# Presentation: 059115

## Telescope Overview

**M. Lampton**
**     ****Space Sciences Laboratory**
**University of California Berkeley**
**11 November 2003**

## Outline

**Overall schedule: space telescopes take time to build and test!**
**Science Driven Requirements**
**Current Baseline Telescope Configuration**
**Telescope Development: previous four years**
**Telescope Development: next two years**
**Acquisition plan**
**Primary mirror material & blank fabrication**
**Primary mirror finisher**
**Telescope contractor issues**
**Continuing trade studies**
**R&D Goals**
**R&D Milestones & Decision points**
**R&D Manpower**
**R&D Deliverables**
**Preparations for Conceptual Design Report**
**Summary**
**Items to be presented & discussed in the breakout session**

## R&D Top-level Schedule

## Telescope: Science Driven Requirements

**Light Gathering Power **
**must measure SNe 4 magnitudes  fainter than 26 magnitude  peak**
**want SNR of  30:1 at peak brightness, aggregate exposure fit**
**presence of zodiacal light foreground radiation**
**time-on-target limited by revisit rate & number of fields**
**spectroscopy demands comparable time-on-target**
**requires geometric diameter ~ 2 meters**
**Angular resolution**
**signal to noise ratio is driver**
**diffraction limit is an obvious  bound**
**Airy disk  at one micron wavelength is 0.12 arcseconds FWHM**
**all blur contributions must be budgeted**
**Field of View**
**determined by required supernova discovery rate**
**volume of space is proportional to field of view**
**Roughly one degree field of view  will deliver the requisite discovery rate**
**Wavelength Coverage**
**0.35 to 1.7 microns requires all-reflector optical train**

## Baseline Configuration: Korsch TMA

**Baseline Configuration: Korsch TMA**

**Prolate ellipsoid concave primary mirror**
**Hyperbolic convex secondary mirror**
**Flat folding mirror with central hole**
**Prolate ellipsoid concave tertiary mirror**
**Flat focal plane**
**Delivers < 0.06 arcsecond FWHM geometrical blur over  annular field 1.37 sqdeg**
**Adapts to focal lengths 15**
**     ****meters through 30 meters; **
**     ****baseline=21.66m**
**Provides side-mounted **
**     ****detector location for best **
**     ****detector cooling**

## Telescope Status: History

**Wide-field high-resolution telescopes are NOT new**
**Schmidt cameras (1930 to present)**
**Wynne cameras (e.g. FAUST)**
**Field-widened cassegrains, Gascoigne (1977-); SDSS**
**Paul three-mirror telescopes (1935) and Baker-Paul**
**Cook three-mirror anastigmats  “TMAs” (1979)**
**Williams  TMA variants (1979)**
**Korsch family of TMAs (1972-1980)**
**Angel-Woolf-Epps three-mirror design (1982)**
**McGraw three-mirror system (1982)**
**Willstrop “Mersenne Schmidt” family (1984)**
**Kodak “IKONOS” Earth Resources telescope = TMA**
**LANL/Sandia/DoE Multispectral Thermal Imager = TMA**

## Milestones Accomplished

**1999**
**Comparisons of many alternative wide field telescope configurations**
**Discarded off-axis designs, TMTs (Willstrop etc) as unpackageable**
**Annular-field TMA concept rediscovered, developed, adopted **
**2000**
**TMA43 (f/10): longitudinal rear axis, separate NIR/VIS;  filter wheel**
**Began mechanical, structural studies w/ subcontractor: SM support, strongback**
**2001**
**Efforts to achieve shorter optical package for stiffness, stray light: TMA55/56**
**Explored longer focal lengths for potentially higher sampling: TMA59 (f/15)**
**Design validated at NASA/GSFC Instrument Systems Analysis Laboratory**
**image quality;  stray light;  metering structure; actuators**

**2002**
**Adopted FIDO detector package, single focal plane, transverse rear axis**
**TMA62 (f/11) with filter wheel and TMA63 (f/11) no filter wheel**
**Major efforts in materials, metering structure,  thermal,  launch loads analysis**
**Detailed mirror FEM analysis, materials & fab/test trades by subcontractor**
**2003**
**Telescope engineering team enlarged**
**Detailed FEMs of PM, SM, TM, folding flat, and metering structure were explored**
**Launch stress**
**Bipod loads and torques**
**1-gravity test plan**
**Thermal analysis of PM and PM environment begun**
**Received top-down fab/test plan from one prospective contractor**
**Stray light analysis begun for outer baffle**
**Fabrication and test planning started: interferometric vs PSF, horiz vs vertical**
**Prospective PM blank manufacturer visited and briefed; cost/schedule being sized**

