept and implementation plan. Proper
technical and organization groundwork during the concept phase will make
possible the straightforward and effective implementation of the project
and enhance the probability for mission success.

Management plays a supportive role in R&D, providing resources as needed
to accomplish an agreed-upon set of research tasks. In contrast to
development phases, management neither dictates the schedule nor demands
strict adherence to a specific budget. Rather, management works with the
PI, Systems Engineering and R&D developers to prioritize those results
that can be obtained for the project with reasonable overall cost and
appropriate schedule. It is management's goal to achieve the lowest
possible mission risk by thoroughly investigating new technologies and
assessing their readiness before proceeding into fabrication.

In parallel with the R&D activities, SNAP management is responsible for
coordinating the scientists and engineers in the development of the SNAP
project concept, in determining its schedule and its implementation
cost. These tasks are further defined below. The groundwork is prepared
for successful project execution during the conceptual design phase.

# Management Elements

Project management during the conceptual design phase is carried out by
the Project Directorate, including: advisory bodies, the Systems
Engineering Office, and the Project Office.

## The Principal Investigator

The Principal Investigator / Project Scientist has principal
responsibility for the project with regard to its scientific mission.
The Principal Investigator (PI) is the spokesperson for the R&D project
and oversees the scientific planning. He leads the activities of the
collaboration, the scientific working groups and receives advice from
the Institutional Board. The PI will maintain frequent contact with the
members of the Institutional Board, from whom he will obtain advice on
all major collaboration issues.

## The Project Directorate

The body responsible for technical, scope, schedule and cost execution
of the project is the Project Directorate. It consists of the Project
Director, the Project Manager, and the Deputy Project Manager. The
Project Office and the Systems Engineering Office assist the Project
Directorate in this regard. The Project Directorate works closely with
the Principal Investigator to maximize the scientific return of the
project.

The Project Director has responsibility for the direction of SNAP
project activities. The Project Director coordinates the R&D, conceptual
design process, and works closely with the Principal Investigator to
ensure the scientific success of the project. On financial matters, the
authority of the PD will be consistent with the requirements of the
funding agencies and will include responsibility for keeping the
agencies informed about the status of the project.

The SNAP Project Manager (PM) is responsible to the PD for the execution
of the project within the schedule, cost and resource constraints
available. With support from the Project Office (PO) and the Systems
Engineering Office (SEO), the PM will establish tasks, work statements,
Memoranda of Understanding (MOU), deliverables, schedules, and changes
to those elements.

The PM is responsible for day-to-day project management, including:
project status, risk management, documentation, cost and schedule
tracking, subcontract management, and coordination with team members.
The PM is the focal point of communication for project construction.

The PM will be responsible for establishing a baseline project plan.
This plan will establish the schedule, cost phasing, and resource needs
to carry out the SNAP project consistent with experiment science
requirements. This proposal is the first step in defining the project
plan. The complete project plan requires the approval of the PD and the
agencies before being accepted as the baseline.

The SNAP Project Systems Engineer (PSE) is responsible to the Project
Manager and Project Director for the engineering of the project and the
organization and management of the engineering resources. The PSE is
responsible for the development of cost and schedule and the
optimization of the project through the Systems Engineering Office
(SEO). The PSE will establish specifications, requirements, interfaces,
and work statements, and is responsible for overseeing configuration
tracking as appropriate during the conceptual design.

## Management Panel

The management of SNAP currently is composed of scientists and engineers
from both the University of California Space Sciences Laboratory (SSL)
and from Lawrence Berkeley National Laboratory (LBNL). In order to
permit the efficient functioning of the management team and provide a
unified approach to oversight and access to resources, and to enhance
communication, the two institutions have formed a Management Panel for
SNAP. Currently serving on this board from SSL are: Prof. Robert Lin
(Director SSL), Henry Heetderks (SNAP Project Manager), and David Pankow
(SNAP Systems Engineer). Serving from LBNL are Saul Perlmutter (SNAP
Project Scientist and PI), Michael Levi (SNAP Project Director and
Co-PI), Pier Oddone (Deputy Director LBNL), Jay Marx (Project
Integration & Management Officer), Prof. James Siegrist (Physics
Division Director), Richard DiGennaro (Deputy Project Manager). This
board currently meets weekly to discuss joint organizational and
planning issues.

