Transcript A noiseless 512 x 512 detector for AO with kHz frame rates John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund Space Sciences Laboratory, University.

A noiseless 512 x 512
detector for AO with kHz
frame rates
John Vallerga, Jason McPhate, Anton Tremsin
and Oswald Siegmund
Space Sciences Laboratory, University of California, Berkeley
Bettina Mikulec and Allan Clark
University of Geneva
LLNL AO talk - Aug. 17, 2004
Future WFS Requirements*
• High (~80%) optical QE
• Lots of pixels - eventually 512x512
• Very low readout noise (< 3 e-)
• kHz frame rates
The last three are not simultaneously
achievable with the current generation
of CCDs
*Angel et al “A Road Map for the Development of Astronomical AO”
LLNL AO talk - Aug. 17, 2004
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
LLNL AO talk - Aug. 17, 2004
Why would you want one?
• No readout noise penalty
– Use as many pixels as you wish
• Continuous temporal sampling to ~
nsecs
– Choose integration period(s) after the fact or on
the fly
• Other advantages
–
–
–
–
Large area, curved focal planes
Cosmic ray = 1 count
LN2 not required
Low dark current (0.16 attoamps cm-2)
LLNL AO talk - Aug. 17, 2004
Spatial Resolution
Cross Strip readout of Glass MCPs
12 µm pore glass MCPs
7 µm pore glass MCPs
LLNL AO talk - Aug. 17, 2004
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)
LLNL AO talk - Aug. 17, 2004
GaAs Photocathodes (GenIII)
• Developed for night
vision tubes
• Slight cooling
required (104 cps at
room temp)
• Only fabricated in
USA and Japan
LLNL AO talk - Aug. 17, 2004
GaN UV Photocathodes, 1000- 4000Å
LLNL AO talk - Aug. 17, 2004
Advantages of multi-pixel sampling of
Shack Hartman spots
5x5 algorithm error for Gaussian input
Quad cell (2x2) algorithm error for Gaussian input
1
0.5
=
=
=
=
=
Sigma
Sigma
Sigma
Sigma
Sigma
0.2
0.4
0.6
0.8
1.0
Calculated position
Calculated position
Sigma
Sigma
Sigma
Sigma
Sigma
0
0
-0.5
-1
-0.5
= 0.2
= 0.4
= 0.6
= 0.8
= 1.0
0
0.5
-1
Gaussian Centroid true position
2x2
-0.5
0
Gaussian Centroid true position (center pixel)
5x5
Linear response off-null
Insensitive to input width
More sensitive to readout noise
LLNL AO talk - Aug. 17, 2004
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
LLNL AO talk - Aug. 17, 2004
Our AO detector concept
An optical imaging tube
using:
– GaAs photocathode
– Microchannel plate to
amplify a single
photoelectron by 104
Photocathode
Photon
e-
Q = 104e-
Pij = Pij + 1
– ASIC to count these
events per pixel
Window
LLNL AO talk - Aug. 17, 2004
MCP
Medip ix2
Medipix2 ASIC Readout
 Pixellated readout for x and gamma ray semiconductor
sensors (Si, GaAs, CdTe etc)
 Developed at CERN for Medipix collaboration
 55 µm pixel @ 256x256 (buttable to 512 x 512).
 Pixel level amp, discriminator, gate & counter.
 Counts integrated at pixel
No charge transfer!
16mm
Applications: Mammography, dental radiography,
dynamic autoradiography, gamma imaging, neutron
imaging, angiography, xray diffraction, dynamic
defectoscopy, etc.
LLNL AO talk - Aug. 17, 2004
14mm
Single Medipix2 pixel
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
Digital
Each 55µm Pixel has ~ 500 transistors using 0.25µm CMOS technology
LLNL AO talk - Aug. 17, 2004
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
• Choice of serial or 32 bit parallel
output
• Maximum designed bandwidth is
100MHz
• Corresponds to 266µs frame
readout
256 bit fast shift register
32 bit CMOS output
LVDS out
LLNL AO talk - Aug. 17, 2004
“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
What is the best strategy to remove/measure
parallax?
LLNL AO talk - Aug. 17, 2004
First test detector
• Demountable detector
• Simple lab vacuum, no photocathode
• UV sensitive
LLNL AO talk - Aug. 17, 2004
Initial Results
It Works!
Lower gain, higher
rear field
First light!
