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

4. Centroid in presence of noise:
8 x 8 Noiseless 35% QE
10 photons -
100 photons
1000 photons
8 x e- rms 90% QE
6 x e- rms 90% QE
4 x e- rms 90% QE

5. Centroid error vs. input fluence

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

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. Sub-pixel spatial linearity
Lamp
Pinhole
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/Medipix Status
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. Acknowledgements
This work was funded by an AODP grant managed by NOAO and funded by NSF
Thanks to the Medipix Collaboration:
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

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

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

28. 256 bit fast shift register
Readout Architecture
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
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. 256 bit fast shift register
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 ( nm)

32. GaN UV Photocathodes, 100- 400 nm
The sensitivity of the activated photocathode is considerably higher, with a quantum efficiency of as high as 37% at about 450 Å and a sensitivity cut off energy of about 6.2 eV. The observed decrease of sensitivity at 988 and 520 Å (and probably not observed at ~670 Å) is likely to correspond to 2xEgap+Eaff,, 4xEgap+Eaff,, and 3xEgap+Eaff,, respectively. A much better spectral continuity of the illumination source is required in order to study the UV absorption fine structure.

33. Hayashida et al. Beaune 2005 NIM
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
CCD
Medipix-MCP
Binning
2 x 2
6 x 6
8 x 8
QE (%) 90 35
Readout noise
2.5 e-
Seeing width (pxls FWHM)
0.75
2.25 3
Diffract. width (pxls FWHM)
0.5
1.5 2

38. Gaussian weighted center of gravity algorithm:
From Fusco et al SPIE , 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
Linear response off-null
Insensitive to input width
More sensitive to readout noise

41. Technology advantage High QE CCDs Number of pixels CCDs, Medipix
Readout noise
APD, Medipix, L3CCD
Frame rate
Medipix, CCD
Gating
Medipix

42. Soft X-Ray Photocathodes