spie 2005 - san diego - j. vallerga john vallerga, jason mcphate, anton tremsin and oswald siegmund...
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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
32 x 32
SPIE 2005 - 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 2005 - San Diego - J. Vallerga
Imaging, Photon Counting DetectorsCharge distribution on stripsCharge CloudMCP stackTube Window withphotocathodeγ
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
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)
Input
Preamp
Disc.
Disc. logic Mux. 13 bit
counter –ShiftRegister
Clock out
Shutter
Lower Thresh.
Disc.
Mux.
Previous Pixel
Mask bit
Analog Digital
Upper Thresh.
Next Pixel
Mask bit
Polarity
~ 500 transistors/pixel
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
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
CCDMedipix-
MCP
Binning 2 x 2 6 x 6 8 x 8 8 x 8
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€
(σ Δφ2 )ph =
π 2
2ln(2)(Nph )•NTND
⎛
⎝ ⎜
⎞
⎠ ⎟
2
•NT
2 + NW2
2NT2 + NW
2
⎛
⎝ ⎜
⎞
⎠ ⎟
2
(σ Δφ2 )det =
π 3
32(ln(2))2•σ det
(Nph )
⎛
⎝ ⎜
⎞
⎠ ⎟
2
•NT
2 + NW2
ND
⎛
⎝ ⎜
⎞
⎠ ⎟
2
(σ Δφ2 )tot = (σ Δφ
2 )det +(σ Δφ2 )ph
Gaussian weighted center of gravity algorithm:
From Fusco et al SPIE 5490. 1155, 2004
SPIE 2005 - San Diego - J. Vallerga
Centroid error vs. input fluence
Centroid estimator error vs. technique
0.100
1.000
10.000
100.000
1 10 100 1000
Input number of photons
Centroid Error (rms, radians)
CCD Quad cellCCD 8x8 weightedCCD 6x6 weightedMedipix 8x8 weighted
SPIE 2005 - 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 1st half of 2006
SPIE 2005 - 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 2005 - San Diego - J. Vallerga
UV photon counting movie
QuickTime™ and aYUV420 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
LampPinhole
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 ke-
Gap: 500µm
SPIE 2005 - San Diego - J. Vallerga
Avg. movement of 700 spots
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
0 5 10 15 20 25
Lamp Position (mm)
Centroid Position (µm)
Delta X
Delta Y
1 pixel
SPIE 2005 - San Diego - J. Vallerga
Position error (550 events/spot)
0
5
10
15
20
25
30
35
40
45
50
-20 -15 -10 -5 0 5 10 15 20
Centroid difference (microns)
Number of centroids
rms = 2.0 µm
SPIE 2005 - San Diego - J. Vallerga
Flat Field
1200 cts/bin - 500Mcps
MCP deadspots
Hexagonal multifiber boundaries
SPIE 2005 - San Diego - J. Vallerga
Flat Field (cont)
Histogram of Ratio consistent with counting
statistics (2% rms)Ratio Flat1/Flat2
SPIE 2005 - San Diego - J. Vallerga
Readout Architecture33
28 b
it P
ixel
Col
umn
0
3328
bit
Pix
el C
olum
n 25
5
3328
bit
Pix
el C
olum
n 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
SPIE 2005 - 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