LLNL AO talk - Aug. 17, 2004
Photon counting movie
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
LLNL AO talk - Aug. 17, 2004
Spatial Resolution
100 µs
1s
LLNL AO talk - Aug. 17, 2004
Group 3-2 visible
9 lp/mm = 55µm
(Nyquist limit)
Optimizing charge cloud size
• Medipix2 “non-photometric”
• MCP-anode gap
• Rear field voltage
Relative value
• # pixels per photon
dependent on:
Charge distribution
-600.0
• MCP gain and threshold
of Medipix pixel amp
LLNL AO talk - Aug. 17, 2004
-400.0
-200.0
0.0
x (um)
200.0
400.0
600.0
MCP event spot area
200V
Rear Field = 1600V
20
Mean Spot Area (pixel)
18
16
G=20k, Area
G=20k, Area
14
G=50k, Area
G=50k, Area
12
G=100k, Area
G=100k, Area
10
G=200k, Area
G=200k, Area
8
6
4
2
0
0
5
10
15
20
25
30
-
Lower
Lower Threshold
Threshold (ke
(ke )
LLNL AO talk - Aug. 17, 2004
35
40
MCP charge cloud size
1600V rear field
160
Normalized Charge
140
200K
100K
50K
20K
120
100
80
60
40
20
0
0
20
40
60
80
R (microns)
LLNL AO talk - Aug. 17, 2004
100
120
140
Modeling Optimum sampling
• Generate N photons
per frame
Counts
• Input spot size,
charge cloud size
25
Histogramofofmeasured
measurederrors
errors
Histogram
15µmstar
starrms
rms
15µm
pxlcharge
chargecloud
cloud 20
5 1pxl
100
events
100 events
50tries
tries
50
15
10
5
-30
• Calculate spot
centroid
Measured
Measured Xc
Ideal Xc
-20
0
-10
0
10
Xc - XcMeasured (um)
20
1.50E+01
6.00E+00
4.00E+00
1.00E+01
• Repeat M times and
plot error distribution
2.00E+00
5.00E+00
0.00E+00
0.00E+00
-2.00E+00
-5.00E+00
-4.00E+00
-1.00E+01
-6.00E+00
-1.50E+01
LLNL AO talk - Aug. 17, 2004
1.00E-05
2.00E-05
3.00E-05
4.00E-05
4.00E-05
5.00E-05
5.00E-05
6.00E-05
6.00E-05
30
30
Spot size vs gain
Pinhole grid mask
(0.5 x 0.5 mm)
Gain: 200,000
Rear Field: 1600V
Threshold: 3 keGap: 500µm
LLNL AO talk - Aug. 17, 2004
Spot size vs gain
Pinhole grid mask
(0.5 x 0.5 mm)
Gain: 20,000
Rear Field: 1600V
Threshold: 3 keGap: 500µm
LLNL AO talk - Aug. 17, 2004
Example of sub pixel resolution
• Calculate centroids of each event
• Accumulate event x,y list
• 2-d histogram on finer pitch
9 lp/mm
LLNL AO talk - Aug. 17, 2004
Example of sub pixel resolution
• Calculate centroids of each event
• Accumulate event x,y list
• 2-d histogram on finer pitch
16 lp/mm
LLNL AO talk - Aug. 17, 2004
Flat Field
MCP deadspots
Hexagonal multifiber
boundaries
1200 cts/bin - 500Mcps
LLNL AO talk - Aug. 17, 2004
Flat Field (cont)
Ratio Flat1/Flat2
Histogram of Ratio
consistent with counting
statistics (2% rms)
LLNL AO talk - Aug. 17, 2004
Future Work (3 yr. NOAO grant)
• Optimize MCP-Medipix2 interface design
• Design and build tube with Medipix2 and GaAs
• Develop parallel readout with European
collaborators
• Develop FPGA to reduce output bandwidth
– 5 million centroids/s vs. 262 million pixels/s.
• Test at AO laboratory at CFAO, U.C. Santa Cruz
• Test at telescope
LLNL AO talk - Aug. 17, 2004
Vacuum Tube Design
LLNL AO talk - Aug. 17, 2004
Vacuum Tube Design
LLNL AO talk - Aug. 17, 2004
Vacuum Tube Design
LLNL AO talk - Aug. 17, 2004
Vacuum Tube Design
LLNL AO talk - Aug. 17, 2004
Issues/Concerns
• QE !
• Throughput
– global
– local
• Detector Lifetime
• Downstream interface
• Cost
LLNL AO talk - Aug. 17, 2004
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
LLNL AO talk - Aug. 17, 2004
Quantum Detection Efficiency (%)
Soft X-Ray Photocathodes
100
CsBr
KI
80
60
40
20
0
0.1
Energy (keV)
LLNL AO talk - Aug. 17, 2004
1
EUV and FUV
CsI 1985 vs 1999
0.7
CsI 1985 30°
CsI 1985 20°
CsI #3 2/99 20°
CsI #3 2/99 30°
CsI #2 1/99 20°
CsI #2 1/99 30°
0.6
0.5
QDE
0.4
0.3
0.2
0.1
0
0
500
1000
Wavelength (Å)
LLNL AO talk - Aug. 17, 2004
1500
2000