A noiseless, kHz frame rate,
imaging detector base on
MCPs readout with a Medipix2
Jason McPhate, John Vallerga, Anton Tremsin
and Oswald Siegmund
Space Sciences Laboratory, University of California, Berkeley
Bettina Mikulec and Allan Clark
University of Geneva
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
WFS detector for future AO systems*
• kHz frame rates
– Match atmospheric timescales
• Many pixels - eventually 512 x 512
– More subapertures and more pixels per subaperture
• Very low readout noise (< 3 e-)
– Lower penalty for more pixels per subaperture
• High (~80%) optical QE
– Use dimmer guide stars or higher frame rates
*Angel et al., “A Road Map for the Development of Astronomical AO”
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
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,
Count stored in digital histogram
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
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)
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
What’s the Catch?
• Global Counting Rates
– 1000 Shack-Hartmann spots per WFS
– Kilohertz feedback rates
– 1000 counts per spot for sub-pixel centroids
1 Gigahertz countingdetector
Requires integrating rate!
• Quantum Efficiency
– Historically Optical Photocathodes < ~15%
– Silicon devices (CCDs) can get ~90%
– Noiseless helps, but not that much
Requires GaAs Photocathode
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Our AO detector concept
An optical imaging tube
using: Photocathod e
• GaAs photocathode
Photon
• MCPs to amplify to ~104 e- Q = 104e-
Pij = Pij + 1
• Medipix2 ASIC readout
Medipix2
Window MCP
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Medipix2 ASIC Readout
Pixelated readout for x and gamma ray semiconductor
sensors (Si, GaAs, CdTe etc)
Developed at CERN for Medipix collaboration
55 µm pixel @ 256 x 256 (abutable to 512 x [n x 256]).
Pixel level amp, discriminator, gate & counter. 14mm
Counts integrated at pixel
No charge transfer!
16mm
Applications: Mammography, dental radiography,
dynamic autoradiography, gamma imaging, neutron
imaging, angiography, x-ray diffraction, dynamic
defectoscopy, etc.
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Readout Architecture
• Pixel values are digital (14 bit)
• Bits are shifted into fast shift
register
3584 bit Pixel Column 255
• Choice of serial or 32 bit parallel
3584 bit Pixel Column 0
3584 bit Pixel Column 1
output
• Maximum designed bandwidth
is 100MHz
• Corresponds to 286µs frame
readout in parallel
256 bit fast shift register
32 bit CMOS output LVDS out
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
First test detector
• Demountable detector
• Simple lab vacuum (~10-7 Torr)
• UV sensitive, no photocathode
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Lab Detector Lessons
• Medipix ASIC works well as MCP readout
• Sub-pixel centroiding of Shack-Hartmann like spots
was achieved
• Optimized parameters for use in optical tube
– Chevron stack of 10 µm pore MCPs (protect cathode from ion feedback)
– MCP gain of about 104 (longer tube life and higher counting rates)
– MCP to Medipix gap of 300 to 500 µm (Medipix wirebond clearance)
– Approximately 1600 V rear field (minimize MCP charge cloud spread)
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
No GaAs capability at UCB
So GaAs photocathode by
industrial vendor:
Means using “standard” size
tube
Only marginally larger than the
Medipix2 device
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Thick Film Ceramic Header
• Internal mounting/GND surface for Medipix
• Route ~60 Medipix signals out of vacuum
• Multi-layered to better match Medipix pitch
• Maintain hermetic seal of tube to ≤10-9 Torr
• Provide land pads for external I/F connectors
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Medipix on a Header
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
MCP/Medipix Serial I/F Board
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Vacuum Tube Design
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Parallel Readout Design
• Development by ESRF
• 1 to 5 Medipix2 chips
• FIFO for each chip
• Flat field, deadtime
corrections
• Optional centroid
calculation
• High speed serial out
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
Future Work (3 yr. NOAO grant)
• Seal a MCP/Medipix tube with a GaAs photocathode
• Perhaps a multi-alkali photocathode tube (@UCB)
• Finalize and build parallel readout
• Test at AO laboratory at CFAO, U.C. Santa Cruz
• Test at telescope
SPIE IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate
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 IR and Photoelect. Imagers and Detector Devices - 2005 - J. McPhate