1. SNAP Telescope M.Lampton f, C.Akerlof b, G.Aldering a, R.Amanullah c, P.Astier d, E.Barrelet d, C.Bebek a, L.Bergstrom c, J.Bercovitz a, G.Bernstein e, M.Bester f, A.Bonissent g, C.Bower h, W.Carithers a, E.Commins f, C.Day a, S.Deustua i, R.DiGennaro a, A.Ealet g, R.Ellis j, M.Eriksson c, A.Fruchter k, J-F.Genat d, G.Goldhaber f, A.Goobar c, D.Groom a, S.Harris f, P.Harvey f, H.Heetderks f, S.Holland a, D.Huterer l, A.Karcher a, A.Kim a, W.Kolbe a, B.Krieger a, R.Lafever a, J.Lamoureux f, M.Levi a, D.Levin b, E.Linder a, S.Loken a, R.Malina m, R.Massey n, T.McKay b, S.McKee b, R.Miquel a, E.Mortsell c, N.Mostek h, S.Mufson h, J.Musser h,P.Nugent a, H.Oluseyi a, R.Pain d, N.Palaio a, D.Pankow f, S.Perlmutter a, R.Pratt f, E.Prieto m, A.Refregier n, J.Rhodes o, K.Robinson a, N.Roe a, M.Sholl f, M.Schubnell b, G.Smadja p, G.Smoot f, A.Spadafora a, G.Tarle b, A.Tomasch b, H.von der Lippe a, R.Vincent d, J-P.Walder a and G.Wang a a Lawrence Berkeley National Laboratory, Berkeley CA, USA, b University of Michigan, Ann Arbor MI, USA, c University of Stockholm, Stockholm, Sweden, d CNRS/IN2P3/LPNHE, Paris, France, e University of Pennsylvania, Philadelphia PA, USA, f University of California, Berkeley CA, USA, g CNRS/IN2P3/CPPM, Marseille, France, h Indiana University, Bloomington IN, USA, i American Astronomical Society, Washington DC, USA, j California Institute of Technology, Pasadena CA, USA, k Space Telescope Science Institute, Baltimore MD, USA, l Case Western Reserve University, Cleveland OH, USA, m CNRS/INSU/LAM, Marseille, France, n Cambridge University, Cambridge, UK, o NASA Goddard Space Flight Center, Greenbelt MD, USA, p CNRS/IN2P3/INPL, Lyon, France

2. 2 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 —need to match this to pixel size of VIS and NIR detectors Field of View —determined by required supernova discovery rate —volume of space is proportional to field of view —one degree field of view will deliver the requisite discovery rate Wavelength Coverage —0.35 to 1.7 microns requires all-reflector optical train

3. 3 Wide Field Telescopes 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 annular-field TMAs (1977) —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

4. 4 Baseline Configuration Prolate ellipsoid concave primary mirror Hyperbolic convex secondary mirror Flat folding mirror with central hole Prolate ellipsoid concave tertiary mirror Delivers < 0.06 arcsecond FWHM geometrical blur over annular field 1.37 sqdeg Flat focal surface Adapts to focal lengths 15 meters through 30 meters; baseline=21.66m Provides side-mounted detector location for best detector cooling * Telephoto advantage = 7

5. 5 Baseline Characteristics Aperture: 2 meters Annular Field of View: ~1 sq deg Wavelength Range: 0.35 to 1.7 microns Strehl: >90% at 1.0 microns WFE: <50 nm RMS Focal surface: flat EFL: 21.66 meters, f/11 Stray light: << Zodiacal foreground

6. 6 Optical Prescription

7. 7 Optical Configuration 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 Few moving parts in payload two-blade shutter for Gigacam focussers/adjusters at secondary & tertiary

8. 8 Metering Configuration

9. 9 Payload Layout 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 Solid-state recorders ACS CD & H Comm Power Data CCD detectors NIR detectors Spectrograph Focal Plane guiders Cryo/Particle shield

