**SNAP Mission Definition and Requirements Document**

Drawn by: M. Levi

Cognizant Engineer: M.Levi

Date: 2002-06-25

Document Type: RQMT

Category Code: PS2200

Release History:

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| R | Description                      | R | C  | S | M | C | Date  |
| e |                                  | e | he | y | . | . |       |
| v |                                  | v | ck | s | A | M |       |
|   |                                  |   |    |   | . | . |       |
|   |                                  | b |    | E |   |   |       |
|   |                                  | y |    | n |   |   |       |
|   |                                  |   |    | g |   |   |       |
|   |                                  |   |    | r |   |   |       |
+---+----------------------------------+---+----+---+---+---+-------+
| A | Release of revision presented at | m |    |   |   | h | 2003- |
|   | 2002 Lehman Review               | e |    |   |   | d | 10-08 |
|   |                                  | l |    |   |   | h |       |
+---+----------------------------------+---+----+---+---+---+-------+
| B | Release of revision presented at | m |    |   |   | h | 2003- |
|   | 2003 internal Review             | e |    |   |   | d | 10-30 |
|   |                                  | l |    |   |   | h |       |
+---+----------------------------------+---+----+---+---+---+-------+
| C | Release of revision supporting   | m | p  |   |   | h | 2004- |
|   | March 2004 SAGENAP whitepaper:   | l | nj |   |   | d | 03-24 |
|   | L2 orbit, link margins           | l |    |   |   | h |       |
+---+----------------------------------+---+----+---+---+---+-------+

**MISSION DEFINITION AND REQUIREMENTS DOCUMENT**

for the

**Supernova / Acceleration Probe (SNAP)**

**University of California, Berkeley**

**Lawrence Berkeley National Laboratory**

**Last Modified March 24, 2004**

# 1. Science Objectives

## 1.1 Overview

Recent measurements carried out by the Supernova Cosmology Project (SCP)
have made the startling discovery that the expansion of the universe is
accelerating. This result is based on the Hubble diagram for Type Ia
supernovae, and has been corroborated by results from several
experiments, both similar and complementary. Einstein's General Theory
of Relativity requires that some mechanism must drive this expansion
rate either through a new form of energy, such as a new vacuum energy
density (cosmological constant), or a yet unknown kind of particle or
field fundamental to the creation and formation of the universe. The
source of this acceleration is more powerful than the gravity from all
seen and unseen forms of matter and known energy. Theorists are unable
to explain the observed effect and so follow-up measurements would have
a tremendous impact on the field of fundamental physics. The Supernova /
Acceleration Probe (SNAP) Mission is expected to provide a understanding
of the mechanism driving the acceleration of the universe. Over the
mission lifetime, the satellite observatory is capable of measuring at
least 2,000 distant Type Ia supernovae. These measurements will map out
in detail the expansion rate of the universe at epochs varying from the
present to 10 billion years in the past. SNAP will measure the key
cosmological parameters ~m~ and as well as determine the spatial
curvature of the universe and thus provide a fundamental test of the
theory of inflation -- the theoretical mechanism that drove the initial
formation of the universe. This sensitive experiment uses Type Ia
supernovae as an astronomical standard candle to provide a distance
scale, which, combined with the redshift obtained from the spectral
lines from the supernova and its host galaxy, determine the cosmological
parameters and ultimately the nature of the "missing energy" in the
universe.

## 1.2 Summary Description of the Science Instruments

The SNAP observatory will have an approximately 2 meter diameter rigid
lightweight primary mirror with a focal plane covering one degree. The
camera will have approximately one-half billion pixels, representing the
largest imager ever fabricated. In order to facilitate thermal
management of the observatory and instruments, and reduce light
emissions from the earth's limb, the spacecraft is placed an orbit about
the second Earth-Sun Lagrange Point (L2). SNAP would be launched from a
Delta IV-M or equivalent rocket. Our baseline science mission plan calls
for continuous operation of SNAP for four years. This provides time to
classify and accurately measure 2,000 distant Type Ia supernova, obtain
final reference images and spectra, while interleaved with wide-field
weak lensing surveys.

**2. Reference Mission**

2.1 Baseline Science Mission

The SNAP reference science mission assumes a 2.0 meter primary mirror
with low obscuration and a large 1.36° field-of-view with
diffraction-limited images (in I-band). The mission repeatedly steps
through its survey zone 1.36 deg x 8 deg in scan length. Owing to end
effects, the fully scanned area is 8.4 sq deg. Each step is 22mm on the
focal plane or 205 arc seconds on the sky. Each step comprises four
dither exposures of 300 sec + 30 sec for readout, for 1320 sec per step.
The 6 visible-band filters are spaced 22mm on the focal plane, so that
each step corresponds to four dithered exposures in one band. In the
near infrared, the filters are 44mm in size and eight dither samples are
taken for each of the three NIR bands. A complete sweep then requires
140 steps and about 51 hours of observing time, or about 57 hours of
mission time accounting for downlink periods. Of the overall 96 hour
observing cadence, the remaining 39 hours of mission time (35 hours of
on-target astronomy time) is available for follow up spectroscopy of
selected supernovae near peak brightness.

The discovered supernovae are then photometrically sampled every four
days for the next four to eight months while the luminosity waxes and
wanes. We plan to survey two zones, one near the north ecliptic pole and
another near the south ecliptic pole. Each survey is expected to take
about 16 months. Our selected observing fields minimize zodiacal light
background and obscuration due to dust in our Galaxy. The satellite is
expected to be able to analyze up to 2,000 supernovae with redshifts
ranging from 0.3 to 1.7.

The optical configuration for the SNAP telescope will be an
annular-field three-mirror anastigmat as originally described by Korsch
(Appl.Optics v.11 2986 1972; Appl.Optics v.19 3640 1980) and detailed by
Lampton et al (SPIE v.4849 215 2002; SPIE v. 5166 202 2003). This
decision has been driven by the need for a diffraction limited angular
field size of the order of one degree, combined with the need for rather
large telephoto advantage: to match the pixel sizes to the diffraction
spot size a focal length of the order of 20 meters is required, while
the entire optical package must fit within the size constraints of
available launch vehicle shrouds. A further advantage of the Korsch TMA
configuration is that the detector is located to the side of the
principal optical axis, where its size does not block any light from the
secondary or tertiary, and where direct passive cooling for the detector
array can be provided.

