SPIE Instr. for Astronomy, Marseille, John Vallerga, [email protected] Optically sensitive MCP...

SPIE Instr. for Astronomy, Marseille, John Vallerga, [email protected]

Optically sensitive MCP image tube with a Medipix2 ASIC readout

John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund

Space Sciences Laboratory

University of California, Berkeley


Motivation for new wavefront sensor detector

• kHz frame rates

• Larger format (> 256 x 256)

– More accuators

– More complex LGS images (parallax, etc)

– Off null / open loop operation

• Very low (or zero!) readout noise

• High dynamic range and gated


Centroid in presence of noise:

8 x 8Noiseless35% QE

10 photons

- - -

100 photons

1000 photons

8 x 82.5 e- rms90% QE

6 x 62.5 e- rms90% QE

4 x 42.5 e- rms90% QE


Photon Counting

QADC

V v

EventsEvents

Charge integrating

Threshold

EventsCount(x,y,t)


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


GaAsP Photocathodes

Hayashida et al. Beaune 2005 NIM


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


Medipix/Timepix ASIC readout

• 256 x 256 array of 55 µm pixels

• Integrates counts, not charge

• 100 kHz/pxl

• Frame rate: 1 kHz

• Low noise (100e-) = low gain operation (10 ke-)

• GHz global count rate

• ~1 W watt/chip, abuttable

• Developed at CERN

~ 500 transistors/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


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


Vacuum Tube Design


Difficulties over last two years

• Finding industrial partner to help fabricate vacuum tube and develop GaAs photocathode Burle merged with Photonis merged with DEP

ITT - busy with military night vision

Hamamatsu - too small a project

• Use in-house tube facilities with multi-alkali photocathode New brazing technique compatible with ceramic header materials

(vacuum brazing)

Slow leaks: tube failure

Photocathode inconsistency


Second tube process

Success!

Qualification: poor optical QE


Quantum efficiency (factor of 4 too low)

AO Tube #2 - Quantum efficiency

0

0.5

1

1.5

2

2.5

3

3.5

4

350 400 450 500 550 600 650 700

Wavelength (Å)

QE (%)


First tests in darkroom


Linearity and ResolutionResidual to linear fit

-6

-4

-2

0

2

4

6

8

0 2 4 6 8 10 12 14 16

pinhole

Residual (microns)

Resolution vs. Position

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 50 100 150 200 250

X Position (pixels)

Resolution (FWHM, pixels)

Projected pinhole pattern

(1 x 2 mm)


Flat field

• White light

• 66 MHz input rate

• No optic

• No “hex” pattern

• Black pixels are masked in Medipix2

• Locally uniform

20 µs 50 s


Fixed pattern noise

Single Flat Histogram

0

200

400

600

800

1000

1200

1400

1600

1800

0 20000 40000 60000 80000 100000

Events per pixel

Frequency

Ratio Histogram

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 20000 40000 60000 80000 100000

Events per pixel

Frequency

Ratio Histogram

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0.95 0.97 0.99 1.01 1.03 1.05

Ratio (Flat1/Flat2)

Frequency

SNR > 200


Old WWII watch movie


Old WWII watch movie 2

(radium dial)

Bkgd

.002 ct/pxl/s

Room Temp


Gain and Event Threshold

AO-02 tube Gain vs. Voltage

0

5

10

15

20

25

1600 1650 1700 1750 1800 1850

Voltage across MCPs

Gain (kiloelectrons)


Gain and Event Threshold

AO-02 tube Detected events/frame

0

50

100

150

200

250

300

350

400

450

1600 1650 1700 1750 1800 1850

Voltage across MCPs

Events/frame (10 microsec)


Electronic shutter and diode laser

No shutter

All images: room lights, 1kHz pulsed laser and 1 sec integ.

Stretched by 400

1.5 µs shutter


Medipix3 - late 2008

• 0.13 µm CMOS technology

• Twice as many transistors in pixel

• Concurrent readout/integration

• Serial readout at 250MHz clock

• Up to 10,000 frames/sec


Summary and future work

• Demonstration of successful use of Medipix2 in sealed MCP tube

• More laboratory and telescope tests to be done

• Working with Photonis to incorporate our various readout technologies into their tubes– Delayline, cross strip, Medipix, Timepix, etc.– Standard industrial design– Better and more consistent photocathodes

• UV, neutron imaging tubes


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


City Movie - 1 Cycle Line Frequency

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.


Extra slides


Timepix version of Medipix

Amplitude rather than counts using “time over threshold’ technique

If charge clouds are large, can determine centroid to sub-pixel accuracy

Tradeoff is count rate as event collisions in frame destroy centroid information

Single UV photon events


Original Medipix mode readout (UV)

Zoom 256 x 256

(14 mm)


Factor of 8 improved resolution!

256 x 256 converted to 4096x4096 pixels (3.4µm pixels)


Centroid error vs. input fluence

Centroid estimator error vs. technique

0.100

1.000

10.000

100.000

1 10 100 1000

Input number of photons

Centroid Error (rms, radians)

CCD Quad cellCCD 8x8 weightedCCD 6x6 weightedMedipix 8x8 weighted

SPIE Instr. for Astronomy, Marseille, John Vallerga, [email protected] Optically sensitive MCP...

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Transcript of SPIE Instr. for Astronomy, Marseille, John Vallerga, [email protected] Optically sensitive MCP...