## Payload Layout   1

**Payload Layout   1**

- Main Baffle Assembly

- Door Assembly

- Solar Array, ‘Sun side’

- Secondary Mirror
- Hexapod
- Bonnet

- Secondary Metering structure

- Primary Mirror

- Optical Bench

- Instrument Metering Structure

- Tertiary Mirror

- Fold-Flat Mirror

- Spacecraft

- Shutter

- Instrument Bay

- Instrument Radiator

- Solar Array, ‘Dark side’

- Hi Gain Antenna

- Mass Memory

- ACS
- CD & H
- Comm
- Power
- Data

- CCD detectorsNIR detectorsSpectrographFocal Plane guiders
- Cryo/Particle shield

## Payload Layout continued

## Telescope: Image Quality Issues

**Image quality drives science SNR, exposure times, ....**
**Many factors contribute to science image quality**
**diffraction: size of aperture, secondary baffle, struts, ...**
**aberrations:  theoretical imaging performance over field**
**manufacturing errors in mirrors**
**misalignments & misfocussing of optical elements**
**dirt, contamination, or nonuniformity in mirror coating **
**guiding errors and spacecraft jitter**
**detector issues**
**constancy of the PSF is important to the weak lensing science**
**Work has begun on a comprehensive budget**
**ongoing simulation team efforts**
**Bernstein’s “Advanced Exposure Time Calculator” PASP**
**telescope studies feed into the simulations**

## Pixel sizes and blur sizes

**Pixel sizes and blur sizes**

- Aberrations

- Attitude control jitter

- Si CCD pixels

- HgCdTe pixels

- Silicon diffusion

- diffraction

- Total blur

## Telescope Status: Ray TraceTMA62/TMA63  configuration  

- Airy-disk zero at one micron wavelength
- 26 microns diam=0.244arcsec

## Telescope Status: Pupil Obscuration Trades

## Diffraction

**Diffraction**

- Circular 2m aperture
- central 0.7m obscuration
- Three legs,  50mm x 1meter
- One micron wavelength

## Telescope Status: Stray Light Trades

**Guiding principle: keep total stray light FAR BELOW natural Zodi**
**R.O.M. assessment conducted in 2001-2002**
**Long outer baffle is clearly preferred**
**limit  is  launch fairing and S/C size**
**ASAP software in place**
**Preliminary telescope ASAP model outer baffle has been built**
**Tertiary structure and cold shield in process**
**ASAP illumination environment models being built**
**Our intension is to track hardware & ops changes as they occur, allowing a “system engineering management” of  stray light.**

## ASAP Stray Light Model

- Harvey scattering Earthshine case shown
- Two baffle scatters and one PM scatter to reach focal plane
- Preliminary estimate: 1⁄2 zodi at full Earth, HEO orbit; goes away in L2 orbit
- Anodized Al or Z306 (no exotic coatings assumed)
- Continuing work to develop entire system including interior of cryostat

## Wavefront Error Budget

## Telescope Technology Roadmap

**Existing technologies are suitable for SNAP Optical Telescope Assembly**
**New materials, processes, test & evaluation methods are unnecessary**
**Mirror materials**
**science driver: *stable* figure to guarantee constant focus and PSF**
**Corning ULETM glass: extensive NRO flight history; lightweight.**
**Schott ZerodurTM glass/ceramic composite:  lower cost, widely used in ground based astronomical telescopes; huge industrial base; but no space flight history above 1 meter size.**
**Metering structure materials**
**science driver:  *stable* structure for constant focus and PSF**
**M55J carbon fiber + cyanate ester resin;  epoxy adhesive bonds**
**full report in Pankow presentation**
**Mirror finishing technology**
**conventional grind/polish/figure using abrasives**
**ion-beam figuring available from two vendors**
**Mirror surface metrology**
**same  as  other space telescopes,  e.g. cassegrains**
**standard interferometer setups will do the job for SNAP**