A key to the success of the design is the feedback of a pointing error
signal from the focal plane to the attitude control of a high stability
spacecraft.

The size of the SNAP primary mirror has a dramatic effect on the science
capabilities of the mission. The combination of the light gathering
power of the mirror and the diffraction limit imposed by the aperture
determine the number of supernovae that can be studied in a fixed
interval, and varies as the *fourth power* of the aperture diameter. The
requirement of diffraction limited optics at I-band has been selected to
make best use of the capabilities of the photometric instruments and
minimize exposure times. The wide-field optical photometry will also
perform with the highest accuracy if the star images are properly
sampled. A plate-scale of approximately 10 arcsec/mm has been selected
as a best compromise between a wide field of view and achieving the best
photometric accuracy. The mission is restricted to the minimum fairing
dimensions of the Atlas III, Delta III, and the Delta IV-M composite
fairing. Given the space required for spacecraft bus and telescope tube
the aperture is not fixed by fairing diameter, rather by total length.
Without deployed components, the largest aperture that is likely to fit
is 2.2 meter, at this size the mission would also become mass
restricted. For apertures below 2.0m, detector performance
characteristics become critical. From simulation, it has been found that
the observational redshift limit, z~max~, for well-measured supernovae
drops linearly with the diameter. So that reducing the mirror diameter
from 2.0m to 1.8m reduces the pratical z~max~ from 1.7 to 1.5. The
reference science mission for the observatory is summarized in Table 1.

###  *Table 1. SNAP Reference Mission*

  ---------------------- ------------------------------------------------
  Aperture               2.0 meter

  Field-of-view          0.7 square degrees

  Optical resolution     diffraction-limited at I-band

  Wavelength coverage    350nm - 1700nm

  Solar avoidance        70

  Temperature            Telescope 270 - 290 K (below thermal background)

  Fields of study        North and South Ecliptic Caps: 7.5 sq deg each

  Pointing               Active Image Feedback

  Plate Scale            10.5 arcsec/mm
  ---------------------- ------------------------------------------------

## 2.2 Photometry of Supernovae

The primary measurement of the mission is to obtain the peak brightness
vs. redshift relationship of Type Ia supernovae out to a redshift of
1.7. These data will require taking accurate photometric observations of
the supernova over the light-curve. An example of the restframe B-band
light-curve is shown in Figure 1. It has been found experimentally that
the supernova peak brightness can be standardized when viewed through a
B-band filter where the B-band is defined in the *restframe* of the
supernova (defined at rest with respect to the supernova). Therefore the
photometry must seek to define an appropriate bandpass depending on the
redshift of the supernova. The importance of this bandpass selection can
be seen in Figure 2, where the B-band is superimposed over an example
Type Ia supernova spectrum. The bandpasses are defined by having an
extensive set of "redshifted B-band" filters, one for each small range
of redshift. With the large field-of-view of the imager, it is possible
to "batch process" large numbers of supernovae in a given observation so
that the objectives can be met with a large number of filters. The
filter set shown below in table 2 are Johnson B-band filters with
central wavelength of x^n-1^, where n= 0 to 8.

***Table 2. Redshifted B-band filters***

+----------------+----------------+---------------+--------------------+
| *Filter*       | *Effective*    | *B-band*      | *B-band*           |
|                |                |               |                    |
| *Number*       | *redshift*     | *Center*      | *Bandpass*         |
|                |                |               |                    |
|                |                | *(m)*         | *(m)*              |
+----------------+----------------+---------------+--------------------+
| 1              | 0.00           | 0.44          | 0.11               |
+----------------+----------------+---------------+--------------------+
| 2              | 0.15           | 0.51          | 0.13               |
+----------------+----------------+---------------+--------------------+
| 3              | 0.32           | 0.58          | 0.15               |
+----------------+----------------+---------------+--------------------+
| 4              | 0.52           | 0.67          | 0.17               |
+----------------+----------------+---------------+--------------------+
| 5              | 0.75           | 0.77          | 0.19               |
+----------------+----------------+---------------+--------------------+
| 6              | 1.01           | 0.88          | 0.22               |
+----------------+----------------+---------------+--------------------+
| 7              | 1.31           | 1.02          | 0.25               |
+----------------+----------------+---------------+--------------------+
| 8              | 1.66           | 1.17          | 0.29               |
+----------------+----------------+---------------+--------------------+
| 9              | 2.06           | 1.35          | 0.34               |
+----------------+----------------+---------------+--------------------+

***Figure 1. B-band Light-curve for Type Ia Supernovae corrected for
redshift and lightcurve timescale.***

![](media/image1.png){width="5.84375in" height="3.9611111111111112in"}

***Figure 2. Example Type Ia supernova spectrum with B-band bandpass
superimposed.***

![](media/image2.png){width="5.840972222222222in"
height="4.216666666666667in"}

**3. Science Requirements**

## 3.1 Level 1 & Level 2 Science Requirements

The purpose of this section is to define the objectives of the
investigation and the baseline measurements to be accomplished. The SNAP
baseline science objective is to obtain a high statistics calibrated
dataset of Type Ia supernovae to redshifts of 1.7 with excellent control
over systematic errors. To achieve this, SNAP must perform the
following:

1.  Perform Wide Field Imaging of the celestial poles from 0.35 to 1.7
    microns with I-band diffraction-limited optics.

2.  Perform multiple-filter Wide Band Imaging photometry in multiple
    filter bands to determine supernovae color.

3.  Detect supernovae at redshifts in the range of 0.3 \< z \< 1.7.

4.  Derive supernova peak luminosity to 2% (statistical) or better
    through multiple measurements over the supernovae light-curve.

5.  Obtain supernova spectrographic observations near peak intensity
    with a resolution of 100 () over 0.35 to 1.7 microns.

