LLNL AO talk - Aug. 17, 2004 John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund Space Sciences Laboratory, University of California, Berkeley

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


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 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


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

12 µm pore glass MCPs 7 µm pore glass MCPs

Cross Strip readout of Glass MCPs


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)

• Developed for night vision tubes

• Slight cooling required (104 cps at room temp)

• Only fabricated in USA and Japan


GaN UV Photocathodes, 1000- 4000ÅGaN UV Photocathodes, 1000- 4000Å

0.1

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100

150 200 250 300 350 400

NW-BH071#3

NW-BH071#2

NW-JG238#3

NW-JG238#2

Quantum Efficiency (%)

Wavelength (nm)


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


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

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

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

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


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


“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

• Demountable detector

• Simple lab vacuum, no photocathode

• UV sensitive


Initial Results

It Works!

First light! Lower gain, higher rear field


Photon counting movie

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.


Spatial Resolution

100 µs 1 s Group 3-2 visible 9 lp/mm = 55µm

(Nyquist limit)


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


MCP event spot area Rear Field = 200V

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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

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2

4

6

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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


MCP charge cloud size

0

20

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160

0 20 40 60 80 100 120 140

R (microns)

Normalized Charge

200K100K50K20K

1600V rear field


Modeling Optimum sampling

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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

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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


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 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

9 lp/mm

• Calculate centroids of each event• Accumulate event x,y list• 2-d histogram on finer pitch


• Calculate centroids of each event• Accumulate event x,y list• 2-d histogram on finer pitch

16 lp/mm

Example of sub pixel resolution


Flat Field

1200 cts/bin - 500Mcps

MCP deadspots

Hexagonal multifiber boundaries


Flat Field (cont)

Histogram of Ratio consistent with counting

statistics (2% rms)Ratio Flat1/Flat2


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 Design


Vacuum Tube Design


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

Thanks to the Medipix Collaboration:

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


Soft X-Ray Photocathodes

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CsBr

KI

Energy (keV)


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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

Documents

LLNL AO talk - Aug. 17, 2004 John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund Space Sciences Laboratory, University of California, Berkeley