1. SNAP OTA Baseline TMA62 M.Lampton Jan 2002 UC Berkeley Space Sciences Lab

2. SNAP Mission Plan Preselect ~20 study fields, both NEP and SEP Discoveries & photometric light curves from repeated deep images –huge multiplex advantage with “batch” observations, 1E9 pixels Spectroscopy near maximum light from followup pointings Deep Surveys: Followup spectroscopy: NS NS ~4 day period

3. SNAP Simple Observatory consists of : 1) 3 mirror telescope w/ separable kinematic mount 2) Baffled Sun Shade w/ body mounted solar panel and instrument radiator on opposing side 3) Instrument Suite 4) Spacecraft bus supporting telemetry (multiple antennae), propulsion, instrument electronics, etc No moving parts (ex. filter wheels, shutters), rigid simple structure.

4. Payload Layout *transverse rear axis *shortest length

5. Annular Field Three Mirror Anastigmat Aperture: 2 meters Field of view: > 1 square degree –1.37 square degrees in TMA62 Diffraction limited longward of one micron –2 microns RMS, 15microns FWZ geometric Flat field Folded to obtain short overall length –3.3 meters in TMA62

6. Wide-Field Telescope: History Wide-field high-resolution telescopes are NOT new –Schmidt cameras (1930 to present) –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 (1972...) –Angel-Woolf-Epps three-mirror design (1982) –McGraw three-mirror system (1982) –Willstrop “Mersenne Schmidt” family (1984) –Dark Matter Telescope (1996+) –New Planetary Telescope (1998) –IKONOS Earth resources satellite (1999) –FAME astrometric TMA –Multispectral Thematic Imager (1999)

7. Three-mirror anastigmat (TMA) Identified as best choice for SNAP Can deliver the required FOV Can deliver the required resolution Inherently achromatic, no correctors needed Inherently flat field Inherently elastic: 9 d.o.f. to meet 4 Seidel conditions plus focus & focal length Can meet packaging requirements

8. Telescope: Downselection 1999-2001: Suitability Assessments –sought 1 sq deg with diffraction limited imaging (< 0.1 arcsec) –low obscuration is highly desirable –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; lateral axis between tertiary & detector –TMA55 (f/10): improved performance, margins positive, common axes for pri, sec, tertiary. –TMA56 (f/10) like TMA55 but stretched –TMA59 (f/15): same but with longer focal length –TMA62 (f/10.5) lateral axis between tertiary & detector

9. Baseline Telescope Baseline Optical System: Annular Field TMA62 –prolate ellipsoid concave primary mirror –hyperbolic convex secondary mirror –flat annular folding mirror –prolate ellipsoid concave tertiary mirror –flat focal plane –provides side-mounted detector location for best detector cooling –EFL = 21.66m matches 10.5 micron SiCCD pixel to 0.1 arcsec angular scale plate scale is 105 microns per arcsecond –delivers annular field 1.37 sqdeg –average geometrical blur 2.5umRMS = 6umFWHM; 16um worst case FWZ compare: SiCCD pixel = 10.5 um; HgCdTE pixel 18.5um –angular geometrical blur 0.023arcsecRMS =0.06arcsecFWHM compare: Airy disk, 1um wavelength: FWHM=0.12arcsec=13um

11. Annular Field Dimensions Outer radius: 0.745 degrees –corresponds to 283.56 mm at detector Inner Radius: 0.344 degrees –corresponds to 129.1 mm at detector Sky coverage 1.37 square degree –corresponds to 1957 cm2 detector area Field Blockages-- none Can go to larger radii but image quality degrades rapidly Can go to smaller radii but vignetting becomes severe

12. TMA62 Optics Prescription Primary Mirror (concave prolate ellipsoid) located at origin: –diameter= 2000 mm; hole= 450mm –curvature= -0.2037586, radius=4.907768m; shape=+0.0188309, asphericity= -0.981169 Secondary Mirror (convex hyperboloid) located at Z=-2.000 meters: –diameter= 450mm –curvature= -0.9103479, radius=1.0984811m; shape= -0.8471096, asphericity= -1.8471096 Folding flat mirror located on axis, Z=+0.91 meters: –oval, 700mm x 500mm; central hole 190 x 120mm Tertiary Mirror (concave prolate ellipsoid) located at Z=+0.91, X= -0.87meters: –diameter=680mm –curvature= -0.7116752, radius=1.405135m; shape=+0.40203, asphericity= -0.59797 Filter/Window located along beam toward detector –nominal thickness 5mm, fused silica Annular Detector Array located at Z=+0.91, X=+0.90 meters: –inner diameter 129mm, outer diameter 283.6mm