6.  Measure supernovae host galaxy redshifts.

7.  Identify and analyze over 2000 "branch normal" Type 1a supernovae.

The statistical sample is to be 2 orders of magnitude greater than the
current published set of \~42 supernovae, and is to extend much farther
in distance and time. From this dataset we expect to obtain a 2%
measurement of the mass density of the universe, a 5% measurement of the
vacuum energy density, a 5% measurement of the curvature, and a 5%
measurement of the equation of state of the \"dark energy\" driving the
acceleration of the universe. Systematic studies will include a
measurement of the \"reddening\" of spectra from \"ordinary dust\" in
the hosts galaxies of supernovae up to redshifts of 1.7, and detection
of potential \"grey dust\" sources. Using Type Ia supernovae as standard
candles will require measurement of the key luminosity indicators: the
lightcurve peak and width. The redshift of the host galaxy of the
supernova needs to be measured, supernova type identified, and spectral
features studied. Effects correlated with host galaxy morphology and the
position of the supernova in the host galaxy can also be studied due to
the excellent resolution possible from space. These properties may
indicate differences in stellar population from which the supernova came
and therefore can be used to test whether the intrinsic brightness of
the supernova changes systematically with redshift.

These objectives are achieved through the definition of the SNAP Level 1
and Level 2 science requirements. The key Level 1 requirements meet the
objectives of supernovae quantity, quality, spectroscopy, and survey
size:

***Table 3. Draft Level 1 Science Requirements***

+-----+----------------------------------------------------------------+
| 1.1 | Obtain over 2000 classified Type Ia supernovae for analysis in |
|     | the redshift range 0.3 \< z \< 1.7                             |
+-----+----------------------------------------------------------------+
| 1.2 | Derive supernova color and relative peak luminosity on average |
|     | to 2% (statistical)                                            |
+-----+----------------------------------------------------------------+
| 1.3 | Obtain supernova spectrographic observations near peak         |
|     | intensity with a resolution R\~100 over 0.35 to 1.7 microns    |
|     | wavelength.                                                    |
+-----+----------------------------------------------------------------+
| ##  | Perform deep multi-color photometric and weak lensing surveys  |
| 1.4 | with field sizes of approximately 15 and 300 square degrees    |
+-----+----------------------------------------------------------------+

From simulation studies the Level 2 requirements are derived. Many of
these requirements assume an optimization of both the science objectives
of measuring the properties of the dark energy along with resulting
trade-studies that have evaluated analysis objectives.

***Table 4. Draft Level 2 Science Requirements***

  --------- ---------------- -----------------------------------------------------------------
  **1.1**                    **Obtain over 2000 classified Type Ia supernovae for analysis in
                             the redshift range 0.3 \< z \< 1.7**

  1.1.1                      Observatory to be a 2 meter diameter telescope with I-band
                             diffraction limited optics sensitive from 0.35 to 1.7 microns.

  1.1.2                      Instrumented field of view of telescope to be approximately one
                             degree (\~0.7 square degrees)

  1.1.3                      Perform wide field imaging of regions of low dust extinction near
                             the ecliptic poles with a solar avoidance angle of 70 degrees.

  1.1.4                      Photometric observations are to be zodiacal light limited.

  **1.2**                    **Derive supernova color and relative peak luminosity on average
                             to 2% (statistical)**

  1.2.1                      Obtain photometric measurements in redshifted B-band broadband
                             filters.

  1.2.2                      Obtain peak and off-peak (plus and minus 4 days rest-frame)
                             multi-band photometric measurements of SNe with S/N=30

  1.2.3                      Measure risetime with detection at average 2 days after explosion
                             at 3.8 magnitudes below peak with S/N\>3, and peak-to-tail ratio
                             of SNe.

  1.2.4                      Obtain peak and off-peak multi-band photometric measurements with
                             at least 10 points on the SNe lightcurve

  1.2.5                      Measure SNe color and extinction with up to six visible light and
                             three infra-red broadband filters from 0.35 to 1.7 microns

  1.3       **Obtain         
            supernova        
            spectrographic   
            observations     
            near peak        
            intensity with a 
            resolution       
            R\~100 over 0.35 
            to 1.7 microns   
            wavelength.**    

  1.3.1     Measure          
            supernova peak   
            spectrum to      
            identify and     
            classify SNe     

  1.3.2     Measure          
            supernova        
            spectra vs.      
            epoch for subset 
            of SNe with      
            z\<0.7           

  1.3.3     Measure the      
            broad (200A)     
            Silicon (6150A   
            rest-frame) and  
            Sulfur (5350A    
            rest-frame)      
            features         

  1.3.4     Obtain           
            spectroscopic    
            measurement of   
            calibration      
            standards        

  1.4                        **Capable of performing deep multi-color photometric surveys with
                             field sizes of approximately 15 and 300 square degrees.**

  1.4.1                      Mission operations and avoidance angles to permit wide field
                             surveys up to 300 square degrees

  1.4.2                      Minimum four visible broadband filters for photo-z measurements
                             to facilitate weak-lensing surveys.
  --------- ---------------- -----------------------------------------------------------------

## 3.2 Instrumentation Requirements

The satellite is expected to carry two key instruments, a 1° FOV wide
field imager sensitive to wavelengths from 350 nm -- 1700 nm, and a low
resolution 350 nm -- 1700nm spectrograph. The capabilities of these
instruments must be a good match to the science requirements for the
satellite. In particular, the instrumentation must provide all the
elements of the supernova studies, namely: 1) early detection of
supernovae, 2) U,B,V,R & I-band restframe photometry (up to m) to follow
the photometric lightcurve of the supernova as it waxes and wanes and
measure the supernova color, 3) spectra at peak brightness to classify
the supernova and provide spectral diagnostics, and 4) photometric
redshifts of the host galaxies in advance of supernova follow-up. Each
of these elements can be expressed as a set of explicit requirements for
the data products:

***Detection***. Early detection of Type Ia supernovae with S/N\>3,
would be obtained by comparing current images to a set of reference
images. Early detection is required to measure the risetime in a model
independent fashion within a few days of explosion and to allow
identification well before peak brightness, approximately 3.8 magnitudes
below peak. Note that the early discovery mitigates fast spacecraft
scheduling. Twenty-two fields totaling 7.5° square degrees would be
repeatedly studied with a repetition rate of every four days in the
north field (and later repeated in the south field).

