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Telescope Optical Performance Breakout Session

Telescope Optical Performance Breakout Session. M.Lampton UCBerkeley Space Sciences Lab 10 July 2002. Optical Performance: Overview. Review Image quality Diffracted Starlight Stray (scattered) Light Acquisition Plan Materials, manufacturing etc will be discussed in Pankow’s talk.

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Telescope Optical Performance Breakout Session

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

  2. Optical Performance: Overview • Review • Image quality • Diffracted Starlight • Stray (scattered) Light • Acquisition Plan • 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 1TMA62/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 Strehl @0.633um 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 diam @633nm or EE=80% at 0.23arcsec diam @633nm • 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 mirrorExample assumes 3 micron growth in image blur

  15. Tolerance to misplaced tertiary mirrorExample 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 = 0.002 • Starlight+Zodi scattered off support spider < 0.001 • 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. 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

  31. 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

  32. 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

  33. Reflex Test Configuration

  34. 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

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