## Mirror Materials Trade

**Corning ULETM ultra-low expansion glass (20-50 ppb/degC)**
**Space flight experience: e.g. HST**
_**[http://www.](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[gsfc](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[.](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[nasa](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[.](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[gov](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[/](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[gsfc](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[/service/gallery/fact_sheets/](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[spacesci](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**__**[/hst3-01](http://www.gsfc.nasa.gov/gsfc/service/gallery/fact_sheets/spacesci/hst3-01)**_
**face sheets bonded to honeycomb core**
**achieves >85% lightweight; anticipate 220kg primary**
**high stiffness eases gravity unloading accuracy**
**Can be rear-supported at 70% radius zone**
**Good strength margins against launch stress**
**Excellent interferometric test fringe density**
**70% is highly compatible with vertical testing & gravity unloading**
**But... has internal bonds (frit or LTF); need verification**
**Schott ZerodurTM glass/ceramic material (20-50 ppb/degC)**
**solid blank, weight relieved by milling backside, open back**
**Can achieve 85% lightweight; anticipate 240kg primary**
**But... open-back construction is not as stiff as ULE sandwich**
**And...open back construction exposes fragile thin ribs to fracture**
**Worse...customary support is at edge, 100% radius **
**poorer launch stress margin**
**Poorer interferometric test fringe pattern: edge concentration**
**ULETM preferred technically: stronger, stiffer, heritage**
**ZerodurTM possibly preferable on schedule/cost grounds**

## Radius of curvature changes due to bulk temperature swings are main concern

**TELESCOPE OPTICS****Mirror thermal control requirements**

## SNAP Technology Mirror

**Demonstrate all technical issues are solved, to buy down risk**
**Make progress towards flight:**
**Material specifications OK?**
**Fabrication plan OK?**
**Test plan OK?**
**Utilize conservative design**
**Stress margins**
**Test margins**
**Closed-back ULETM, frit bonded**
**Open-back ZerodurTM, monolith **
**Quotation preps are underway**
**Pre-release review of RFQ scheduled Feb 2004**
**Proceed with material selection, vendor selection, pilot fabrication schedules as far as resources permit through R&D phase**

## Telescope Trade Studies 

**Trade Studies worked during  Pre-R&D Phase**
**Optical configuration:                                      >> TMA**
**Warm optics vs cold optics:                             >> warm**
**Secondary support: 3 or 4 legs?                      >> three**
**Aperture of telescope?                                     >> 2 meters**
**FIDO integrated sensor array vs separated:  >> integrated**
**Trade Studies continuing through  R&D Phase**
**SNAP Technology Mirror: definition, industry coordination, contract award and monitoring**
**Telescope acquisition plan: vendor experience, facilities**
**Test plan & gravity unloading technique**
**Test plan: full aperture vs partial aperture**
**Wavefront error: cost vs  performance**
**focal length: is 21.66m the best choice?**
**Best apportionment of WFE among contributors**
**pupil obscuration, diffraction, stray light...**
**Kinematic bipod mounts for PM: details + consequences**
**Primary mirror detailed analysis**
**resonance freqs, sag under 1G testing, launch margins**
**Secondary, tertiary, flat mirrors: requirements etc**
**Protoflight vs Prototype + Flight metering structures**

## Overall Risk Assessment

**Schedule risks: OTA is a long lead item!**
**Mirror fab/test risks: metrology, figure control**
**Somewhat less demanding than HST: we are NIR not NUV**
**yet -- need comprehensive  test  plan.**
**Use interferometric *and* PSF acceptance tests**
**Use built-in test equipment for frequent end-to-end checks**
**Mechanical structural risks**
**comprehensive test plan:  static, dynamic,  etc **
**Margins established for glass elements **
**Highly conservative design selected for SNAP Technology Mirror**
**Thermal and mechanical  disturbance  issues**
**“****Easy” thermal  environment: HEO has few eclipses**
**Use active thermal control for mirrors and structure**
**Error budget: fixturing, optical test equipment, etc**
**Full-aperture reference test flat vs partial aperture**
**Contamination control: materials & test plan**
**Stray light control: management & test plan**