***Photometry.*** The supernova lightcurve is photometrically studied by
obtaining at least ten data points along the development of the
lightcurve at fairly uniform intervals. Photometry for the supernova is
obtained primarily in the rest-frame B-band of the supernova using a
filter set that approximates redshifted B-band filters. An overall fit
to the lightcurve in both time and color space (using U, V, R, & I
rest-frame filters) will be required to obtain the lightcurve parameters
and to correct the data for dust extinction. To achieve this
requirement, rest-frame B-band data points would be taken at peak
brightness (S/N\>30), at 0.5 magnitude below peak (S/N\>30) (two
measurements at rise and fall of lightcurve), at 1.0 magnitude below
peak (S/N\>20), at 1.5 and 2.0 magnitude below peak (S/N\>15), and at
2.5 magnitude below peak on trailing edge of lightcurve (S/N\>10) except
in the IR channel where this last requirement does not apply.
U,V,R&I-band color photometry is obtained simultaneously. Restframe
photometry in B-band is redshifted into the observer frame by (1+*z*).
An optimal redshifted B-band filter set would eliminate a principal
source of systematic error from the k-corrections. The photometry will
require both optical and NIR coverage.

***Spectra***: In order to classify the supernova as Type Ia a spectrum
is obtained at peak magnitude. The defining signature of a Type Ia
supernova is the SiII feature at 6250A. Spectral luminosity indicators
include the SiII features and the CaII features near 3950A, the
observing time requirements are based on the CaII features for
simplicity. Beyond a certain minimum resolution, there is a range of
resolution and signal-to-noises that will satisfy this observing
requirement: (\>60, S/N\>20), (\>70, S/N \> 14), or ( \> 100, S/N \>10)
in the supernova-frame 3500-4200A. This leaves some flexibility in the
design of the different arms of the spectrograph. In the optical regime,
a single medium resolution spectrograph extending from 3500A to 10000A
would satisfy SiII line measurement to z\< 0.6, and the CaII feature
requirement to z\<1.5. A medium resolution NIR arm would satisfy the
supernovae with redshift silicon-feature requirement beyond z\>0.6, and
the CaII feature in the range 1.5\<z\<1.7.

***Redshift***. Obtain pre-explosion host galaxy redshift estimate based
on photometric redshifts (only once per field) based on images in
multiple filters to S/N\>30. Obtain accurate host galaxy red-shift from
at-peak supernova spectrum or final reference spectrum; this requires
access to redshifted H-alpha or 4000A break. These features are close to
the key supernova features (CaII, SiII), so the requirements for the
spectroscopy are sufficient.

***Off-peak spectra (evolution)***. On a small (eg. 10%) subset of the
data obtain a spectrum at off-peak brightness along lightcurve for
z\<0.7 to check for evolution of spectral features. Wavelength range is
from 3000A to 6500A in the restframe of the supernova.

***Host galaxy spectroscopy:*** Obtain host-galaxy final reference
spectrum, and measure redshift from at-peak supernova spectrum and/or
final reference spectrum. The spectrograph will sample host-galaxy light
underneath and around the SN spectrum obtained at peak. This light must
be subtracted, and in some cases this will require a final reference
spectrum of the host-galaxy after the SN has faded. Individually or in
combination these spectroscopic observations will be used to measure the
host galaxy redshift when possible. Strong galaxy spectral features are
H-alpha (6567A) and NII (6587A) for star-forming galaxies, and the
spectral break at 4000A for quiescent galaxies. These spectral features
are near in wavelength to key Type Ia supernova spectral features (SiII
and CaII, respectively), and therefore do not impose additional
requirements on the spectral coverage. Host galaxy spectroscopy benefits
from the ability of the integral field unit to collect light over all or
most of the face of the (resolved) galaxy. Host galaxy spectroscopy will
benefit from the lowest possible readnoise and dark current from the
spectrograph detectors. Ground-based spectroscopic observations will
help support this requirement.

***3.3 Optical Photometry***

The requirement for supernova detection and photometry in the visible
light domain is fulfilled by a large field imager based on CCD
technology. The pixel size is chosen to be as low as attainable in
science grade imagers to minimize the overall size of the focal plane
while providing adequate sampling of the point-spread function. Since
the CCD's occupy the same focal plane as the IR detectors the optimal
pixel size is jointly constrained. The IR detectors under consideration
are currently restricted to 18 m pixel size so that the optimal sampling
in the CCD's is given by the ratio of the longest wavelengths each sees.
For the CCD's this dictates a 10.5 m pixel size. The optical system is
constructed from 36 3.5k x 3.5k devices with a 1.0 m wavelength cut-off.
With this pixel count and size the CCD's are the same size as the IR
devices. Each CCD has a 2x2 array of filters imprinted or mounted
closely on the surface.

The high-resistivity p-channel CCD technology provides high quantum
efficiency at 1000 nm since the fully-depleted devices are 200 m thick
and back-illuminated. The typical exposure time is 300 sec while the
longest single exposure is set during spectroscopy, at 1000 sec.
Multiple frames would be stacked and cleaned of cosmic rays during
ground processing. The longest aggregated image representing one
lightcurve point (summed from many exposures) in the imager is 0.9
hours. For the parameters given in Table 5, which assumes a 2-meter
primary mirror and an overall system detection efficiency of 60%, the
imager sensitivity is limited only by zodiacal light background.

