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.
Download ReportTranscript 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.
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 SPIE 2005 - 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 2005 - San Diego - J. Vallerga Advantages of multi-pixel sampling of Shack-Hartmann spots Quad cell (2x2) algorithm for Gaussian input Calculated position 1 Sigma Sigma Sigma Sigma Sigma = 0.2 = 0.4 = 0.6 = 0.8 = 1.0 0 -1 -1 0 Centroid true position 1 Non-linearity of 2 x 2 binning SPIE 2005 - San Diego - J. Vallerga Advantages of multi-pixel sampling of Shack-Hartmann spots 4x4 6x6 4x4 COG non-linearity for Gaussian input 6x6 COG non-linearity for Gaussian input 1 Sigma = 0.4 Sigma = 0.8 Sigma = 1.2 0.5 Calculated position (center of gravity) Calculated position (center of gravity ) 1 0 -0.5 Sigma = 0.4 Sigma = 0.8 Sigma = 1.2 0.5 0 -0.5 -1 -1 -1 -0.5 0 0.5 Centroid true position 1 -1 -0.5 0 Centroid true position Linear response off-null Insensitive to input width More sensitive to readout noise SPIE 2005 - San Diego - J. Vallerga 0.5 1 Centroid in presence of noise: 1000 photons 100 photons 10 photons 8x8 Noiseless 35% QE 8x8 2.5 e- rms 90% QE SPIE 2005 - San Diego - J. Vallerga - - 6x6 2.5 e- rms 90% QE - 4x4 2.5 e- rms 90% QE Photon Counting Count (x,y,t) Events Threshold Charge integrating Q SPIE 2005 - San Diego - J. Vallerga V sv ADC Events sEvents 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 2005 - San Diego - J. Vallerga APD arrays 32 x 32 Edoardo Charbon Ecole Polytechnique Federale de Lausanne SPIE 2005 - San Diego - J. Vallerga 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 SPIE 2005 - San Diego - J. Vallerga 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 SPIE 2005 - 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 2005 - San Diego - J. Vallerga GaAsP Photocathodes Hayashida et al. Beaune 2005 NIM SPIE 2005 - 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 2005 - San Diego - J. Vallerga 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 SPIE 2005 - San Diego - J. Vallerga Photocathode Photon e- Q = 104e- Pij = Pij + 1 Window MCP Medip ix2 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) Previous Pixel Shut ter Mask bit Lower Thresh. Polarity Mux. Clock out Disc. Disc. logic Input Preamp Disc. Mux. 13 bit counter – Shift Register Upper Thresh. Mask bit Next Pixel Analog SPIE 2005 - San Diego - J. Vallerga Digital ~ 500 transistors/pixel Vacuum Tube Design SPIE 2005 - San Diego - J. Vallerga Vacuum Tube Design SPIE 2005 - San Diego - J. Vallerga Vacuum Tube Design SPIE 2005 - San Diego - J. Vallerga Vacuum Tube Design SPIE 2005 - San Diego - J. Vallerga Technology advantage High QE CCDs Number of pixels CCDs, Medipix Readout noise APD, Medipix, L3CCD Frame rate Medipix, CCD Gating Medipix SPIE 2005 - San Diego - J. Vallerga Assumed performance parameters MedipixMCP CCD Binning 2x2 6x6 8x8 8x8 QE (%) 90 90 90 35 Readout noise 2.5 e- 2.5 e- 2.5 e- 0 Seeing width (pxls FWHM) 0.75 2.25 3 3 Diffract. width (pxls FWHM) 0.5 1.5 2 2 SPIE 2005 - San Diego - J. Vallerga Gaussian weighted center of gravity algorithm: N T N N (s )ph 2ln(2)( N ph ) N D 2N N 2 (s )det 2 2 2 2 T 2 T 2 W 2 W 2 2 2 N T N W s det 2 32(ln(2)) ( N ph ) N D 2 3 (s 2 )tot (s 2 )det (s 2 )ph From Fusco et al SPIE 5490. 1155, 2004 SPIE 2005 - San Diego - J. Vallerga 2 Centroid error vs. input fluence Centroid estimator error vs. technique Centroid Error (rms, radians) 100.000 CCD Quad cell CCD 8x8 weighted CCD 6x6 weighted Medipix 8x8 weighted 10.000 1.000 0.100 1 10 100 Input number of photons SPIE 2005 - San Diego - J. Vallerga 1000 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 1st half of 2006 SPIE 2005 - San Diego - J. Vallerga 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 Napoli • University of Cagliari • NIKHEF • CEA • University of Pisa • CERN • University of Auvergne • University of Freiburg • Medical Research Council • University of Glasgow • Czech Technical University • Czech Academy of Sciences • ESRF • Mid-Sweden University • University of Erlangen-Nurnberg SPIE 2005 - San Diego - J. Vallerga UV photon counting movie QuickTime™ and a YUV420 codec decompressor are needed to see this picture. SPIE 2005 - San Diego - J. Vallerga First test detector • Demountable detector • Simple lab vacuum, no photocathode • Windowless – UV sensitive SPIE 2005 - San Diego - J. Vallerga Sub-pixel spatial linearity Lamp Pinhole Detector SPIE 2005 - 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 keGap: 500µm SPIE 2005 - San Diego - J. Vallerga Avg. movement of 700 spots 0 Delta X Centroid Position (µm) -10 Delta Y -20 -30 1 pixel -40 -50 -60 -70 -80 -90 -100 0 5 10 15 Lamp Position (mm) SPIE 2005 - San Diego - J. Vallerga 20 25 Position error (550 events/spot) 50 Number of centroids 45 40 35 30 rms = 2.0 µm 25 20 15 10 5 0 -20 -15 -10 -5 0 5 10 Centroid difference (microns) SPIE 2005 - San Diego - J. Vallerga 15 20 Flat Field MCP deadspots Hexagonal multifiber boundaries 1200 cts/bin - 500Mcps SPIE 2005 - San Diego - J. Vallerga Flat Field (cont) Ratio Flat1/Flat2 SPIE 2005 - San Diego - J. Vallerga Histogram of Ratio consistent with counting statistics (2% rms) Readout Architecture • Pixel values are digital (13 bit) 3328 bit Pixel Column 255 3328 bit Pixel Column 1 3328 bit Pixel Column 0 • Bits are shifted into fast shift register 256 bit fast shift register 32 bit CMOS output LVDS out SPIE 2005 - San Diego - J. Vallerga • Choice of serial or 32 bit parallel output • Maximum designed bandwidth is 100MHz • Corresponds to 266µs frame readout “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 SPIE 2005 - San Diego - J. Vallerga