SPIE San Diego - J. Vallerga John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund Space Sciences Laboratory, University of California, Berkeley Bettina Mikulec and Allan Clark University of Geneva Photon counting arrays for AO wavefront sensors

SPIE San Diego - J. Vallerga 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

SPIE San Diego - J. Vallerga Advantages of multi-pixel sampling of Shack-Hartmann spots Non-linearity of 2 x 2 binning

SPIE San Diego - J. Vallerga Advantages of multi-pixel sampling of Shack-Hartmann spots  Linear response off-null  Insensitive to input width  More sensitive to readout noise 4 x 46 x 6

SPIE San Diego - J. Vallerga Centroid in presence of noise: 8 x 8 Noiseless 35% QE 10 photons photons 1000 photons 8 x e - rms 90% QE 6 x e - rms 90% QE 4 x e - rms 90% QE

SPIE San Diego - J. Vallerga Photon Counting Q ADC V  v Events  Events Charge integrating Threshold Events Count (x,y,t)

SPIE San Diego - J. Vallerga 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

SPIE San Diego - J. Vallerga APD arrays Edoardo Charbon Ecole Polytechnique Federale de Lausanne 32 x 32

SPIE San Diego - J. Vallerga L3CCD (e2V Technologies) Integrates charge Multiplies charge in special readout register Adjust gain such that  e < 1e - But Multiplication noise doubles photon noise variance Single readout limiting frame rate

SPIE San Diego - J. Vallerga Imaging, Photon Counting Detectors Photocathode converts photon to electron MCP(s) amplify electron by 10 4 to 10 8 Rear field accelerates electrons to anode Patterned anode measures charge centroid

SPIE San Diego - J. Vallerga 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)

SPIE San Diego - J. Vallerga GaAsP Photocathodes Hayashida et al. Beaune 2005 NIM

SPIE San Diego - J. Vallerga 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

SPIE San Diego - J. Vallerga Our concept An optical imaging tube using: –GaAsP photocathode –Microchannel plate to amplify a single photoelectron by 10 4 –ASIC to count these events per pixel

SPIE San Diego - J. Vallerga 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

SPIE San Diego - J. Vallerga Vacuum Tube Design

SPIE San Diego - J. Vallerga Vacuum Tube Design

SPIE San Diego - J. Vallerga Vacuum Tube Design

SPIE San Diego - J. Vallerga Vacuum Tube Design

SPIE San Diego - J. Vallerga Technology advantage High QECCDs Number of pixelsCCDs, Medipix Readout noiseAPD, Medipix, L3CCD Frame rateMedipix, CCD GatingMedipix

SPIE San Diego - J. Vallerga Assumed performance parameters CCD Medipix- MCP Binning2 x 26 x 68 x 8 QE (%)90 35 Readout noise 2.5 e - 0 Seeing width (pxls FWHM) Diffract. width (pxls FWHM)

SPIE San Diego - J. Vallerga Gaussian weighted center of gravity algorithm: From Fusco et al SPIE , 2004

SPIE San Diego - J. Vallerga Centroid error vs. input fluence

SPIE San Diego - J. Vallerga 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/Medipix Status First tube in Fall 2005 GaAs tube in 1 st half of 2006

SPIE San Diego - J. Vallerga Acknowledgements 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 Thanks to the Medipix Collaboration: This work was funded by an AODP grant managed by NOAO and funded by NSF

SPIE San Diego - J. Vallerga UV photon counting movie

SPIE San Diego - J. Vallerga First test detector Demountable detector Simple lab vacuum, no photocathode Windowless – UV sensitive

SPIE San Diego - J. Vallerga Sub-pixel spatial linearity Lamp Pinhole Detector

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

SPIE San Diego - J. Vallerga Avg. movement of 700 spots 1 pixel

SPIE San Diego - J. Vallerga Position error (550 events/spot) rms = 2.0 µm

SPIE San Diego - J. Vallerga Flat Field 1200 cts/bin - 500Mcps MCP deadspots Hexagonal multifiber boundaries

SPIE San Diego - J. Vallerga Flat Field (cont) Histogram of Ratio consistent with counting statistics (2% rms) Ratio Flat1/Flat2

SPIE San Diego - J. Vallerga Readout Architecture 3328 bit Pixel Column bit Pixel Column bit Pixel Column bit fast shift register 32 bit CMOS outputLVDS 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

SPIE San Diego - J. Vallerga “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