***Table 5. Optical Photometry Reference Model Specification***

  -------------------------- --------------------------------------------
  Field-of-view              Approximately 0.34 square degrees

  Plate Scale                0.10 arcsec/pixel

  Wavelength coverage        350nm - 1000nm

  Detector Type              High-Resistivity P-channel CCD's

  Detector Architecture      36 3.5k x 3.5k CCD's with 10.5 micron pixels

  Detector Array Temperature 140 K

  Detector Quantum           \>80% @ 400-800nm
  Efficiency:                

  Read Noise                 4 e- \@100kHz (6 e- required)

  Exposure Time              300 sec (typical single exposures)

  Dark Current               0.005 e-/s/pix (0.03 e-/s/pix required)

  Readout Time               30 sec

  Exposure control           Mechanical shutter

  Filters                    6 bands
  -------------------------- --------------------------------------------

The optical devices will be fabricated by LBNL using a new
state-of-the-art CCD based on ultra-high purity high-resistivity n-type
silicon. These CCD's are fully-depleted and back-illuminated with
superior response. The largest devices currently in operation at Lick
Observatory are 2k x 2k with 15 m^2^ pixels. Currently, larger 2k x 4k
devices are being packaged and devices with smaller 12.0, 10.5, and 9
m^2^ pixels have been tested. The technology has also been moved to a
commercial foundry. Since the devices do not require thinning to obtain
high sensitivity with back-illumination the devices are extremely robust
and easier to fabricate in volume. Measurements at the LBNL 88"
cyclotron also indicate significant enhanced radiation tolerance.
Further information about this technology can be found at URL,
[http://www-ccd.lbl.gov](http://www-ccd.lbl.gov/). LBNL has a long
history in the fabrication of very large silicon array detectors, the
largest contains one-square meter of silicon detectors. A larger
instrument, of forty square meters of silicon detector is currently in
fabrication.

Given the very large number of CCD's, LBNL and IN2P3/France have also
begun development of radiation hard multi-channel
preamplifier/correlated double samplers developed on an integrated
circuit. Appropriate radiation hard 16-bit analog-to-digital converters
are currently being fabricated using a radiation tolerant process.

***3.4 IR Photometry***

The requirement for supernova detection and photometry in the NIR light
domain is fulfilled by a large field imager based on HgCdTe technology.
Since the HgCdTe detectors occupy the same focal plane as the CCD
detectors the optimal pixel size is jointly constrained. The IR
detectors under consideration are currently restricted to 18 m pixels.
The IR system is constructed from 36 2kx2k HgCdTe devices with a 1.7 m
wavelength cut-off (or longer). This cut-off is a good match to the
cut-off the thermal emissions from the approximately room temperature
operating point of the optical surfaces of the telescope and provides
adequate dark current at the 140K operating temperature of the focal
plane. The requirements for the IR photometer are shown in Table 6. The
devices have a single fixed filter on each device.

Following the supernova lightcurve in the infrared will require the
highest possible throughput and quantum efficiency. The parameters given
in Table 5 assume a 2 meter primary mirror and an overall IR system
detection efficiency of 40% including telescope obscuration,
reflectance, optical transport, and detector quantum efficiency. With
these parameters the IR photometer is only limited by zodiacal light
background. HgCdTe arrays are known to have strong intrapixel
sensitivity variations. Therefore, a careful study of the dithering
pattern will be undertaken to ensure that the IR photometer will produce
accurate, unbiased photometry.

Table 6. NIR Photometry Reference Model Specification

  -------------------------- --------------------------------------------
  Field-of-view              Approximately 0.34 square degrees

  Plate Scale                0.17 arcsec/pixel

  Wavelength coverage        900nm - 1700nm (1500nm required)

  Detector Type              HgCdTe Hawaii-2RG (1.7 m cut-off)

  Detector Architecture      36 HgCdTe 2kx2k Array's with 18 micron
                             pixels

  Detector Array Temperature 140 K

  Detector Quantum           \>60% average
  Efficiency:                

  Read Noise                 5 e- (multiple samples) (8 e- required)

  Exposure Time              300 sec (typical single exposures)

  Dark Current               0.02 e/s/pix (0.2 e/s/pix required)

  Readout Time               30 sec

  Exposure control           Electronic

  Filters                    3 bands
  -------------------------- --------------------------------------------

3.5 Spectroscopy

The architecture of both the spectrograph is based on an integral field
spectrograph with an image slicer. The image slicer eliminates the need
for a slit and greatly reduces the pointing accuracy required to place
the supernova within the field of view of the spectrograph while
preserving photometric accuracy because of the 100% filling factor. The
optical spectrograph may require selectable resolution (or binning) in
order to achieve an optimum of performance and exposure times.
Furthermore, the spectral features of the high redshift supernovae are
dilated by 1+*z*, so that reduced resolution is a good match to
standardizing the performance over all supernovae followed. The
performance features of the spectrograph are shown in Table 7. The
spectrograph is assumed to have a visible and NIR arm within a single
instrument. The cross-over wavelength between the arms at 1 m is
obtained with a high throughput dichroic splitter.

The spectrographic images are obtained by pointing the satellite at each
individual supernova one at a time during its peak brightness. The
parameters given in Table 6 assumes a 2 meter primary mirror and an
overall visible light system efficiency of 40% and an IR system
efficiency of 30% including telescope obscuration, reflectance, optical
transport, and detector quantum efficiency.

