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
SPIE Instrumentation for Astronomy AO - June22, 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”
SPIE Instrumentation for Astronomy AO - June22, 2004
Imaging, Photon Counting Detectors
• Detects individual quanta of
light via photoelectric effect
• Microchannel plate amplifies
single electron to large
charge cloud
• Signal per photon >> noise
• Readout gives X,Y of every
event
• Time of every event also
available
SPIE Instrumentation for Astronomy AO - June22, 2004
Microchannel Plates
2 µm pores on 3 µm centers (Burle Industries)
SPIE Instrumentation for Astronomy AO - June22, 2004
MCP Detectors at Berkeley
10 mm
COS FUV for Hubble (2004,2005,???)
400 pxl
85 mm
GALEX
NUV Tube
(in orbit)
14000 pxl
68 mm
SPIE Instrumentation for Astronomy AO - June22, 2004
25 mm Optical Tube
GaAs Photocathodes (GenIII)
• Developed for night
vision tubes
• Slight cooling
required (104 cps at
room temp)
• Only fabricated in
USA and Japan
SPIE Instrumentation for Astronomy AO - June22, 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
Gaussian Centroid true position
2x2
-1
-0.5
0
Gaussian Centroid true position (center pixel)
5x5
Linear response off-null
Insensitive to input width
More sensitive to readout noise
SPIE Instrumentation for Astronomy AO - June22, 2004
Wavefront Sensor Photon Rate
• Future large telescopes need > 5000 actuators
• Kilohertz feedback rates
• 1000 detected events per spot for sub-pixel
centroiding
5000 x 1000 x 1000
 5 Gigahertz counting rate!
• 104 time faster than existing photon counting
imagers
– Requires integrating readout
SPIE Instrumentation for Astronomy AO - June22, 2004
Our 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
SPIE Instrumentation for Astronomy AO - June22, 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.
SPIE Instrumentation for Astronomy AO - June22, 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
SPIE Instrumentation for Astronomy AO - June22, 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
SPIE Instrumentation for Astronomy AO - June22, 2004
First test detector
• Demountable detector
• Simple lab vacuum, no photocathode
• UV sensitive
SPIE Instrumentation for Astronomy AO - June22, 2004
Initial Results
It Works!
First light!
Lower gain, higher
rear field
SPIE Instrumentation for Astronomy AO - June22, 2004
Spatial Resolution
100 µs
1s
SPIE Instrumentation for Astronomy AO - June22, 2004
Group 3-2 visible
9 lp/mm = 55µm
(Nyquist limit)
Flat Field
MCP deadspots
Hexagonal multifiber
boundaries
1200 cts/bin - 500Mcps
SPIE Instrumentation for Astronomy AO - June22, 2004
Flat Field (cont)
Ratio Flat1/Flat2
Histogram of Ratio
consistent with counting
statistics (2% rms)
SPIE Instrumentation for Astronomy AO - June22, 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
SPIE Instrumentation for Astronomy AO - June22, 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
SPIE Instrumentation for Astronomy AO - June22, 2004