Noiseless, high frame rate (> kHz), photon counting arrays for use in the optical to extreme UV

1. Noiseless, high frame rate (> kHz), photon counting arrays for use in the optical to extreme UV 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 detector requirements • High optical QE for fainter guide stars • Lots of pixels - eventually 512 x 512 • More accuators • More complex LGS images (parallax, gated, etc) • Off null / open loop operation • Very low (or zero!) readout noise • kHz frame rates

3. Q V ± sv ADC Events Count (x,y,t) Events ± sEvents Threshold Photon Counting Charge integrating

4. 1000 photons 100 photons 10 photons 8 x 8Noiseless35% QE 8 x 82.5 e- rms90% QE 6 x 62.5 e- rms90% QE 4 x 42.5 e- rms90% QE - - - Centroid in presence of noise:

5. Centroid error vs. input fluence

6. 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

7. Bandpass by photocathode selection

8. 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)

9. 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

10. Our AO detector concept An optical imaging tube using: • GaAs photocathode • MCPs to amplify to ~104 • Medipix2 ASIC readout

11. Medipix2 ASIC Readout • Each pixel has amp, discriminator, gate & counter. • 256 x 256 with 55 µm pixels (buttable to 512 x 512). • Counts integrated at pixel. No charge transfer! • Developed at CERN for Medipix collaboration (xray) ~ 500 transistors/pixel

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

13. UV photon counting movie

14. Lamp Pinhole Sub-pixel spatial linearity Detector

15. Imaged pinhole array Pinhole grid mask (0.5 x 0.5 mm) Gain: 20,000 Rear Field: 1600V Threshold: 3 ke- Gap: 500µm

16. Avg. movement of 700 spots 1 pixel

17. Position error (550 events/spot) rms = 2.0 µm

18. Vacuum Tube Design

19. Vacuum Tube Design

20. Vacuum Tube Design

21. Vacuum Tube Design

22. Medipix on a Header

23. Summary • Noiseless detectors outperform CCDs at low fluence per frame • Photocathode choice to fit application • Medipix ASIC readout allows for a huge dynamic range, fast frame rate. MCP/MedipixStatus • First tube in Fall 2005 • GaAs tube in 1st half of 2006

24. Future Possibilities • Medipix 3 now being discussed • 130 nm CMOS technology • Faster front end for less deadtime per pixel • Faster readout rate (10 kHz frame rate) • Radiation hard • Si APDs rather than MCPs as photon converter/amplifier • Higher optical QE • Near IR response • Cooling will be required to reduce dark count rate

25. 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:

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

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

28. 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 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

29. 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

30. “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

31. EUV and FUV (50 - 200 nm)

32. GaN UV Photocathodes, 100- 400 nm

33. GaAsP Photocathodes Hayashida et al. Beaune 2005 NIM

34. Avalanche Photodiodes (APDs, Geiger mode) • Single photon causes breakdown in over-voltaged diode • QE potential of silicon • Arrays in CMOS becoming available But • Appreciable deadtime • Low filling factor • High dark counts, crosstalk and afterpulsing

35. APD arrays 32 x 32 Edoardo Charbon Ecole Polytechnique Federale de Lausanne

36. L3CCD (e2V Technologies) • Integrates charge • Multiplies charge in special readout register • Adjust gain such that se < 1e- But • Multiplication noise doubles photon noise variance • Single readout limiting frame rate

37. Assumed performance parameters

38. Gaussian weighted center of gravity algorithm: From Fusco et al SPIE 5490. 1155, 2004

39. Advantages of multi-pixel sampling of Shack-Hartmann spots Non-linearity of 2 x 2 binning

40. Advantages of multi-pixel sampling of Shack-Hartmann spots 4 x 4 6 x 6 • Linear response off-null • Insensitive to input width • More sensitive to readout noise

41. Technology advantage

42. Soft X-Ray Photocathodes