#######  **Table 7. Spectrograph Reference Model Specification**

+----------------------------------+---+----------------------------------+---+
| > Spectrograph architecture      |   | Integral field spectrograph, two |   |
|                                  |   | arms                             |   |
+----------------------------------+---+----------------------------------+---+
| > Wavelength coverage            |   | 350-980 nm, 980-1700nm           |   |
+----------------------------------+---+----------------------------------+---+
| > Plate scale                    |   | 0.15 arcsec/pixel                |   |
+----------------------------------+---+----------------------------------+---+
| Spatial resolution of image      | 0 |                                  |   |
| slicer                           | . |                                  |   |
|                                  | 1 |                                  |   |
|                                  | 5 |                                  |   |
|                                  | a |                                  |   |
|                                  | r |                                  |   |
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|                                  | l |                                  |   |
|                                  | i |                                  |   |
|                                  | c |                                  |   |
|                                  | e |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Pixel size                       | H |                                  |   |
|                                  | g |                                  |   |
|                                  | C |                                  |   |
|                                  | d |                                  |   |
|                                  | T |                                  |   |
|                                  | e |                                  |   |
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|                                  | D |                                  |   |
|                                  | : |                                  |   |
|                                  | 1 |                                  |   |
|                                  | 5 |                                  |   |
|                                  | - |                                  |   |
|                                  | 2 |                                  |   |
|                                  | 0 |                                  |   |
|                                  | m |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Field-of-View                    | R |                                  |   |
|                                  | e |                                  |   |
|                                  | d |                                  |   |
|                                  | u |                                  |   |
|                                  | n |                                  |   |
|                                  | d |                                  |   |
|                                  | a |                                  |   |
|                                  | n |                                  |   |
|                                  | t |                                  |   |
|                                  | 3 |                                  |   |
|                                  | x |                                  |   |
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|                                  | c |                                  |   |
|                                  | s |                                  |   |
|                                  | e |                                  |   |
|                                  | c |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Location                         | M |                                  |   |
|                                  | o |                                  |   |
|                                  | u |                                  |   |
|                                  | n |                                  |   |
|                                  | t |                                  |   |
|                                  | e |                                  |   |
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|                                  | l |                                  |   |
|                                  | a |                                  |   |
|                                  | n |                                  |   |
|                                  | e |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Resolution                       |   |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Detector Type                    | C |                                  |   |
|                                  | C |                                  |   |
|                                  | D |                                  |   |
|                                  | ' |                                  |   |
|                                  | s |                                  |   |
|                                  | ( |                                  |   |
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|                                  | a |                                  |   |
|                                  | r |                                  |   |
|                                  | m |                                  |   |
|                                  | ) |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Detector Architecture            | 1 |                                  |   |
|                                  | k |                                  |   |
|                                  | x |                                  |   |
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|                                  | t |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Detector Array Temperature       | 1 |                                  |   |
|                                  | 4 |                                  |   |
|                                  | 0 |                                  |   |
|                                  | K |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Detector Quantum Efficiency:     | 8 |                                  |   |
|                                  | 0 |                                  |   |
|                                  | % |                                  |   |
|                                  | \ |                                  |   |
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|                                  | d |                                  |   |
|                                  | T |                                  |   |
|                                  | e |                                  |   |
+----------------------------------+---+----------------------------------+---+
| Read Noise                       | 2 |                                  |   |
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+----------------------------------+---+----------------------------------+---+
| Dark Current                     | 0 |                                  |   |
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+----------------------------------+---+----------------------------------+---+
| Exposure Time                    | V |                                  |   |
|                                  | i |                                  |   |
|                                  | s |                                  |   |
|                                  | : |                                  |   |
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|                                  | s |                                  |   |
+----------------------------------+---+----------------------------------+---+

## 

## 3.6 Summary Rates

In sixteen months of study, as shown in Table 8, the satellite can
discover, follow the lightcurve, and obtain spectra at peak brightness
for 2000 Type Ia supernovae \[not accounting for analysis or other on
the ground induced efficiencies\].

###  Table 8. Summary of North (or South) Field Survey

+------+--------+-----+-----------+-----------+------------+--------+
| #    | *Peak  | *Zo | *SNe      | *#SNe     | *#S        | *      |
| ###  | SNe    | dia | Rat       | follo     | Ne/survey* | Spectr |
| Reds | flux   | cal | e/deg/yr* | wed/year* |            | oscopy |
| hift | \[     | \[  |           |           |            | \[hrs  |
|      | e/s\]* | e/s |           |           |            | /yr\]* |
|      |        | \]* |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.1  | 842.7  | 0   | 1.1       | 8         | 11         | 0      |
|      |        | .11 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.2  | 202.2  | 0   | 3.5       | 26        | 35         | 2      |
|      |        | .13 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.3  | 87.0   | 0   | 6.5       | 48        | 64         | 5      |
|      |        | .15 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.4  | 47.8   | 0   | 9.6       | 71        | 95         | 11     |
|      |        | .17 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.5  | 30.0   | 0   | 12.5      | 93        | 124        | 22     |
|      |        | .19 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.6  | 20.6   | 0.2 | 15.0      | 112       | 150        | 39     |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.7  | 15.0   | 0   | 17.2      | 128       | 171        | 64     |
|      |        | .21 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.8  | 11.4   | 0   | 18.9      | 137       | 183        | 96     |
|      |        | .22 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 0.9  | 9.0    | 0   | 20.3      | 134       | 179        | 130    |
|      |        | .23 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 1    | 7.3    | 0   | 21.3      | 127       | 170        | 168    |
|      |        | .23 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 1.1  | 6.0    | 0   | 22.0      | 116       | 155        | 205    |
|      |        | .24 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 1.2  | 5.1    | 0   | 22.4      | 106       | 142        | 248    |
|      |        | .24 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 1.3  | 2.9    | 0   | 22.6      | 97        | 130        | 297    |
|      |        | .49 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 1.4  | 2.5    | 0   | 22.6      | 89        | 119        | 351    |
|      |        | .50 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 1.5  | 2.2    | 0   | 22.5      | 80        | 107        | 403    |
|      |        | .50 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 1.6  | 2.0    | 0   | 22.3      | 70        | 94         | 447    |
|      |        | .50 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| 1.7  | 1.8    | 0   | 22.0      | 60        | 80         | 480    |
|      |        | .51 |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+
| *    |        |     |           | **1502**  | **2009**   | **     |
| *tot |        |     |           |           |            | 2968** |
| al** |        |     |           |           |            |        |
+------+--------+-----+-----------+-----------+------------+--------+

**4. Mission Requirements**

The purpose of this section is to define the mission requirements placed
upon SNAP organizations.

## Launch.

The SNAP Observatory shall be launched to the vicinity of the L2
Lagrange point of the Earth-Sun system. A lunar fly-by, or a direct
ascent, may be utilized. The motion about the L2 point may be a halo
orbit or a Lissajous orbit.

## Orbital Adjustment.

The SNAP Observatory shall have on-board propulsion capable of: (i)
offsetting initial injection errors, (ii) maneuvers to place into L2
orbit; (iii) set and maintain station keeping near L2; (iv) unload
angular momentum from the reaction wheels.

## DeOrbit.

The SNAP Observatory will not require disposal from its L2 location.

## Lifetime.

The SNAP Observatory shall be designed for a six-year lifetime.

