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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. Future WFS Requirements*. High (~80%) optical QE

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a noiseless 512 x 512 detector for ao with khz frame rates
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

future wfs requirements
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”

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

Imaging, Photon Counting Detectors
why would you want one
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)
spatial resolution
Spatial Resolution

Cross Strip readout of Glass MCPs

12 µm pore glass MCPs

7 µm pore glass MCPs

mcp detectors at ssl berkeley
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)

gaas photocathodes geniii
GaAs Photocathodes (GenIII)
  • Developed for night vision tubes
  • Slight cooling required (104 cps at room temp)
  • Only fabricated in USA and Japan
advantages of multi pixel sampling of shack hartman spots
Advantages of multi-pixel sampling of Shack Hartman spots

5 x 5

2 x 2

  • Linear response off-null
  • Insensitive to input width
  • More sensitive to readout noise
wavefront sensor event rates
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
our ao detector concept
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
medipix2 asic readout
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.

single medipix2 pixel
Single Medipix2 pixel

Each 55µm Pixel has ~ 500 transistors using 0.25µm CMOS technology

readout architecture
Readout Architecture

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

3328 bit Pixel Column 0

3328 bit Pixel Column 255

3328 bit Pixel Column 1

256 bit fast shift register

32 bit CMOS output

LVDS out

built in electronic shutter
“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?

first test detector
First test detector
  • Demountable detector
  • Simple lab vacuum, no photocathode
  • UV sensitive
initial results
Initial Results

It Works!

Lower gain, higher rear field

First light!

spatial resolution1
Spatial Resolution

Group 3-2 visible 9 lp/mm = 55µm

(Nyquist limit)

100 µs

1 s

optimizing charge cloud size
Optimizing charge cloud size
  • Medipix2 “non-photometric”
  • # pixels per photon dependent on:
    • MCP-anode gap
    • Rear field voltage
    • MCP gain and threshold of Medipix pixel amp
modeling optimum sampling
Modeling Optimum sampling

Input spot size, charge cloud size

Generate N photons per frame

Calculate spot centroid

Repeat M times and plot error distribution

spot size vs gain
Spot size vs gain

Pinhole grid mask

(0.5 x 0.5 mm)

Gain: 200,000

Rear Field: 1600V

Threshold: 3 ke-

Gap: 500µm

spot size vs gain1
Spot size vs gain

Pinhole grid mask

(0.5 x 0.5 mm)

Gain: 20,000

Rear Field: 1600V

Threshold: 3 ke-

Gap: 500µm

example of sub pixel resolution
Example of sub pixel resolution

Calculate centroids of each event

Accumulate event x,y list

2-d histogram on finer pitch

9 lp/mm

example of sub pixel resolution1
Example of sub pixel resolution

Calculate centroids of each event

Accumulate event x,y list

2-d histogram on finer pitch

16 lp/mm

flat field
Flat Field

MCP deadspots

Hexagonal multifiber boundaries

1200 cts/bin - 500Mcps

flat field cont
Flat Field (cont)

Histogram of Ratio consistent with counting statistics (2% rms)

Ratio Flat1/Flat2

future work 3 yr noao grant
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
issues concerns
Issues/Concerns
  • QE !
  • Throughput
    • global
    • local
  • Detector Lifetime
  • Downstream interface
  • Cost
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

Acknowledgements

This work was funded by an AODP grant managed by NOAO and funded by NSF

Thanks to the Medipix Collaboration:

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