1 Telescope Optical Performance Breakout Session M.Lampton UCBerkeley Space Sciences Lab 10 July 2002

2 Optical Performance: Overview l Review l Image quality l Diffracted Starlight l Stray (scattered) Light l Acquisition Plan l Materials, manufacturing etc will be discussed in Pankow’s talk

3 Review Telescope is a three-mirror anastigmat 2.0 meter aperture 1.37 square degree field Lightweight primary mirror Low-expansion materials Optics kept near 290K Transverse rear axis Side Gigacam location passive detector cooling combines Si & HgCdTe detectors Spectrometers share Gigacam focal plane Minimum moving parts in payload shutter for detector readouts

4 Image Quality 1 TMA62/TMA63 configuration Airy-disk zero at one micron wavelength 26 microns diam=0.244arcsec

5 Image Quality 2

6 Image Quality 2 continued Although the range of radii in use within the focal plane is the nominal design range 129 to 283mm, the extremes are poorly populated with pixels

7 Image Quality: Distortion

8 Image Quality 3 Science SNR drives Strehl ratio —Imperfections in delivered wavefront cause central PSF intensity to be less than ideal diffraction-limited PSF —This ratio is the “Strehl Ratio” Systems Engineer manages WFE budget —geometrical aberrations —manufacturing figure errors & cost —alignment errors in 1-g environment —gravity release in mirrors & structure —launch induced shifts & distortions —on-orbit thermal distortion —ageing & creep of metering structure —how many on-orbit adjustments? Primary mirror dominates WFE budget because it is the most expensive to figure. Non-optical factors: —Attitude control system stability —Transparency & optical depth in silicon Marechal’s equation relates WFE and Strehl

9 Image Quality 4 For diffraction-limited optics, rmsWFE or is usually the governing procurement specification SNAP exposure-time-critical science is at wavelengths > 0.63um Science team needs to be aware of cost/schedule/quality trades

10 Image Quality 5

11 Image Quality 6 Example: overall telescope 43 nm RMS WFE —gives Strehl= 0.93 at 1000 nm —gives Strehl=0.90 at 830 nm —gives Strehl=0.83 at 633 nm Example: overall telescope 50 nm RMS WFE —gives Strehl=0.91 at 1000 nm —gives Strehl=0.87 at 830 nm —gives Strehl=0.77 at 633 nm WFE to be budgeted among pri, sec, flat, and tertiary mirrors —detailed breakdown to be determined How sensitive are cost & schedule to our WFE specification? Encircled Energy specification needs to be defined —central obstruction 40% radius, 16% area —with this obstruction alone, EE=50% at 0.088arcsec or EE=80% at 0.23arcsec —Budget lower EE for aberrations, spider, figuring, thermal, gravity..

12 Image Quality 7 Strehl vs Aperture Trade —Strehl (image quality) costs time & money —Aperture (image quantity) costs time & money —Central obscuration trades off with stray light —NIR (not visible) is where SNR demands the most observing time —Is 77% Strehl and 2.0 meters aperture the right mix? Encircled Energy Specification —High spatial frequency figure errors lose photons —Low spatial frequency figure errors broaden the encircled energy —Steeper EE curves demand absence of LSF amplitudes —Is 70% EE at 0.1 arcsecond the right target? Quantitative answers require modelling Our sim team can deal with image quality trades We expect to resolve these issues during R&D phase

13 Tolerance to Primary curvature

14 Tolerance to misplaced secondary mirror Example assumes 3 micron growth in image blur

15 Tolerance to misplaced tertiary mirror Example assumes 3 micron growth in image blur

16 Diffracted Starlight 1

17 Diffracted Starlight 2 (Four vanes)

18 Diffracted Starlight 3 (Eight vanes)

19 Circular 2meter aperture 5 x 5 arcsec

20 Circular 2meter aperture 0.7 meter central obscuration

21 Circular 2m aperture Three radial legs, 50mm x 1 meter

22 Circular 2m aperture central 0.7m obscuration Three legs, 50mm x 1meter

23 Diffracted Starlight 8

24 Diffracted Starlight 9

25 Diffracted Starlight 10

26 Diffracted Starlight 11 Extensive work with sim team Modelling PSF for SNR, exposure times... Modelling wings of diffraction pattern Algorithms for photometry in presence of diffraction Determination of effective SNR Inputs from our known sky, down to V=19 (SDSS) How well can these effect be modelled?

27 Stray Light 1 Guiding principle: keep total stray light FAR BELOW natural Zodi R.O.M. assessment gives... —Natural Zodi (G.Aldering) = 1 photon/pixel/sec/micron —Starlight+Zodi scattered off primary mirror = —Starlight+Zodi scattered off support spider < —Sunlight scattered off forward outer baffle edge = 2E-5 —Earthlight scattered off forward outer baffle inner surface = 0.02 —Total stray = 0.02 photon/pixel/sec/micron ISAL conclusion: “manageable” Long outer baffle is clearly preferred —limit is launch fairing and S/C size ASAP software in place ASAP training begun Preliminary telescope ASAP models being built ASAP illumination environment models not yet started Our intension is to track hardware & ops changes as they occur, allowing a “system engineering management” of stray light.

28 Stray Light 2

29 Stray Light 3: Reverse Trace

30

31 Optical Performance: Throughput Protected silver —provides highest NIR reflectance currently available —durability is an issue: 3 years at sea level prior to launch —this is our baseline —new developments at LLNL: Thomas & Wolfe process Protected aluminum —highly durable coating —slight reflectance notch at 0.8 microns wavelength —after four reflections, amounts to 30-40% loss at 0.8 um —prefer to retain high reflectance at 0.8 microns —not our first choice

32 Telescope Acquisition Plan Potential Vendors Identified —Ball Aerospace Systems Division (Boulder) —Boeing-SVS (Albuquerque/Boulder) —Brashear LP (Pittsburgh) —Composite Optics Inc (San Diego) —Corning Glass Works (Corning NY) —Eastman Kodak (Rochester) —Goodrich (Danbury) —Lockheed-Martin Missiles & Space Co (Sunnyvale) —SAGEM/REOSC (Paris) These vendors have been briefed on SNAP mission Each has responded to our Request for Information Identify a route (materials, fabrication, test, integration, test) —Milestones with appropriate incentives —Visibility into contractor(s) activities

33 Test Plans Individual Mirror Testing Assembly into metering structure Assembled optical testing —interferometric —reflex testing against reference flat Integration with focal plane assembly End-to-end testing —in air at room temperature —in vacuum or dry N2 with cold focal plane —reflex testing against reference flat

34 Reflex Test Configuration

35 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 —engineering trade studies and “budgets” —manufacturing process risk assessments —test plans and associated cost/risk trades facilities; equipment —prepare the acquisition plan —performance specifications & tolerance analysis —create draft ICDs —develop preliminary cost & schedule ranges