13. TMA62 Prescription -- BEAM FOUR format 8 surfaces TMA62.OPT f/10.83, optim 6 to 14mrad, use 6 to 13mrad index X Z pitch Curvature shape Diam diam Mirr? ------:--.-------:--.--:-----:---.-------:---.-------:------:----:----------: : 0 : 0.0 : : -0.2037586: 0.0188309: 2.01 : :mir pri : : 0 :-2.0 : : -0.9103479: -0.8471096: : :mir sec : : 0 : 0.1 : : : : : :iris : : 0 : 0.91: 45 : : : : :mir fold : :-0.87 : 0.91:-90 : -0.7116752: 0.4020288: : :mir tert : : 0.25 : 0.91: 90 : 0 : : 0.3 : :lensFilter: 1.456: 0.255 : 0.91: 90 : 0 : : 0.3 : :lensFilter: : 0.9 : 0.91: 90 : 0 : : 0.65 : :CCDarray : : : : : : : : : : : EFL=21.66meters : : : : : : : : : : : : : : :

14. TMA62 spot diagrams

15. TMA55 Vignetting?

16. Ray Trace Results Five radii: +X, +XY, +Y, -XY, -X Transmission vs off-axis angle,milliradians

17. TMA62 Vignetting and Image quality issues Nominal annulus 6 to 13mrad –no vignetting, but little or no tolerance –2 um rms average image blur over this field At 5mrad: approx 50% of rays are lost at edge of hole in 45deg flat mirror At 14mrad: vignetting losses depend critically on element sizing; geometrical blur about 40um FWZ.

18. TMA56 sensitivity coefficients -secondary mirror-

19. TMA56 sensitivity coefficients -fold mirror & detector-

20. Glare & Stray Light Sources Ecliptic Poles places Sun 70 to 110deg off axis –sunshade design “straightforward” Earth, moon can be up to 15 deg off axis –needs careful baffle study, now in work Stars, Zodiacal dust, diffuse Galactic light –concerns are optics scatter, dirt, structure Stray light specification: must be small compared to natural NIR foreground Thermal emission from optics must also be small

21. Baffle treatment: outer tube, secondary cone, inner tube

22. Stray Light Baffle Concept

23. Diffuse NIR foreground

24. Mirror emissivity

25. Optical Mirror Technologies Open-back weight relieved Zerodur or silica –offers 75% to 80% LW –potentially quicker procurement cycle Ultralight core+face+back: 90-95%LW –typically use Corning ULE –requires ion milling –requires in-chamber metrology SiC technologies –evolving; under study

26. Materials http://www.minerals.sk.ca/atm_design and other sources

27. Primary Mirror Substrate Key requirements and issues –Dimensional stability over time –Dimensional stability in thermal gradient –High specific stiffness (1g sag, acoustic response) –Stresses during launch –Design of supports Prefer < 100kg/m2 Variety of materials & technologies Initial design for primary mirror substrate: 334 kg

28. Primary Mirror Substrate Stresses from pseudo-static launch loads –6.5g axial, 0.5g transverse –3-point supports Baseline –Face sheets (12 mm) –Locally thickened web walls (10 mm) –Thicker outer ring (8 mm) Mass (330 kg) Fundamental mode 360 Hz Conclusions –80% lightweighted design is workable –3 pt support may be usable for launch –Vertical axis airbag support required for figuring Deformations of mirror top face under pseudo-static launch loads: peak deflection = 20  m Design with locally thicker web plates Standard web thickness = 5 mm (orange) Thickened plates = 10 mm (red)

29. Primary Mirror Substrate Free-free modes Sag during 1g figuring –Sag is too large (>0.1  m) on simple supports (3 pt vertical, strap horizontal) –Will likely require vertical axis figuring on airbag supports 1g sag on 3pt support vertical axis P-P Z deflection = 2.3  m 1g sag in 180º strap support horizontal axis P-P Z deflection = 0.5  m Fundamental mode: 360 Hz Second mode: 566 Hz 1g front face ripple on perfect back- side support P-P Z deflection = 0.018  m

30. Secondary Metering Structure Key requirements: –Minimize obscuration (<3.5%) & interference spikes –Dimensional stability –35 Hz minimum fundamental frequency Baseline design: hexapod truss with fixed end –Simple design with low obscuration (3.5%) –6-spiked diffraction pattern –Ø 23 mm by 1 mm wall tubular composite (250 GPa material) struts with invar end-fittings.

31. Secondary Metering Structure

32. Tertiary Metering Structure Key requirements: –Dimensional stability –35 Hz minimum fundamental frequency Easier design problem than secondary metering structure –Overall dimensions much smaller than secondary metering truss –No obscuration concerns –Use strut design from secondary metering structure (cost effective) Lowest global mode of tertiary metering truss: 110Hz

33. Telescope: Focussing 13 mechanical adjustments is minimum set –focussing –collimation –centering –alignment –on orbit, may only need secondary to be articulated Least squares optimization for focussing and collimation Alternatives: Zernike defocus analysis

34. GIGACAM 1 billion pixel detector 132 large format silicon CCDs 25 2Kx2K HgCdTe NIR detectors Larger than SDSS array Smaller than BABAR silicon vertex detector Outside diameter 480mm Each chip has dedicated bandpass filter Located within 150K cryostat Accommodates guiding and spectroscopy feeds