## Telescope: Main R&D Tasks

**Critical path: telescopes take time!**
**Continue development of the OTA requirements document **
**Need to refine performance specifications**
**optical image quality**
**stability of focus, point spread function, ...**
**Need to understand & communicate tolerances**
**Need to prepare draft Interface Control Documents**
**optical**
**thermal**
**mechanical**
**electrical**
**Need to assess risks and take steps to minimize them**
**Need to perform  trade studies (outlined above)**
**Pursue SNAP Technology Mirror**
**Begin with a detailed and complete specification**
**Get alternative vendor quotations**
**Evaluate fabrication schedule trades, risks, cost controls**

## Telescope Acquisition Plan

**Potential Materials for PM:**
**Corning ULE glass**
**Schott Zerodur**
**Potential PM blank manufacturers:**
**Corning (ULE only)**
**Eastman Kodak (ULE or Zerodur)**
**Goodrich (ULE or Zerodur)**
**Brashear LP (ULE or Zerodur)**
**SAGEM/REOSC (Zerodur)**
**Potential PM finisher/testers, and Telescope integrators**
**Brashear LP**
**Eastman Kodak**
**Goodrich **
**SAGEM/REOSC**
**These vendors have been briefed on SNAP mission & requirements**
**Each has responded to our Requests for Information**
**Identify a route  (materials, fabrication, test, integration, test)**
**Milestones with appropriate incentives**
**Visibility into contractor(s) activities**

## Fabrication/Test Flow

**Specifications reviewed for completeness & accuracy by experienced industry personnel**
**Select primary mirror material and configuration via competitive bid process**
**Select vendor and place contract for primary mirror blank**
**Material quality, quantity; lightweighting; fabrication; test**
**Select primary mirror finisher, place contract, coordinate with blank fabrication**
**Telescope overall design specifications, review, competitive bid solicitation**
**Structural, thermal, stability, actuators, stray light baffles....**
**Secondary, flat, tertiary, GSE mirror fabrication & test**
**Integrate optics with metering structure           <<< Level 5**
**Include built-in optical test equipment**
**KiloCam for autocollimation imaging**
**Interferometer ports for pupil WFE maps**
**Horizontal beam, if preferred by integrator**
**Vertical beam with gravity unloaders**
**Integration with focal plane instrumentation   <<< Level 4**
**Integration with outer baffle and aft closure   <<< Level 4**
**Integration with spacecraft                                <<< Level 3**
**Qualification test plan                                        <<< Level 2**
**EMC, acoustic, vibration, thermal-vacuum **
**Mate with booster & fairing                              <<< Level 1**
**Launch **
**On-orbit commissioning tests**
**Image quality PSF maps; stability; calibration throughput coefficients**

## OTA SpecificationDraft Oct 2003,  CM 00008 rev C (in work)

**Scope**
**Applicable Documents**
**General **
**Project level publications**
**Performance Requirements**
**Science drivers**
**Optical performance**
**Environmental**
**Operational**
**Structural qualification**
**Optical Specification**
**Baseline configuration**
**Rationale and system trades**
**Optical system**
**Coatings**
**Waveront error budget**
**Mechanical tolerances**
**Thermal effects**
**Focussing mechanism**
**Stray light design**
**Built in test equipment**
**Contamination control**

**Verification plan**
**Quality assurance**
**Requirements verification**
**Definitions**
**Analysis**
**Demonstration**
**Inspection**
**Horizontal beam testing**
**Vertical beam testing**
**Autocollimation flat**
**Interferometry**
**PSF analysis **

## Primary Mirror SpecificationDraft Sept 2003,  CM 00034

**Mirror Substrate**
**Thickness**
**Stability**
**Coefficient of thermal expansion**
**Substrate inclusions**
**Environments**
**Radiation**
**Dynamics**
**Mass**
**Interfaces**
**Venting**
**Cleanliness levels**
**Coating**
**Spectral characteristics**
**Uniformity**
**Humidity resistance**
**adhesion **
**Contamination control**
**Environmental Requirements**
**Verification**

**Scope**
**Applicable Documents**
**General **
**Government documents**
**Project level publications**
**Definitions**
**Nomenclature**
**Coordinates**
**Design practices**
**Aperture definitions**
**Conic constant**
**Surface error**
**Thermal definitions**
**PM Optical Requirements**
**Optical prescription**
**Surface definition**
**Reference fiducials**
**Metrology**
**Error budget**