## Institutional Board

The SNAP Institutional Board is composed of a lead scientist from each
of the collaborating research institutions. The Institutional Board
assists the PI in establishing scientific goals and objectives of the
SNAP project. It advises the PI on all collaboration and collaborating
institution matters of the project. It will develop a policy for
membership and for publication. The Board will decide on controversial
issues within the collaboration by consensus or by voting.

The PI and the PD chair the Board. Official Board meetings will be held
on an as needed basis and no less frequently than twice a year.

## Project Technical Board

The SNAP Project Technical Board (PTB) is responsible for working with
and advising the Project Directorate with respect to the execution of
the project as a whole. Its membership consists of the PM and PSE, leads
from the Systems Engineering Office, the Project Office, the Safety,
Quality, and Reliability Office, the individual systems managers, and
additional members as deemed appropriate by the Project Directorate.

The PTB shall meet on a regular basis to discuss technical, cost and
schedule issues and will form the basis of the Change Control Board
(CCB). Its main goal is to help ensure that all systems of the project
are being adequately integrated and executed toward the scientific and
technical goals of the project within the constraints of budget and
schedule.

## Systems Engineering Office

The Systems Engineering Office (SEO) is charged with developing and
maintaining the system hardware and software requirements and
specifications. The SEO reports directly to the PSE within the SNAP
Project Directorate. The SEO has responsibility for system level issues.
During conceptual design, the SEO primarily will be concerned with
top-level requirements flow-down so that design and planning decisions
are made with overall systems considerations in mind. Its other key role
is the development of interfaces between the various systems. As such,
the SEO will be working continuously with system managers to ensure that
interfaces are properly defined and that technical issues affecting more
than one system are resolved efficiently and effectively.

Requirements, specifications, and Interface Control Documents (ICD) will
be entered into configuration management by the end of the conceptual
design phase. Systems management will be performed through regular
reviews of the design activity.

## Project Office

The Project Office (PO) is responsible for monitoring the technical
scope, cost and schedule performance of all portions of the SNAP project
and providing assessments to the Project Directorate. The Project
Directorate then provides timely and complete reports to the sponsoring
entities. The Project Office will maintain the SNAP master
resource-loaded schedule that is capable of determining and monitoring
the progress of the project.

# Management Processes

Beyond day-to-day oversight of the R&D program, the main activities of
the management group and systems engineering group include requirement
specifications and development, development of collaboration agreements,
systems engineering, oversight of cost and schedule development, risk
assessment, and generation of regular progress reports. Each of these
areas is discussed below, followed by a summary of the major milestones
and deliverables during the conceptual design phase.

## Requirements and specifications development and control

A top-level project scientific requirements document will be developed
during the initial phase of the program by the project science team.
Systems requirements and definition are the responsibility of the
Project Systems Engineer and the Systems Engineering Office. The SEO
will lead the definition of lower level system and subsystem
requirements using a documented flow-down process. All hardware and
software elements of the SNAP system will be defined in these
specifications. At the appropriate point in the program, the
specifications will be placed under configuration control, after which
changes will be made by means of formally controlled Engineering Change
Orders (ECO) that assure proper review by all affected project elements.
Updated documentation will be made available to all affected parties. A
straightforward system of verification of the latest current version of
any project document will be continuously maintained and readily
accessible to all.

## Management of collaboration agreements

Memoranda of Understanding (MOU) will be written between the
institutional members of the SNAP collaboration and the Project
Directorate. These agreements will cover the cost, schedule, and
technical scope of deliverables, and resources to be provided by
respective collaborators and the resources to be provided by the Project
Directorate. The MOUs will specify inter-institutional conduct within
the collaboration as well as the scope of efforts for the respective
collaborators. The MOUs will be reviewed along with progress each year
as part of the yearly financial planning cycle.

