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Photon counting arrays for AO wavefront sensors

Photon counting arrays for AO wavefront sensors. John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund Space Sciences Laboratory, University of California, Berkeley Bettina Mikulec and Allan Clark University of Geneva. Future WFS detector requirements.

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Photon counting arrays for AO wavefront sensors

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  1. Photon counting arrays for AO wavefront sensors 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. Advantages of multi-pixel sampling of Shack-Hartmann spots Non-linearity of 2 x 2 binning

  4. 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

  5. 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:

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

  7. 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

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

  9. 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

  10. 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

  11. 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)

  12. GaAsP Photocathodes Hayashida et al. Beaune 2005 NIM

  13. 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

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

  15. 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

  16. Vacuum Tube Design

  17. Vacuum Tube Design

  18. Vacuum Tube Design

  19. Vacuum Tube Design

  20. Technology advantage

  21. Assumed performance parameters

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

  23. Centroid error vs. input fluence

  24. Summary • Noiseless detectors outperform CCDs at low fluence • “Crossover” point at 90 photons for 8x8 binning using best performance values • Higher if weighting/correlation schemes not used MCP/MedipixStatus • First tube in Fall 2005 • GaAs tube in 1st half of 2006

  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. UV photon counting movie

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

  28. Lamp Pinhole Sub-pixel spatial linearity Detector

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

  30. Avg. movement of 700 spots 1 pixel

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

  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. 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

  35. “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

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