Telescope Optical Performance M.Lampton UCBerkeley Space Sciences Lab 10 July 2002 draft 6, 7 June 2002

Optical Performance: Overview l Status Review l Image quality l Diffracted Starlight l Stray (scattered) Light l Plans, Contracts, Manpower l Materials, manufacturing etc will be discussed in Pankow’s talk

Optical Performance: Review 1 Light Gathering Power —must measure SNe 4 magnitudes fainter than 26 magnitude peak —require SNR of 50:1 at peak brightness —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 imposes upper bound —Airy disk at one micron wavelength is 0.12 arcseconds FWHM —other blur contributions must be kept well below this limit. Field of View —determined by required supernova discovery rate —volume of space is proportional to field of view —one square degree area will deliver the requisite discovery rate Wavelength Coverage —0.35 to 1.7 microns requires all-reflector optical train

Optical Performance: Review 2 Aperture: 2 meters Field of View: 1 sq deg Wavelength Range: 0.35 to 1.7 microns Strehl: >91% at 1.0 microns WFE: <50 nm RMS Focal surface: flat EFL: meters, f/11 Stray light: << Zodiacal

Optical Performance: Review 3 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 (1979) —Williams TMA variants (1979) —Korsch family of TMAs (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

Optical Performance: Review : Suitability Assessments —off-axis designs attractive but unpackagable; rejected —four, five, and six-mirror variants explored; rejected —eccentric pupil designs explored; rejected —annular field TMA concept rediscovered & developed —TMA43 (f/10): satisfactory performance but lacked margins for adjustment —TMA55 (f/10): improved performance, shorter pri-sec, margins OK —TMA59 (f/15): same but with longer focal length —TMA62 (f/11): transverse rear axis with filter wheel —TMA63 (f/11): transverse rear axis, no filter wheel

Payload Layout 1 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

Payload Layout 2

Optical Performance: Review 5 Existing technologies are suitable for SNAP Optical Telescope Assembly New materials, processes, test & evaluation methods are unnecessary Mirror materials —Corning ULE glass: extensive NRO flight history, but expensive —Schott Zerodur glass/ceramic composite: lower cost, widely used in ground based astronomical telescopes; huge industrial base —Astrium/Boostec SiC-100: newcomer; unproven in space optics; higher CTE; adoped for Herschel/FIRST in infrared Structural materials —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 —no unusual accuracy drivers have been encountered

Optical Performance: Review 6 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 20 meters through 30 meters; baseline=21.66m Provides side-mounted detector location for best detector cooling

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

Image Quality 2 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

Image Quality 3 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

Image Quality 4

Mirror surface specification 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..

Image Quality 5 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

Diffracted Starlight 1

Diffracted Starlight 2 (Four vanes)

Diffracted Starlight 3 (Eight vanes)

Circular 2meter aperture 5 x 5 arcsec

Circular 2meter aperture 0.7 meter central obscuration

Circular 2m aperture Three radial legs, 50mm

Circular 2m aperture central 0.7m obscuration Three 50mm legs

Diffracted Starlight 8

Diffracted Starlight 9

Diffracted Starlight 10

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?

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.

Stray Light 2

Stray Light 3: Reverse Trace

Optical Performance: Throughput Protected silver —provides highest NIR reflectance currently available —durability is an issue: 3 years at sea level prior to launch —needs further study 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 —needs further study

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

Telescope: R&D Phase Management Management objective: biddable Requirements Document that reflects all science requirements and trades Experienced team has been assembled Have begun dialogs with prospective vendors Have begun examining potential fab/test flows No need for high-risk “advanced” materials or processes Emphasize proven manufacturing & test techniques We plan on selection of contractor(s) with sufficient experience to bring successful delivery cost & schedule This contractor mix defines the overall acquisition plan

Telescope: CDR Preparations Plan Fully complete and document all trade studies Supplement these with industry commentary Use system-engineering “budgets” to identify optimum allocation of tolerances & resources Identify materials and fabrication alternatives taking into account schedule risk and overall cost Detail the acquisition plan and milestones Prepare acceptance test plan, including flight qual 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 —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