SPIE 2005 - 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
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SPIE 2005 - San Diego - J. Vallerga John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund Space Sciences Laboratory, University of California,
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SPIE 2005 - 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 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
Non-linearity of 2 x 2 binning
Quad cell (2x2) algorithm for Gaussian input
-1
0
1
-1 0 1 Centroid true position
Calculated position
Sigma = 0.2
Sigma = 0.4
Sigma = 0.6
Sigma = 0.8
Sigma = 1.0
SPIE 2005 - San Diego - J. Vallerga
Advantages of multi-pixel sampling of Shack-Hartmann spots
Linear response off-nullInsensitive to input widthMore sensitive to readout noise
4 x 4 6 x 64x4 COG non-linearity for Gaussian input
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
Centroid true position
Calculated position(center of gravity )
Sigma = 0.4Sigma = 0.8Sigma = 1.2
6x6 COG non-linearity for Gaussian input
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
Centroid true position
Calculated position(center of gravity)
Sigma = 0.4Sigma = 0.8Sigma = 1.2
SPIE 2005 - San Diego - J. Vallerga
Centroid in presence of noise:
8 x 8Noiseless35% QE
10 photons
- - -
100 photons
1000 photons
8 x 82.5 e- rms90% QE
6 x 62.5 e- rms90% QE
4 x 42.5 e- rms90% QE
SPIE 2005 - San Diego - J. Vallerga
Photon Counting
QADC
V v
EventsEvents
Charge integrating
Threshold
EventsCount(x,y,t)
SPIE 2005 - 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 2005 - San Diego - J. Vallerga
APD arrays
Edoardo CharbonEcole Polytechnique Federale de Lausanne