10. 10 Payload Layout

11. 11 Focal plane concept Coalesce all sensors at one focal plane. —Imager sensors on the front. 36 HgCdTe 2kx2k 18  m 36 CCD 3.5kx3.5k 10.5  m —Filters 1 of 3 per HgCdTe 4 of 6 per CCD —Spectrograph on the back with access ports through the focal plane. Common 140K operating temperature. Dedicated CCDs for guiding from the focal plane. Exposure times of 300 s with four/eight exposures in CCDs/HgCdTe. 20 s readout slow enough for CCD noise and 4 post exposure and 4 pre exposure reads of HgCdTe. r in =6.0 mrad; r out =13.0 mrad r in =129.120 mm; r out =283.564 mm

12. 12 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 —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

13. 13 Example WFE Budget Primary figure33 nm Secondary position5 nm zeroable by telecommand Secondary orientation5 nm zeroable by telecommand Secondary figure10 nm Folding flat position5 nm Folding flat orientation5 nm Folding flat figure10 nm Tertiary position5 nm zeroable by telecommand Tertiary orientation5 nm zeroable by telecommand Tertiary figure 10 nm Detector position10 nm Detector orientation10 nm Detector flatness10 nm Manager's reserve15 nm --------------------------------------------------------------------- TOTAL root sum square43 nm Corresponding Strehl = 83% at 0.633um, or 93% at 1.0um

14. 14 Ray Trace Examples TMA62/TMA63 configuration Airy-disk zero at one micron wavelength 26 microns diam=0.244arcsec

15. Pupil Obscuration Trades

16. 16 Circular 2m aperture central 0.7m obscuration Three legs, 50mm x 1meter Diffraction

17. 17 Stray Light Trades 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 Long outer baffle is clearly preferred —limit is launch fairing and S/C envelope We will be starting detailed stray light assessment in 2003 Our intension is to track hardware & ops changes as they occur, allowing a “system engineering management” of stray light.

18. 18 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 ULE glass: extensive NRO flight history; lightweight —Schott Zerodur glass/ceramic composite: lower cost, widely used in ground based astronomical telescopes; huge industrial base —fused silica also a possibility, but CTE issues 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

19. 19 Mirror Materials Trade Corning ULE ultra-low expansion glass —extremely low CTE: 20-50 parts per billion per dec C —face sheets bonded to honeycomb core —achieves 85-90% lightweight; anticipate 200kg primary —extensive manufacturing & test history —attractive for its high natural resonance frequencies, low sag —eases mass margin for entire mission Fused Silica —large supplier base —extensive manufacturing and test history —CTE issue is a potential problem Schott Zerodur glass/ceramic material —extremely low CTE: 20-50 parts per billion per deg C —solid blank, weight relieved by milling backside —achieves 70-85% lightweight; anticipate 250kg primary —extensive manufacturing & test history

20. 20 Trade Studies Summary Trade Studies worked during Pre-R&D Phase —Optical configuration: >>TMA —Warm optics vs cold optics: >>warm —integrated sensor array vs separated: >>integrated Trade Studies continuing through R&D Phase —Exact aperture: cost & schedule vs aperture —Wavefront error: cost vs performance focal length: is 21.66m the best choice? pupil obsuration, diffraction, stray light... —Primary mirror thickness, stiffness, mass trade resonance freqs, sag under 1G testing,... —Protoflight vs Prototype + Flight metering structures —Vendor-dependent issues mirror material: ULE? FS? Zerodur? test: gravity unloading scheme test: full aperture vs partial aperture

21. 21 Overall Schedule draft May 28 2002

22. 22Conclusions Baseline Requirements —Far less demanding than HST: we are NIR not NUV Baseline Design —annular field three-mirror anastigmat Working towards a biddable requirements document —vendor participation in 2003-2004 Schedule risks: OTA is a long lead item! No new technology Budgets and Plans —tolerances —stray light —fab plan —test plan