A noiseless 512 x 512 detector for AO with kHz frame rates

1. A noiseless 512 x 512 detector for AO with kHz frame rates 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. Future WFS Requirements* • High (~80%) optical QE • Lots of pixels - eventually 512x512 • Very low readout noise (< 3 e-) • kHz frame rates The last three are not simultaneously achievable with the current generation of CCDs *Angel et al “A Road Map for the Development of Astronomical AO”

3. 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

4. Why would you want one? • No readout noise penalty • Use as many pixels as you wish • Continuous temporal sampling to ~ nsecs • Choose integration period(s) after the fact or on the fly • Other advantages • Large area, curved focal planes • Cosmic ray = 1 count • LN2 not required • Low dark current (0.16 attoamps cm-2)

5. Spatial Resolution Cross Strip readout of Glass MCPs 12 µm pore glass MCPs 7 µm pore glass MCPs

6. 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)

7. GaAs Photocathodes (GenIII) • Developed for night vision tubes • Slight cooling required (104 cps at room temp) • Only fabricated in USA and Japan

8. GaN UV Photocathodes, 1000- 4000Å

9. Advantages of multi-pixel sampling of Shack Hartman spots 5 x 5 2 x 2 • Linear response off-null • Insensitive to input width • More sensitive to readout noise

10. Wavefront Sensor Event Rates • 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

11. Our AO detector concept An optical imaging tube using: • GaAs photocathode • Microchannel plate to amplify a single photoelectron by 104 • ASIC to count these events per pixel

12. Medipix2 ASIC Readout • Pixellated readout for x and gamma ray semiconductor sensors (Si, GaAs, CdTe etc) • Developed at CERN for Medipix collaboration • 55 µm pixel @ 256x256 (buttable to 512 x 512). • Pixel level amp, discriminator, gate & counter. • Counts integrated at pixel No charge transfer! 14mm 16mm Applications: Mammography, dental radiography, dynamic autoradiography, gamma imaging, neutron imaging, angiography, xray diffraction, dynamic defectoscopy, etc.

13. Single Medipix2 pixel Each 55µm Pixel has ~ 500 transistors using 0.25µm CMOS technology

14. Readout Architecture Pixel values are digital (13 bit) Bits are shifted into fast shift register Choice of serial or 32 bit parallel output Maximum designed bandwidth is 100MHz Corresponds to 266µs frame readout 3328 bit Pixel Column 0 3328 bit Pixel Column 255 3328 bit Pixel Column 1 256 bit fast shift register 32 bit CMOS output LVDS out

15. “Built-in” Electronic Shutter • Enables/Disables counter • Timing accuracy to 10 ns • Uniform across Medipix • Multiple cycles per frame • No lifetime issues • External input - can be phased to laser What is the best strategy to remove/measure parallax?

16. First test detector • Demountable detector • Simple lab vacuum, no photocathode • UV sensitive

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

18. Photon counting movie

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

20. Optimizing charge cloud size • Medipix2 “non-photometric” • # pixels per photon dependent on: • MCP-anode gap • Rear field voltage • MCP gain and threshold of Medipix pixel amp

21. MCP event spot area

22. MCP charge cloud size

23. Modeling Optimum sampling Input spot size, charge cloud size Generate N photons per frame Calculate spot centroid Repeat M times and plot error distribution

24. Spot size vs gain Pinhole grid mask (0.5 x 0.5 mm) Gain: 200,000 Rear Field: 1600V Threshold: 3 ke- Gap: 500µm

25. Spot size vs gain Pinhole grid mask (0.5 x 0.5 mm) Gain: 20,000 Rear Field: 1600V Threshold: 3 ke- Gap: 500µm

26. Example of sub pixel resolution Calculate centroids of each event Accumulate event x,y list 2-d histogram on finer pitch 9 lp/mm

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

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

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

30. 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

31. Vacuum Tube Design

32. Vacuum Tube Design

33. Vacuum Tube Design

34. Vacuum Tube Design

35. Issues/Concerns • QE ! • Throughput • global • local • Detector Lifetime • Downstream interface • Cost

36. 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:

37. Soft X-Ray Photocathodes

38. EUV and FUV