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Photon CountingPhoton Countingwith a 512 x 512 Pixel Arraywith a 512 x 512 Pixel Array

at 1 kHz Frame Rateat 1 kHz Frame Rate--

A New Wavefront SensorA New Wavefront Sensorfor Adaptive Opticsfor Adaptive Optics

Bettina Mikulec, Allan ClarkUniversity of Geneva

John Vallerga, Jason McPhate, Anton Tremsin, Oswald Siegmund

Space Science Laboratory, University of California

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Introduction into Adaptive Optics*Introduction into Adaptive Optics*

• Turbulence in the earth’s atmosphere makes stars twinkle

• More importantly, turbulence spreads out the star light making it a blob rather than a point– Temperature fluctuations in the

air cause changes in the index of refraction

– light rays are no longer parallel when they reach telescope and can therefore not anymore be focused to a point

Even the largest ground-based astronomical telescopes have no better resolution than an 8"

telescope!

Even the largest ground-based astronomical telescopes have no better resolution than an 8"

telescope!*adapted from AO lectures of Claire Max, Astro289C, UC Santa Cruz

blurPoint focus

Parallel light raysLight rays affected by turbulence

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Adaptive OpticsAdaptive Optics

proposal for anew WFS -

Optical Medipix tube

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Example for the enormous improvements using AO(Lick Observatory).

Adaptive OpticsAdaptive Optics

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The new generation: adaptive optics on 8-10 m The new generation: adaptive optics on 8-10 m telescopestelescopes

Summit of Mauna Kea volcano in Hawaii:

Subaru

2 Kecks

Gemini North

And at other places: MMT, VLT, LBT, Gemini South

ESO VLT

Gemini South

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30 m diameter:– California Extremely Large Telescope (CELT) -– Thirty Meter Telescope (TMT)

50 m diameter:– EURO50 on La Palma

100 m diameter:– European Southern Observatory’s “OverWhelmingly Large

Telescope” (OWL)

All propose AO systems with > 5000 actuators

Palomar Hale 5m telescope

Next Generation of Large Telescopes Next Generation of Large Telescopes (proposed)(proposed)

TMT

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• Determine the distortions with the help of a natural or laser guide star and a lenslet array (one of the methods). Deviations of the spot positions from a perfect grid is a measure for the shape of the incoming wave-front.

Adaptive OpticsAdaptive Optics

Shack-Hartmann wavefront sensor

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Wavefront Sensor RequirementsWavefront Sensor Requirements

• High QE for dimmer guide stars (~80% optical QE)• Gate the detector in 2-4 s range for operation with laser

guide starso Many pixels in the order of 512 x 512; future large

telescopes will have about 5000 actuators (controlled via 70 x 70 centroid measurements)

• 1000 photons per spot to get a 3% centroid rms error with respect to the stellar image size.

o 1 kHz frame rate (light integration, readout, calculations, send out 5000 signals and ready for new frame); faster than the timescale of the atmospheric turbulences

o Very low readout noise (< 3e-)

Large pixel array, high frame rate and no readout noise

not simultaneously achievable with CCDs!

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2 µm pores on 3 µm centers (Burle Industries)

Proposal for a New Wavefront SensorProposal for a New Wavefront Sensor

High-QE GaAs photo-cathode Matched pair of microchannel plates (MCP) with 10 m

pore diameter in chevron configuration Medipix2 counting CMOS pixel chip Integrate photon events on pixel,

noiseless chip readout

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The Medipix2 Photon Counting ChipThe Medipix2 Photon Counting Chip

0.25 m CMOS technology (33M transistors/chip)square pixel size of 55 µm 256 x 256 pixelssensitive to positive or negative input charge (free choice of different detector materials)pixel-by-pixel detector leakage current compensation window in energydiscriminators designed to be linear over a large range14-bit counter per pixel count rate: ~1 MHz/pixel (0.33 GHz/mm2)3-side buttableserial or parallel I/O (min. readout time of full matrix 266 µs)

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Measurement SetupMeasurement Setup

• A Medipix2 photon counting chip • A matched pair of MCPs:

– Photonis MCPs with 33 mm diameter– 10 m hole diameters, L/D = 40/1– low resistivity (~22 MOhms per plate)– gain was varied between 20k and 200k (1430 - 1680 V)

• Vacuum tank pumped down to ~10-6 torr• Hermetic feed-throughs (50-pin connector for Medipix

signals)• A standard UV Hg pen-ray lamp with collimator (~10

counts/s -500M counts/s)

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Feasibility TestsFeasibility Tests

single photonevents

gain 106, rear field 427 V gain 50k, rear field 980 V

It works!It works!

