SNAP Telescope Status

1. SNAP Telescope Status M. Lampton & M. Sholl SNAP Collaboration Meeting October 2007

2. Outline • Three SPIE papers this August • Lampton & Sholl Proc. SPIE 6687 #23 • Sholl et al Proc. SPIE 6675 #12 • Besuner et al Proc. SPIE 6687 #28 • Telescope Requirements Review • Ongoing work

3. Lampton & Sholl“Comparison of on-axis three-mirror anastigmat telescopes” • Field of View • survey speed requirement • the bigger, the faster! • drives telescopes towards fully centered fields. • Light Gathering Power • directly driven by survey speed requirement • depth and signal to noise ratio • the bigger, the faster! • drives survey telescopes towards fully centered pupils. • Size and Stability of the Point Spread Function • size and stability directly influence signal to noise ratio • usual requirement is to be “diffraction limited” • further requirements from pixel size & image sampling • diffraction = lambda * f/number ≈ pixel size: • 10µm pixel / 1µm wavel f/10 • Broad Wavelength Coverage • drives towards all-reflector designs • Stray Light, Stray Heat • drives towards well baffled designs • NIR emphasizes need for a well defined real exit pupil • Limited package diameter and length • favors on-axis centered pupils • drives telephoto ratio Survey Rate to a given flux F

4. Lampton & Sholl (cont’d) Telescope Landscape f/1 - PAUL BAKER Off Axis TMAs Korsch SCHMIDT SPHERE + 4 MIR CORR SCHMIDT CASS R-C + FLATTENER Speed CASSEGRAIN RITCHEY CHRETIEN f/10 - PARABOLOID f/100 - | 0.1 | 0.3 | 1 | 3 | 10 Full field angle, degrees TMA group can have Seidel aberrations = 0 and a flat field without refractive elements

5. Lampton & Sholl (cont’d) With nine adjustables, meeting six optical requirements (Seidels=0, focal length, field curvature) leaves a 3-space of design freedom.

6. Lampton & Sholl (cont’d) • Korsch 1972; 1977; 1980 explored a portion of TMA space... • Retained the requirement for a centered pupil • Retained the requirement for a centered field • Retained the requirement for a telephoto advantage >> 1 that is essential for space astronomy • Retained the requirement for a real exit pupil, by arranging that the tertiary mirror light crosses the TM axis en route to its focus • Immediately faced the blockage problem • Explored various ways to extract all the light from the principal axis • Came up with four alternative optical layouts.

7. Lampton & Sholl (cont’d) • Using the flexibility of TMAs, arrange for the Cassegrain focus and the exit pupil to be colocated along their common axis. • Using the flexibility of TMAs, arrange the magnifications so the Cassegrain field size is smaller or larger than the exit pupil. • if CF is smaller: put an extraction mirror there to separate the Cassegrain light from the final focus light. This will punch a hole in the center of the exit pupil. But there’s already a hole there – the shadow of the secondary mirror! Nothing is lost. => Full Field group. • if CF is larger: put an extraction mirror there anyway. This will punch a hole in the center of the field of view. Some portion of the survey field is lost, but full throughput is obtained throughout the remaining annular field. => Annular Field group. Korsch’s two key ideas:

8. Lampton & Sholl (cont’d) Korsch Full-field Configuration II Korsch Full-field Configuration I SNAP Korsch Annular-field Configuration II Korsch Annular-field Configuration I

9. Lampton & Sholl (cont’d)

10. Lampton & Sholl (cont’d) • Full Field TMAs • no hole in the middle of the field • very wide range of EFLs available • Annular Field TMAs • slower Cass section => somewhat more tolerant PM-SM despace • inherent baffling, particularly with II’s complete cold stop • For low geometric distortion.... • need large tertiary magnification and/or concave focal plane • final f/number = Cass f/number * tertiary magnification • achievable with the FFTMAs whose tertiary magnification is large • can also be had with the AFTMAs provided system is slow enough • concave focal plane can improve distortion performance • example: Grange et al 2006, Refregier et al 2006 “DUNE”

11. Lampton & Sholl (cont’d) • FFTMA configuration I • ESA “Wide Field Imager” telescope project, European Space Agency CDF Study Report CDF-46(A), 2006. • AFTMA configuration I • CNES “DUNE” project, R. Grange et al., Proc. SPIE 6265, #49, 2006; . A. Refregier et al., Proc SPIE 6265 #1Y, 2006. • AFTMA configuration II • US DoE LBNL “SNAP” project, M.Lampton et al Proc. SPIE 4849 215-226, 2002; M.Lampton et al Proc. SPIE 5166, 113-123, 2003; M. Sholl et al., Proc. SPIE 5487, 73-80, 2004; M. Sholl et al., Proc. SPIE 5899, 27-38, 2005; Sholl et al this conference; Besuner et al this conference. • NAOJ project, K. Nariai and M. Iye, astro-ph/0504514; Pub.Astron.Soc.Jap. 57, 391-397, 2005. • CNES Pleiades project,D. Fappani and H. Ducollet, Proc. SPIE 6687 (this conference), 2007.