## Pointing.

During science observations the SNAP Observatory shall be stable in yaw
and pitch to better than ±0.02 arcsec (1 sigma rms) \[requirement\],
±0.01 arcsec (1 sigma rms) \[specification\] and stable in roll to
better than ±0.8 arcsec (1 sigma rms) \[requirement\], ±0.4 arcsec (1
sigma rms) \[specification\]

## Engineering.

The SNAP Observatory shall be designed with S-band communications for
commanding and engineering data downlink on a daily basis.

## Science Telemetry

The L2 orbit location is visible from Earth for an 8-14 hour interval
every 24 hours from any given ground site such as Goldstone Tracking
Station. Science telemetry will take place once per 24 hours in a single
time allocation block for which the appropriate spacecraft attitude
maneuver will be established to aim the onboard K-band antenna towards
Earth.

Onboard data storage will be required owing to this periodic downlink
and the fact that downlink attitudes are incompatible with science
attitudes. The minimum onboard bit capacity OBBC is given by

OBBC = (1+ohead) \* (Nvis + 2\*Nnir) \* 16 \* compress \* Tsurvey/Texp

where ohead is the needed data storage overhead (\~1%), Nvis is the
number of visible sensor pixels (36 \* 3512^2^ = 444 Mpix), Nnir is the
number of near infrared pixels (36\*2048^2^ = 151Mpix), 16 is the number
of bits per pixel, compress is the compression factor (\~0.5),
Tsurvey=22h=79200sec per day), and Texp is the time separating
successive exposures = 330 sec. These factors determine OBBC= 1.44
terabits. A safety factor of two or three will allow ground system
outages to be accommodated.

In addition, the satellite must maintain a capability to transmit
occasional instrument data to establish and maintain optical focus, and
provide for check-out.

A preliminary downlink and uplink analysis is given in the table below,
based on a 20watt transponder, a 1.2meter onboard transmit antenna,
K-band downlink at 26GHz, and one 34m ground downlink receiving antenna.
A satisfactory signal to noise ratio margin is obtained at a 235
Mbit/sec downlink data rate. The downlink contact time can be estimated
with the formula:

Tcontact = (1 + DLohead) \* OBBC / DLBR

where DLohead is the downlink data overhead for headers and data
redundancy checking (assumed to be 20% here) and DLBR is the downlink
bit rate, assumed here to be 235Mbit/sec. At this rate, approximately 2h
of downlink time per 24h of mission time will be required to accommodate
the nominal 1.45 terabits stored during the previous 22 hours of science
data acquisition time.

###  Table 9. Ka Band Telemetry Analysis

  -------------------------- ---------- --- ----------------------- ------------
  **DOWNLINK ANALYSIS**                     **UPLINK ANALYSIS**     

                                                                    

  Frequency \[MHz\]:         26000          Frequency \[MHz\]:      2040

  Wavelength \[m\]:          0.012          Wavelength \[m\]:       0.147

                                                                    

  Sat Transmit Power \[W\]:  20             Gnd Transmit Power      1000
                                            \[W\]:                  

  Sat Component Loss \[dB\]: 4              Gnd Component Loss      0.5
                                            \[dB\]:                 

  Sat Antenna Size \[m\]:    1.2            Gnd Antenna Size \[m\]: 34

  Sat Antenna Efficiency     55             Gnd Antenna Efficiency  64
  \[%\]:                                    \[%\]:                  

  Sat Antenna Gain \[dBi\]:  47.69          Gnd Antenna Gain        55.291
                                            \[dBi\]:                

  Sat EIRP \[dBW\]:          56.7           Gnd EIRP \[dBW\]:       84.791

                                                                    

  Range \[km\]:              1800000        Range \[km\]:           1800000

  Free Space Loss \[dB\]:    245.853        Free Space Loss \[dB\]: 223.746

  Atmospheric Loss \[dB\]:   1              Atmospheric Loss        0
                                            \[dB\]:                 

  Rain Loss \[dB\]:          0              Rain Loss \[dB\]:       0

  Radome Loss \[dB\]:        0              Radome Loss \[dB\]:     0

  Polarization Loss \[dB\]:  0.5            Polarization Loss       0.2
                                            \[dB\]:                 

  Pointing Loss \[dB\]:      0.5            Pointing Loss \[dB\]:   0.1

                                                                    

  Gnd Antenna Size \[m\]:    34             Sat Antenna Size \[m\]: 0.033

  Gnd Antenna Efficiency     64             Sat Antenna Efficiency  100
  \[%\]:                                    \[%\]:                  

  Gnd Antenna Gain \[dBi\]:  77.397         Sat Antenna Gain        -3
                                            \[dBi\]:                

  Gnd Component Loss \[dB\]: 0.2            Sat Component Loss      4.5
                                            \[dB\]:                 

  Gnd Rcvr Input Power       -83.955        Sat Rcvr Input Power    -116.755
  \[dBm\]:                                  \[dBm\]:                

  Gnd Rcvr Noise Figure      0.4            Sat Rcvr Noise Figure   2.5
  \[dB\]:                                   \[dB\]:                 

  Gnd Antenna Temperature    20             Sat Antenna Temperature 100
  \[K\]:                                    \[K\]:                  

  Gnd System Temperature     149.178        Sat System Temperature  1263.443
  \[K\]:                                    \[K\]:                  

  Gnd Receiver G/T \[dB/K\]: 54.46          Sat Receiver G/T        -38.516
                                            \[dB/K\]:               

                                                                    

  Modulation Type:           QPSK           Modulation Type:        PCM/PSK/PM

  Modulation Function:       Sq. Wave       Modulation Function:    SINE WAVE

  Modulation Index \[rad\]:  1.57           Modulation Index        1.3
                                            \[rad\]:                

  Modulation Loss \[dB\]:    0              Modulation Loss \[dB\]: 2.636

  Bit Error Rate:            1.00E-06       Bit Error Rate:         1.00E-07

  Coding Type: RS+R1/2CVSDD                 Coding Type:            NO CODING

  Coding Gain \[dB\]:        8              Coding Gain \[dB\]:     0

  Data Rate \[kbps\]:        235000         Data Rate \[kbps\]:     2

  Bandwidth \[kHz\]:         235000         Bandwidth \[kHz\]:      2

  Required Eb/No \[dB\]:     2.5            Required Eb/No \[dB\]:  11.3

                                                                    