The members of the SNAP collaboration will subcontract to LBNL, or, if
required, to UCB. The subcontract will be established by means of a
proposal that contains a statement of work, technical requirements,
specifications (where appropriate), schedule, and cost of elements of
the deliverables. A subcontract manager affiliated with the SNAP project
office will monitor progress and serve as the point of contact for
contractual matters between the project office and collaboration
institutions. The subcontract manager reports to the Project Manager.

## Management and performance of systems engineering

The SNAP Project Systems Engineer has primary responsibility for
assessment of all systems-level issues (the Project Director is
ultimately responsible for decisions, with input from the PI). It is
critical in this process to view all elements of the SNAP project as a
combined entity and to properly allocate requirements and design
approaches across the entire system. As part of the conceptual design
study, the Project Systems Engineer will develop a Systems Engineering
Management Plan that will serve as the framework and control for all
subsequent work and trade-offs. The evaluation criteria for examining
the trade-offs will be developed during this same phase, very early in
the design process. Additionally, the SEO will be primarily concerned
with the development and analysis of top-level requirements and their
flow-down. Systems models will be developed and the initial trade
studies will be performed. System operation will be considered so that
design decisions are made with the end user in mind. The other key role
of the SEO is in the development of interfaces between subsystems. As
such, the SEO will work continuously with system and subsystem managers
to ensure that interfaces are properly defined and that technical issues
affecting more than one system are resolved efficiently and effectively.
An important product of the initial phase is the establishment of
initial system architecture and initial subsystem interface definition.

## Cost and schedule development

During the conceptual design phase, several critical activities must
occur. The SEO management plan must be developed. The requirements
development and analysis must be started and the appropriate evaluation
criteria must be identified. A number of important trade-off studies
must be performed and analyzed, as they will determine basic aspects of
the system. The risk management plan with ties to the R&D plan needs to
be developed and implemented. The initial system architecture and
interface definitions must be established.

During the conceptual design phase, the initial R&D and technology plans
must be perfected and executed including the necessary prototyping
efforts and associated test plans. The engineering concepts must be
developed, and prototype development and tests must occur. Also crucial
during the conceptual design is the development of the detailed R&D
plans that extend beyond the conceptual design phase into the
preliminary design phase.

Additionally, the SNAP Project Office will develop the management and
staffing plans; MOUs between collaborators for the R&D phase; the
complete Work Breakdown Structure (WBS); the cost estimates; and the
integrated schedule.

With the movement into Conceptual Design, activities shift from a
definition and exploratory phase to one on concerted systems, project,
and technology design and development. The conceptual design of all
subsystems is completed and a Conceptual Design Report is reviewed. The
final technology and development plans with milestones and decision
points are completed.

Within the SEO and PO areas the system architecture is finalized. The
subsystem requirements are assigned. The subsystem interface definitions
and formal risk analysis are completed. The configuration management and
change control plans are finalized and implemented. The final project
planning, system safety, reliability and quality assurance plans are
completed. The cost estimate (cost bracket), contingency analysis,
master resource loaded project schedule, and WBS are completed and
prepared for review.

During conceptual design, the acquisition strategy is prepared. This
strategy sets forth the management approach that will be used to ensure
that the project contract or system of project contracts satisfies the
approved mission need. The acquisition strategy can be part of the
mission need document, a separate document, or a part of the Acquisition
Plan.

## Risk assessment

The first step in the management of risk is its assessment. This is done
initially in conjunction with the estimation and determination of the
work to be done. Each element used in costing is assessed and scored as
to its stage of development and potential impact on the project.
Specifically, each element is rated for design/approach maturity,
complexity, dependency, technical development, cost uncertainty, and
potential schedule variance.

The data thus obtained are then scored following procedures adapted from
previous large H.E.P. project and NASA approaches. Calculations are then
done on the assessed risk score to determine an appropriate level of
cost and schedule contingency. In parallel, possible scope contingencies
are identified with decision points established where technical
trade-off choices must be made.

