Optically sensitive Medipix2 detector for adaptive optics in very large telescopes

1. Optically sensitive Medipix2 detector for adaptive optics in very large telescopes John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund Space Sciences Laboratory, University of California, Berkeley Bettina Mikulec and Allan Clark University of Geneva

2. Adaptive optics tutorial* Turbulence in earth’s atmosphere makes stars twinkle More importantly, turbulence spreads out light; makes it a blob rather than a point *Adapted from AO lectures of Claire Max, Astro 289C, UC Santa Cruz Even the largest ground-based astronomical telescopes have no better resolution than a 20 cm telescope!

3.  blur Optical consequences of turbulence • Temperature fluctuations in small patches of air cause changes in index of refraction (like many little lenses) • Light rays are refracted many times (by small amounts) • When they reach telescope they are no longer parallel • Hence rays can’t be focused to a point:  Point focus Parallel light rays Light rays affected by turbulence

4. How a deformable mirror works (idealization) BEFORE AFTER Incoming Wave with Aberration Deformable Mirror Corrected Wavefront

5. Adaptive optics increases peak intensity of a point source Lick Observatory No AO With AO Intensity With AO No AO

6. Optical Medipix tube goes here Schematic of adaptive optics system Feedback loop: next cycle corrects the (small) errors of the last cycle

7. Lick adaptive optics system at 3m Shane Telescope DM Wavefront sensor Off-axis parabola mirror IRCAL infra-red camera

8. How to measure turbulent distortions (one method among many) “Shack-Hartman” WFS

9. ESO VLT Gemini South The new generation: adaptive optics on 8-10 m telescopes Summit of Mauna Kea volcano in Hawaii: Subaru 2 Kecks Gemini North And at other places: MMT, VLT, LBT, Gemini South

10. Neptune in infra-red light (1.65 microns) With Keck adaptive optics Without adaptive optics 2.3 arc sec May 24, 1999 June 27, 1999

11. VLT NAOS AO first light Cluster NGC 3603: IR AO on 8m ground-based telescope achieves same resolution as HST at 1/3 the wavelength Hubble Space Telescope WFPC2,  = 800 nm NAOS AO on VLT  = 2.3 microns

12. Faint companions around bright stars Two images from Palomar of a brown dwarf companion to GL 105 End Tutorial Credit: David Golimowski

13. Vision Science End Tutorial

14. Next generation of large telescopes (proposed) 30 m diameter: • California Extremely Large Telescope (CELT) - • Thirty Meter Telescope (TMT) 50 m diameter: • EURO50 on La Palma 100 m diameter: • European Southern Observatory’s “OverWhelmingly Large Telescope” (OWL) All propose AO systems with > 5000 actuators

15. WFS detector requirements • High optical QE for dimmer guide stars • Lots of pixels - eventually 512 x 512 • Very low readout noise • kHz frame rates The last three are not simultaneously achievable with the current generation of CCDs

16. MCP Detectors at SSL Berkeley COS FUV for Hubble (200 x 10 mm windowless) 25 mm Optical Tube GALEX 68 mm NUV Tube (in orbit)

17. Photocathode converts photon to electron MCP(s) amplify electron by 104 to 108 Rear field accelerates electrons to anode Patterned anode measures charge centroid Imaging, Photon Counting Detectors

18. GaAs Photocathodes (GenIII) Photocathode type determines wavelength response • Soft x-ray to near IR

19. Wavefront Sensor Event Rates(example for big telescope) • 5000 centroids • Kilohertz feedback rates (atmospheric timescale) • 1000 detected events per spot for sub-pixel centroiding • 5000 x 1000 x 1000 = 5 Gigahertz counting rate! • Requires integrating detector

20. Our concept • An optical imaging tube using: • GaAs photocathode • Microchannel plate to amplify a single photoelectron by 104 • Bare Medipix2 to count these events per pixel

21. Vacuum Tube Design

22. Vacuum Tube Design

23. Vacuum Tube Design

24. Vacuum Tube Design

25. First test detector • Demountable detector • Simple lab vacuum, no photocathode • Windowless – UV sensitive

26. Initial Results It Works! Lower gain, higher rear field First light!

27. MCP event spot area

28. MCP charge cloud size

29. Spatial Resolution Group 3-2 visible 9 lp/mm = 55µm (Nyquist limit) 100 µs 1 s

30. Interesting tangent - sub pixel resolution Use single spot events and calculate centroids Accumulate event x,y list 2-d histogram on finer pitch 9 lp/mm

31. Interesting tangent - sub pixel resolution Calculate centroids of each event Accumulate event x,y list 2-d histogram on finer pitch 16 lp/mm

32. Flat Field MCP deadspots Hexagonal multifiber boundaries 1200 cts/bin - 500Mcps

33. Flat Field (cont) Histogram of Ratio consistent with counting statistics (2% rms) Ratio Flat1/Flat2

34. Future Work (3 yr. NOAO grant) • Optimize MCP-Medipix2 interface design • Design and build tube with Medipix2 and GaAs • Develop parallel readout with European collaborators • Develop FPGA to reduce output bandwidth • 5 million centroids/s vs. 262 million pixels/s. • Test at AO laboratory at CFAO, U.C. Santa Cruz • Test at telescope

35. Univ. of Barcelona University of Cagliari CEA CERN University of Freiburg University of Glasgow Czech Academy of Sciences Mid-Sweden University University of Napoli NIKHEF University of Pisa University of Auvergne Medical Research Council Czech Technical University ESRF University of Erlangen-Nurnberg Acknowledgements This work was funded by an AODP grant managed by NOAO and funded by NSF Thanks to the Medipix Collaboration:

36. Soft X-Ray Photocathodes

37. EUV and FUV

38. GaN UV Photocathodes, 1000- 4000Å

39. h Isoplanatic Angle (0) & Sky Coverage Bright stars + 0 =1% sky coverage Telescope Primary mirror

40. Can achieve>70% skycoverage withlaserguidestaradaptive optics!

41. 589.2 nm L d Laser Guide Star Parallax • “Star” more of a streak • Shape changes over pupil • Can use pulsed laser to limit spatial extent • Requires gated detector

42. Deformable mirrors come in many sizes • Range from 13 to > 900 actuators (degrees of freedom) ~ 300mm ~ 50 mm Xinetics

43. 30 m telescope capability