  Predicted Eb/No \[dB\]:    7.386          Predicted Eb/No \[dB\]: 19.682

  Implementation Loss        1.5            Implementation Loss     2
  \[dB\]:                                   \[dB\]:                 

  Predicted Link Margin      3.386          Predicted Link Margin   6.382
  \[dB\]:                                   \[dB\]:                 
  -------------------------- ---------- --- ----------------------- ------------

**5. Ground Systems Requirements**

## 5.1 Mission Operations

### Mission Operations Center 

The SNAP Observatory shall be controlled from a Mission Operations
Center (MOC) capable of monitoring both spacecraft and instrument health
and safety, while validating and uplinking SNAP observing schedules
generated by the Science Operations Center (SOC).

The MOC shall be capable of receiving SNAP science and engineering
telemetry at the full downlink rate of 300 Mbps, and temporarily storing
at least one orbit's worth of data.

The MOC shall transfer science and selected engineering data to the
Science Operations Center (SOC) concurrent with spacecraft contacts, and
shall be capable of retransmitting science data to the SOC from its
temporary storage in the event data are corrupted in transit to the SOC.

The MOC shall deliver to the SOC at least 98% of all SNAP available
science data.

### Ground Station.

The SNAP Ground Station shall be capable of S-Band communications with
the SNAP Observatory at engineering rates. The Ground Station shall be
capable of receiving Ka-Band science telemetry at 235 Mbps or greater.

## 5.2 Science Operations

### Science Data Storage and Archiving

The Science Operations Center shall receive and store all SNAP
Observatory Science Data and all necessary ancillary engineering data
from the Mission Operations Center, and provide copies as required by
the Data Management Plan to other facilities.

### SuperNova Detection and Selection.

The Science Operations Center shall detect Supernovae in the images and
select Supernovae for follow up measurements.

### Guest Observations

The Science Operations Center shall provide access to available imaging
time on the SNAP Observatory through an organized science program run by
the PI institution.

### Observation Schedule Management

The Science Operations Center shall optimize the SNAP Observing schedule
in order to (i) achieve a relatively uniform distribution of Supernovae
redshifts; (ii) meet the science requirements in minimum time; and (iii)
provide access by the guest observer program.

## 5.3 Mission Data Requirements

### Science Data Management

The SNAP Principal Investigator team shall be responsible for initial
analysis of the data, its subsequent delivery to an appropriate data
repository, the publication of scientific findings, and communication of
results to the public. Additionally, the SNAP PI team shall be
responsible for collecting engineering, and ancillary information
necessary to validate and calibrate the scientific data prior to
depositing it into an approved data repository. The dataset would likely
be split into: 1) a co-added deep static dataset, and 2) a transient
dataset. Data release will include making data available on-line at the
DOE National Energy Research Scientific Computing Center located at LBNL
and/or through other centers. The SNAP archival data will be made
available to the public and the science community. Additionally, we
expect to be able to provide a significant and ever increasing fraction
of the SNAP observing time in a restricted mode to meritorious survey
programs.

### 

### Data Management Plan

SNAP Management shall develop a data management plan to address the
total activity associated with the flow of science data, from
acquisition, through processing, data product generation and validation,
to archiving and preservation.

**6. Education and Public Outreach**

The SNAP project shall develop and execute an Education and Public
Outreach Plan. The program will make use of the intense public interest
in the accelerating universe as a unique opportunity to engage students,
educators, and the general public in an enhanced understanding of our
physical world. The program shall address grades K-14 students and
teachers, the general public, as well as upper-level undergraduates and
graduate students pursuing advanced degrees in the sciences and
engineering disciplines.

**7. Spacecraft**

In Table 10, the critical specifications for the SNAP spacecraft are
given. Many of these specifications are still under development to allow
design trade-off flexibility.

####### Table 10. Spacecraft Specifications

  ------------------------- ---------------------------------------------
  Launch Vehicle            Delta 4040 or equivalent, maximum lift to
                            orbit 2780 kg (direct injection to L2)

  Mission Lifetime          6 years

  Orbit                     L2 Lagrange zone of Earth-Sun system

  Telemetry                 235 Mbit/s Ka-band, \~2 hr. pass each 24h

  Spacecraft attitude       3-axis stabilized, low jitter
  control                   

  Pointing jitter z axis    Focal plane stabilized by ACS using focal
  (rotation)                plane feedback to ±0.4 arcsec in roll (1
                            sigma rms) \[spec\].

  Pointing jitter x-y axis  Focal plane stabilized by ACS using focal
                            plane feedback to ±0.01 arcsec in yaw & pitch
                            (1 sigma rms) \[spec\].

  Exposure time             Maximum single exposure 3000 sec, 300 sec
                            typical.

  Bore of Spectrograph      3 arcsec x 3 arcsec (FOV)

  Pointing accuracy         Place supernova within bore of spectrograph
  (instrument)              

  Pointing knowledge        Place supernova within bore of spectrograph
  (instrument)              

  Slew Rate to new field    \>3 degrees/minute

  Acquisition time new      \< 30 seconds for field within 3 arcminutes
  field                     

  Observing Duty-Factor     \>80%

  Total Mass (Spacecraft &  2020 kg (wet)
  Payload)                  

  Total Power (Spacecraft & 460W (average)
  Payload)                  

  Solid State Recorder      350 Gbytes = two nominal 22h data takes
  Capacity                  

  Propulsion system         for orbit insertion, momentum management, and
                            station keeping

  Thermal control           telescope \< 290K, focal plane array ≤140K

  Solar Avoidance           70 degrees

  Field of View             nominally centered near north and south
                            ecliptic poles

  Thermal management        passive coatings, thermal insulation,
                            heaters, radiators

  Redundant architecture    Failure mitigation & identification of
                            critical components
  ------------------------- ---------------------------------------------

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