## Telescope Optical Test Plan

**Each component is tested alone, 1-G environment, by manufacturer**
**Mirrors, structure will have vendor test protocols**
**Entire telescope ****interferometric**** tests can be run vertical or horizontal**
**Interferometric tests yield pupil WFE distribution**
**If interferometer resolution is good enough, +1G and –1G interferograms can be combined to give a 0-G interferogram   <<ACCURACY IS THE ISSUE**
**no flotation system would then be needed, horiz beam perhaps OK**
**Entire telescope ****autocollimation imaging**** tests yield PSF information**
**Secondary, flat, tertiary mirrors have fractional-micron 1-G deformation when mounted on flight supports; unloaders are fairly simple.**
**Primary mirror deforms several microns (huge!) on flight support bipods**
**Therefore primary mirror needs a GSE flotation system to verify the 0-G performance for autocollimation test**
**Would also improve sensitivity of interferometric tests.**
**We want both interferometry and imaging.**

## Gravity unloader concept

## Telescope Acquisition Plan

- This existing TPF/TDM Oversight Team is the logical choice for SNAP

## Optical Telescope AssemblySchedule Summary

## R&D Phase Manpower

**Dr M. Lampton**
**Optics team lead**
**Dr M. Sholl**
**Analysis**
**Synthesis**
**Test planning**
**Dr D. Pankow**
**Mechanical, thermal, structural**
**Systems Engineering**
**Mr R. Besuner**
**Finite element modelling**
**Mr M. Krim**
**Consultant: manufacturability and test planning**
**Dr B. Bigelow**
**Optomechanics**
**Test planning**
**System Engineering Staff**
**Administrative Staff**

## R&D Phase Deliverables

**Telescope Requirements Specification Document (CM 00008)**
**Primary mirror specifications  (CM 00034)**
**manufacturing tolerances**
**alignment specifications**
**SNAP Technology Mirror Progress Report**
**Subcontractors’ reports**
**Metering Structure Concept Trades**
**OTA Thermal Model and Results Summary (CM 00033)**
**Focusing/Alignment Tolerance Analysis**
**Focusing/Alignment Mechanism Trade Study**
**Integration Flow and Test Plan (CM 00015)**
**Preliminary Stray Light Analysis**
**Telescope acquisition plan: schedule/milestones, cost.**
**Conceptual Design Report.**

## Conceptual Design Report Preparation

**Fully complete and document  all trade studies, especially test plan**
**Supplement these with industry commentary**
**Document status & progress in SNAP Technology Mirror**
**Use systems-engineering “budgets” to identify optimum allocation of tolerances & resources**
**Identify materials and fabrication flow alternatives taking into account schedule risk and overall cost**
**Detail the acquisition plan and milestones**
**Prepare acceptance test plan, including acceptance tests:**
**Structural stability, thermal, stiffness, normal modes, creep**
**Alignment and focussing plan**
**Thermal  vacuum  and optical stability**
**Stray light**
**Gravity unloading plan**
**Full-aperture and limited-aperture test opportunities**
**Facilities needed, facilities available**

## Telescope: Summary

**Pre-R&D**
**converted science  drivers  into telescope requirements**
**reviewed existing optical telescope concepts**
**developed  annular-field TMA configuration**
**preliminary  materials  assessment**
**begun to explore vendor capabilities**
**started a budget for image quality **
**R&D Phase**
**Prepare the acquisition plan**
**Pursue the SNAP Technology Mirror **
**engineering  trade studies and “budgets”**
**manufacturing process risk assessments**
**test plans and associated cost/risk trades**
**facilities; equipment; manpower**
**performance specifications & tolerance analysis**
**create draft  ICDs**
**develop cost & schedule ranges**
**Prepare Conceptual Design Report**

## Spacecraft/Telescope Breakout Session

**Mike Lampton**
**Overall integration and test plan**
**David Auslander**
**Attitude control system analysis & predicted performance**
**Industry evaluation of SNAP ACS situation**
**Robert Besuner**
**Observatory mechanical FEM **
**Telescope mechanical FEM**
**Primary mirror stress/strain FEM**
**David Pankow**
**Thermal + structural issues**
**Mike Sholl**
**Optomechanical tolerances & WFE budget**
**Telescope draft specifications walkthrough**
**Built-in test equipment “BITE” presentation**
**Primary mirror draft specifications walkthrough**
**Optical test plan and acceptance criteria**