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

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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


Gan uv photocathodes 1000 4000

GaN UV Photocathodes, 1000- 4000Å


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!


Photon counting movie

Photon counting movie


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


Mcp event spot area

MCP event spot area


Mcp charge cloud size

MCP charge cloud size


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


Vacuum tube design

Vacuum Tube Design


Vacuum tube design1

Vacuum Tube Design


Vacuum tube design2

Vacuum Tube Design


Vacuum tube design3

Vacuum Tube Design


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:


Soft x ray photocathodes

Soft X-Ray Photocathodes


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

EUV and FUV


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