llnl ao talk - aug. 17, 2004 john vallerga, jason mcphate, anton tremsin and oswald siegmund space...
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LLNL AO talk - Aug. 17, 2004
John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund
Space Sciences Laboratory, University of California, Berkeley
Bettina Mikulec and Allan Clark
University of Geneva
A noiseless 512 x 512 detector for AO with kHz
frame rates
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 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
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
12 µm pore glass MCPs 7 µm pore glass MCPs
Cross Strip readout of 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- 4000ÅGaN UV Photocathodes, 1000- 4000Å
0.1
1
10
100
150 200 250 300 350 400
NW-BH071#3
NW-BH071#2
NW-JG238#3
NW-JG238#2
Quantum Efficiency (%)
Wavelength (nm)
LLNL AO talk - Aug. 17, 2004
Advantages of multi-pixel sampling of Shack Hartman spots
Linear response off-nullInsensitive to input widthMore sensitive to readout noise
2 x 2 5 x 5
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
– ASIC to count these events per pixel
LLNL AO talk - Aug. 17, 2004
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!
14mm
16mm
Applications: Mammography, dental radiography, dynamic autoradiography, gamma imaging, neutron imaging, angiography, xray diffraction, dynamic defectoscopy, etc.
LLNL AO talk - Aug. 17, 2004
Single Medipix2 pixel
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
Each 55µm Pixel has ~ 500 transistors using 0.25µm CMOS technology
LLNL AO talk - Aug. 17, 2004
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
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!
First light! Lower gain, higher rear field
LLNL AO talk - Aug. 17, 2004
Photon counting movie
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
LLNL AO talk - Aug. 17, 2004
Spatial Resolution
100 µs 1 s Group 3-2 visible 9 lp/mm = 55µm
(Nyquist limit)
LLNL AO talk - Aug. 17, 2004
Optimizing charge cloud size
-600.0 -400.0 -200.0 0.0 200.0 400.0 600.0
x (um)
Charge distribution
• Medipix2 “non-photometric”
• # pixels per photon dependent on:
• MCP-anode gap
• Rear field voltage
• MCP gain and threshold of Medipix pixel amp
LLNL AO talk - Aug. 17, 2004
MCP event spot area Rear Field = 200V
0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20 25 30 35 40
Lower Threshold (ke-)
Mean Spot Area (pixel)
G=20k, Area
G=50k, Area
G=100k, Area
G=200k, Area
Rear Field = 1600V
0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20 25 30 35 40
Lower Threshold (ke-)
Mean Spot Area (pixel)
G=20k, Area
G=50k, Area
G=100k, Area
G=200k, Area
LLNL AO talk - Aug. 17, 2004
MCP charge cloud size
0
20
40
60
80
100
120
140
160
0 20 40 60 80 100 120 140
R (microns)
Normalized Charge
200K100K50K20K
1600V rear field
LLNL AO talk - Aug. 17, 2004
Modeling Optimum sampling
0
5
10
15
20
25
-30 -20 -10 0 10 20 30
Xc - XcMeasured (um)
Counts
Measured XcIdeal Xc
Histogram of measured errors15µm star rms
1 pxl charge cloud100 events
50 tries
-1.50E+01
-1.00E+01
-5.00E+00
0.00E+00
5.00E+00
1.00E+01
1.50E+01
0.00E+00 1.00E-05 2.00E-05 3.00E-05 4.00E-05 5.00E-05 6.00E-05
0
5
10
15
20
25
-30 -20 -10 0 10 20 30
Xc - XcMeasured (um)
Counts
Measured XcIdeal Xc
Histogram of measured errors15µm star rms
5 pxl charge cloud100 events
50 tries
-6.00E+00
-4.00E+00
-2.00E+00
0.00E+00
2.00E+00
4.00E+00
6.00E+00
0.00E+00 1.00E-05 2.00E-05 3.00E-05 4.00E-05 5.00E-05 6.00E-05
• Input spot size, charge cloud size
• Generate N photons per frame
• Calculate spot centroid
• Repeat M times and plot error distribution
LLNL AO talk - Aug. 17, 2004
Spot size vs gain
Pinhole grid mask
(0.5 x 0.5 mm)
Gain: 200,000
Rear Field: 1600V
Threshold: 3 ke-
Gap: 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 ke-
Gap: 500µm
LLNL AO talk - Aug. 17, 2004
Example of sub pixel resolution
9 lp/mm
• Calculate centroids of each event• Accumulate event x,y list• 2-d histogram on finer pitch
LLNL AO talk - Aug. 17, 2004
• Calculate centroids of each event• Accumulate event x,y list• 2-d histogram on finer pitch
16 lp/mm
Example of sub pixel resolution
LLNL AO talk - Aug. 17, 2004
Flat Field
1200 cts/bin - 500Mcps
MCP deadspots
Hexagonal multifiber boundaries
LLNL AO talk - Aug. 17, 2004
Flat Field (cont)
Histogram of Ratio consistent with counting
statistics (2% rms)Ratio Flat1/Flat2
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
• 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
LLNL AO talk - Aug. 17, 2004
Soft X-Ray Photocathodes
0
20
40
60
80
100
0.1 1
CsBr
KI
Energy (keV)
LLNL AO talk - Aug. 17, 2004
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 500 1000 1500 2000
CsI 1985 vs 1999
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°
Wavelength (Å)
EUV and FUV