Once the project is under way, issues identified with risk to the
project are monitored and contingency is allocated where necessary. In
addition, reliability assessments and trade-off studies also seek to
minimize incidental as well as project risk. It should be stressed that
risk management is not merely the initial allocation of funding
contingency to various tasks and subtasks. Complete risk management is
an ongoing effort throughout the life of the project and involves
developing not only the funding contingency but also the schedule and
technical contingencies.

## Progress reports

Financial and project reports will be submitted to the funding agencies
as required. Technical progress reports will be submitted to summarize
progress, concerns, problems, changes, and plans for the next period. In
addition, frequent contact with the agency technical monitors will be
the project's standard practice.

## Activities

Key activities during the conceptual design phase of the project are:

-   Perform project & detail design phase technical and programmatic
    risk analysis

-   Develop system-level functions and requirements

-   Identify long-lead procurements

-   Develop project execution plan for preliminary design

-   Set project execution strategy

-   Review design alternatives

-   Identify project standards and procedures

-   Develop preliminary design phase budget and schedule

-   Develop total project cost and schedule range

-   Identify current and two fiscal year funding requirements

### Major reviews

Major reviews are:

-   Draft Requirements Review

-   Conceptual Design Review

### Major deliverables at CDR

Major deliverables at CDR are:

-   Acquisition Plan

-   Project Expectations Summary

-   Statement of Work

-   NEPA Documentation

-   Systems Engineering Management Plan

-   Conceptual Design Package

-   Preliminary Project Execution Plan

-   Preliminary Hazard Analysis Report

-   Preliminary Team Execution Plan

-   Risk Management Plan

-   Preliminary Design Phase Budget & Schedule

-   Verification of Mission Need

-   CDR Package

-   Total Project Cost & Schedule Range

[]{#__RefHeading___Toc55464423 .anchor}![](media/image50.wmf)Figure 50.
As can be seen in the R&D phase organization chart, the SNAP project is
divided into its principal systems and subsystems beneath the Project
Directorate. Each major system has a designated System Manager who is
responsible for the successful development and completion within budget
and schedule constraints.

[^1]: S. E. Holland, D. E. Groom, M. E. Levi, N. P. Palaio, S.
    Perlmutter, R. J. Stover, and M. Wei, \"Large Format CCD Image
    Sensors Fabricated on High Resistivity Silicon,\" pp 179-182, Proc.
    1999 IEEE Workshop on Charge-Coupled Devices and Advanced Image
    Sensors, June 10-12, Karuizawa, Nagano, Japan.

[^2]: C. J. Bebek, D. E. Groom, S. E. Holland, A. Karchar, W. F. Kolbe,
    J. Lee, M. E. Levi, N. P. Palaio, B. T. Turko, M. C. Uslenghi, M. T.
    Wagner, G. Wang \"*Proton radiation damage in p-channel CCDs
    fabricated on high-resistivity silicon*,\" LBNL-49316, IEEE Trans.
    Nucl. Sci., Vol. 49, pp. 1221\--1225, (June 2002)

[^3]: L. Cawley, C. Hanley, *WFC3 Detector Characterization Report #1:
    CCD44 Radiation Test Result*, STScI Instrument Science Report WFC3
    2000-05, Oct. 2000

[^4]: T. Hardy, R. Murowinski, and M.J. Dean, "*Charge transfer
    efficiency in proton-damaged CCDs*," IEEE Trans. Nucl. Sci. 45(2),
    pp. 154-163, April 1998.

[^5]: C. Bebek, A. Karcher, W. Kolbe, D. Maurath,V. Prasad, M. Uslenghi,
    M. Wagner, "*Measurement of Lateral Charge Diffusion in Thick, Fully
    Depleted, Back-illuminated CCDs,*" to be submitted to IEEE Trans.
    Nucl. Sci.

[^6]: J.-P. Walder et al., "A Low Power, Wide Dynamic Range Multi-Gain
    Signal Processor for the SNAP CCD," submitted to IEEE Trans. Nucl.
    Sci.

[^7]: C. Akerlof, et al., Supernova/Accelerator: A Satellite Experiment
    to Study the Nature of the Dark Energy, submitted to PASP