12. Sholl, Fleming, Jelinsky & Lampton“Stray light design and analysis of the SNAP Observatory” • A product of the ongoing SNAP telescope development at Ball Aerospace • Makes full use of BATC tools and expertise in this important subject • SNAP Telescope noise floor is in-field Zodiacal radiation • Goal of stray light design: stray-light << Zodi (~1 M=23/arcsec2 in dark survey regions) • In L2 halo orbit, Earth and Moon occasionally illuminate interior of stray light baffle • Requirement: observe supernova light curve to 26th magnitude anywhere within observation region • Extensive analysis of baffling performed with ASAP • PST data indicates that in-field scatter from mirror roughness and dust is main in-field stray light source • In infrared bands, main noise source is thermal emissions from the structure

13. Sholl et al: PST Bounds

14. Sholl et al: Conclusions • SNAP Telescope noise floor is in-field Zodiacal radiation • Goal of stray light design: stray-light << Zodi (~23/arcsec2 in dark survey regions) • In L2 halo orbit, Earth and Moon occasionally illuminate interior of stray light baffle • Extensive analysis of baffling performed with ASAP • PST data indicates that in-field scatter from mirror roughness and dust is main in-field stray light source • System is more sensitive to fold mirror scattering, due to its proximity to the Cassegrain focus, and consequently high local irradiance • In infrared bands, main noise source is thermal emissions from the structure (<10% Zodi) • Design/analysis efforts show that the SNAP observatory meets known requirements across the wavelength band ...details in Steve Kilston’s talk

15. Besuner, Sholl, Lieber, and Kaplan“Integrated modeling of PSF stability of the SNAP telescope” • Point Spread Function stability is important to WL success • Starting 2007: PSF to System Engineering level (Berkeley + BATC) • Apply BATC’s integrated modeling tools to the SNAP PSF questions

16. Besuner et al (cont’d): EOSyM • Ball Aerospace EOSyM integrated modeling toolkit • Realized inside MATLAB/Simulink environment. • 1996 (VLT) to present (JWST), cradle-to-grave support. • Embeds structural dynamics, disturbances (thermal, etc), detectors, signal processing, optics, controls in single environment. • Built-in geometrical and physical optics tools.

17. Besuner et al (cont’d): structural dryout • Graph shows HST PM/SM despace over time after launch. • HST structure is carbon fiber/epoxy. • SNAP uses lower-desorption carbon fiber/cyanate ester. • Conservatively assume same hygrostrain rate for SNAP as HST. • At BOL, 2.5ppb composite shrinkage per hourΔχ≤ 0.017%/hour. • After 2 years, 0.37ppb composite shrinkage per hourΔχ≤ 0.0025%/hour. • GOAL: < 0.1% per hour HST Slope ~106um/yr @BOL SNAP Slope ~16um/yr @ 2yr

18. Besuner et al: Conclusions • Ball Aerospace’s EOSyM integrated modeling tool has been implemented for the SNAP observatory. • Effects of thermal attitude-maneuver environmental transients on the PSF stability of the telescope have been simulated. • Effects of structural dryout & shrinkage have been modelled. • The baseline SNAP telescope meets current weak lensing PSF stability requirements of Δχ ≤ 0.1% over one hour, even from beginning of science mission at Launch+50 days, provided that we exclude the time intervals during downlink attitude maneuvers. • A mission design that merges WL + SN throughout its entire duration will deliver the required WL image stability. ...more details in Steve Kilston’s talk

19. Telescope Requirements Review • Purpose: to discover if the current telescope specifications meet the current group of science driven requirements • Space to explore: telescope aperture, focal length, field of view • Tools to use: simulators • Tasks to model: imaging and spectroscopy • Review Board • Greg Aldering, chair • Jeff Newman • Dave Schlegel • Convened 4 Sept 2007 • WL group presentation (G. Bernstein, organizer) • Aperture is not critical • Field of view is VERY important • PhotoZ and SpectroZ requirements were not considered • SN group presentation (L. Gladney, organizer) • Aperture is only a very mild driver • Angular resolution is only a very mild driver • Spectroscopy was not considered • Conclusion: Need to critically review & update science reqts and then re-review the telescope requirements

20. Current & Future Efforts • TMA73 is about to be released solving one minor problem in TMA72 • J. Fleming (BATC) discovered vignetting of innermost pixels on FP • CNES Thales study group discovered vignetting of innermost pixels on FP • CNES Astrium study group discovered vignetting of innermost pixels on FP • We confirm those findings • TMA73 fixes that: 1% change in prescription • Also provides additional space at exit pupil • No cost or schedule impact to ongoing telescope planning • Continuing to support the CNES telescope studies • next contractor reviews 15-16 October • final reports from contractors due late 2007 • Continuing to work with BATC • mirror design trades, mounting, and test plans • lighter, stiffer metering structure • develop a finer grained telescope integration and test plan • look forward to having a thoroughly reviewable/costable telescope