BigBoss Optical Corrector Intro to UCL

1. BigBoss Optical CorrectorIntro to UCL M. Sholl 16 July 2010

2. Outline • Corrector Design • Tolerance Study • Contact with SESO • Contact with Corning • Contact with Schott • Liens on corrector design • F/8 M2 • Fiber verification camera • Questions on slides 16 & 17 of CD1 presentation

3. Telescope Requirements Table X1: Telescope requirements

4. Layout of Corrector

5. University College London (UCL) Built similar (but smaller) corrector for DES Peter Doel, David Brooks, Optical Sciences Laboratory, UCL LLF6 (used in ADC design) no longer available LLF6 used on existing Mayall ADC Use LLF1 instead Quick check of Zemax prescription reoptimization suggests it will work Schott can do 980mm LLF1 sizes Corning can do fused silica lenses from standard boules Standard size: 1500mm, 180 & 250mm thick Need a homogeneity specification. Coatings may be an issue on such large lenses

6. UCL Suggestions Bottom line: “Cassegrain option would be significantly more expensive (due to the 2m convex aspheric mirror)” Suggest you look into smaller designs, especially first lens

7. Stray Light Instrument must be “Well Baffled” Stars have no direct view of focal plane Prime Focus No direct view of focal plane Only view is reflection off M1 (signal) Cassegrain Focus Requires optimally sized M1 and M2 baffle Sweet spot: M1 and M2 baffles’ area fraction equal Source: Hales, 1992 Prime focus throughput: 72% Cassegrain throughput: 63% Prime focus design superior in throughput!

8. DES Corrector Eric Prieto, in conversation with SESO suggested we look further at DES corrector (f/2.8 into fiber) 2.2º FOV (BigBoss is 3º-3.5º) Band limited by filters ADC: Not present on DES corrector (needed on BigBoss) Add effects of Kitt Peak atmosphere to quantify degradation to spot sizes

9. Spot Sizes Without Atmosphere

10. DES Corrector Spot Sizes With Kitt Peak Atmosphere

11. DES Corrector (cont.) Needs an ADC Focal plane size: Ø0.45m (~Ø1m needed on BigBoss) Should try to investigate design form of DES corrector, but aforementioned items limit direct applicability to BigBoss

12. ADC on BigBoss • Two opposite-rotating Risley prisms • Entrance and exit surfaces of prisms are curved, for aberration control • Schott Materials • LLF1 (available) • N-PSK3 (will need to melt) • ~1° tolerances on prism rotation

13. Spots at 60° from Zenith Risley prism 0° rotation Risley prism 85° rotation

14. Corrector Tolerance Budget • Entire corrector and focal plane moves as a unit (via hexapod). This is system compensator. • Tolerances may be accommodated by compensator • Manufacturing errors • Vertex offset • Figure (measured as surface sag) • Wedge (measured as distance at lens endge) • Lens thickness • Homogeneity (as-built) • Alignment errors • Lateral offset • Despace • Tilt • Thermal drift cannot be accommodated in real-time, so it is budgeted as a fixed and “uncomponsatible” error.

15. Tolerance Starting point (preliminary) • Goal: atmospheric seeing and as-built geometric blur shall have a FWHH < 100 microns • 120 micron fibers • 5 micron positioner tolerances • 5 micron fiber view camera accuracy • 3 micron pointing jitter • Peak geometric blur on “perfect” design: 18.3 microns RMS • Multiply by 2.35 to convert to FWHH (Gaussian approximation) • 28 micron blur on as-built design, adds to atmosphere (32 microns RMS) to make a 100 micron FWHH blur • 282-18.32=450 “quadrature” points, divided 50 ways • For equal pain, each error source can enlarge the spot from 18.3 to 18.4 microns RMS, after hexapod compensation (where appropriate)

16. Manufacturing Errors (compensated) (preliminary)

17. Alignment Errors (Compensated) (preliminary)

18. Stability (no compensation) (preliminary) Invar 36 or GRCE Structure

19. Comment on Tolerances • Preceding three slides came from a manual “equal pain” analysis. • Have already received pushback on uncompensated errors from Eric Anderssen. • More detailed tolerancing underway… • Now is the time to push back on these numbers, if they are unreasonable

20. Hexapod • Motion requirements • Step size: 5 microns (goal) 10 microns (requirement) • Despace: ±2mm • Lateral : ±1mm • Tilt: : ±1° • PI (Germany) contacted • ADS-Int. (Italy) to be contacted

21. Corrector Design Details

22. Contact with SESO • Spoke with Denis Fappani at SPIE Astronomical Telescopes (June, 2010) • Lens manufacturing generally feasible • Recommend you thick (50% at center) three ADC elements to reduce gravity sag • ~2 years to polish, coating extra • Need to provide figure errors

23. Contact with Corning • Asked for quote on blanks Please advise as to the availability, cost and lead time for these boules. Homogeneity improves the corrector performance, of course, so please let us know if higher homogeneity levels are possible, or if the requested levels are difficult to achieve. The corrector is required to work from 0.35 to 1.1 microns, so material suitability (low absorption) is required in that band.

24. Contact with Schott • Please advise as to the availability, cost and lead time for these boules. Homogeneity improves the corrector performance, of course, so please let us know if higher H-levels are available in N-LLF1 and N-PSK3. • Risk (verbal): Melt required for N-PSK3

25. Additional Consideration • Should accommodate existing f/8 M2 • Existing M2 support appears to be 460mm thick • M1-f/8 M2 distance: 7491mm • M1-C1 BigBoss distance: 8800mm • Removable fiber view camera assembly attaches to same (or similar) interface as f/8 M2

26. Liens on Corrector Design • Thicken three ADC surfaces • N-PSK3 lens diameter is larger than ADC elements, can it be reduced? • Adopt sensible curvature (fewer decimal places) for focal plane (constrain during next redesign) • Improve telecentricity (constrain during next redesign) • N-PSK3 may be difficult to obtain (per discussion with Schott), and homogeneity may be “best effort.” Different material selection?

27. Fiber View Camera • Mounted in front of corrector • Back-illuminate fibers • View fibertips with camera to verify location

28. Lens Mounting (Source: DES, P. Doel CD1 presentation, Slide 16) • Baseline athermal elastomeric (RTV rubber) bonding technique • Looking at two cell options • Invar lens cell + flexures + thin RTV layer (see figure) • Steel cell + thick RTV layer LBT lens mounting (from Diolaiti et al. SPIE 4841)

29. Questions on CD1 Slide 16 • What is assembly sequence? • Nonuniform gap an issue? • What is vertical screw (seems to be touching glass) • Are glass/invar gradients an issue? • Large invar difficult to procure? • What decides thick/thin RTV layers • Is gravity sag of lens in RTV an issue?

30. Lens to Cell Alignment (Source: DES, P. Doel CD1 presentation, Slide 16) • Lens to cell • Lens to cell alignment performed using rotary table and digital dial gauges. RTV inserted into gap D.G.I. Cell Adjustment Screws Cell Translation Stage Lens Rotary Table

31. Proposal Inputs • Any advice on this presentation, and design • Help/advice (if possible) with tolerance analysis? • Also homogeneity • Drawings or photos of DES Optical mounts? • Cost • Schedule • Risk • Test sequence, similar to that of DES? (Brooks presentations)