06 April 2004

• Event size function of MCP gain, rear field, MCP-Medipix distance and Medipix threshold

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Flood FieldsFlood Fields

• Take image with collimated UV source at 50ke gain and 1600 V rear field (~5000 counts/pixel). Average single spot area: 2.4 pixels

– Fixed pattern noise from dead spots on the MCPs and MCP multifibres divides out.

take 2 independent uniform illuminations(flood fields at ~500Mcps)

Histogram of ratio consistent with countingstatistics (rms 0.02)

Ratio = flood1 / flood2.

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ResolutionResolution

• The Air Force test pattern was used to demonstrate the imaging properties of the detector, in particular the resolution.

increaseshuttertime

100 s exposure; the spotscorrespond to individualphoton events.

1 s exposure. Group 3-2 visible (~9 lp/mmcorresponding to the Nyquistlimit of 55 m pixels)

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Event CentroidingEvent Centroiding

• Centroiding individual photon events to achieve sub-pixel resolution:– Take many very low count rate images with larger spot area

to avoid overlapping spots. (~100-150 counts/frame; 1000 frames)

– Identify unique spots and reject overlapping events (counts 2), count spots, record their size and calculate the centroids.

Could be useful forlow rate imaging applications!

centroiding

Group 4-2 starts to be resolved(17.95 lp/mm; 55.7 m corresponding to ~28 m pixels).

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UV Photon Counting MovieUV Photon Counting Movie

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Air Force resolution mask, 100 ms exposures

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Spot Size vs. GainSpot Size vs. Gain

Pinhole grid mask (pitch 0.5 mm x 0.5 mm) to simulate Shack-Hartmann spots:

Rear field: 1600V, gap 500 m, threshold 3 ke-

Gain: 200 000 Gain: 20 000

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Sub-Pixel Spatial LinearitySub-Pixel Spatial Linearity

LampPinhole

Detector

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Average Movement of ~700 SpotsAverage Movement of ~700 Spots

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

0 5 10 15 20 25

Lamp Position (mm)

Centroid Position (µm)

Delta X

Delta Y

1 pixel

• Achieved a 2 m rms centroid position error with ~550 events/spot.

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Electron DetectionElectron Detection

• First test results with beta sources

QE ~46% for Ni and ~63% for the Tl image;increasing efficiency with e- energies above ~50 keV consistent with literature.

63Ni, 67 keV max.~300 counts/pixel

Gain ~60k,rear field 1600 VMedipix threshold~38 ke-

204Tl, 764 keV max.~100 counts/pixel

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ConclusionsConclusions

• New detector concept proven to work!• Performed systematic tests varying different

detector parameters• No fixed pattern noise yet detectable except

MCP imperfections• Resolution at Nyquist limit and below (for event-

by-event centroiding) demonstrated• Images presented with both UV and electron

sources detector has a great capacity to be used for various wavelengths and particles

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Ongoing workOngoing work

• Test new ceramic chip carrier (= tube backend); thermal cycling tests with Medipix2 chip mounted

• Tube fabrication at SSL Berkeley and at commercial firm with GaAs photo-cathode

• Test prototype parallel readout board designed in collaboration with ESRF– reduce output bandwidth by using an FPGA; goal: 1 kHz

continuous frame rate with 2x2 chip arrangement

• Test prototype tubes at the AO laboratory at CFAO, U.C. Santa Cruz

• Final test at a telescopeSSL received 3-year NOAO grant, 1.5 more years to go…, some funding for Univ. Geneva from Fonds National and F. Schmidheiny

• Consider other applications for such a detector

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Parallel Readout BoardParallel Readout Board

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Tube DesignTube Design

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Tube DesignTube Design

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Backup Slides!

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The Setup at SSL - PhotosThe Setup at SSL - Photos

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Spot SizeSpot Size

Spot Area vs Rear Field

0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200 1400 1600

Rear Field (V)

Spot Area (pixel)

Gain 25k

Gain 50k

Gain 100k

Gain 200k

Gain 400k

Rear Field = 1600V

0

2

4

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8

10

12

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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

Spot area versus rear field.Spot area versus Medipix2 low threshold.

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Soft X-Ray PhotocathodesSoft X-Ray Photocathodes

0

20

40

60

80

100

0.1 1

CsBr

KI

Energy (keV)

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EUV and FUVEUV and FUV

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 500 1000 1500 2000

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)

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GaN UV Photocathodes, 1000- GaN UV Photocathodes, 1000- 4000Å4000Å

0.1

1

10

100

150 200 250 300 350 400

NW-BH071#3

NW-BH071#2

NW-JG238#3

NW-JG238#2

Quantum Efficiency (%)

Wavelength (nm)

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hBright st

ars + 0

= 1% sk

y

coverage

Isoplanatic Angle (Isoplanatic Angle (00) & Sky Coverage) & Sky Coverage

TelescopePrimarymirror

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Laser Guide StarsLaser Guide Stars

Can achieve>70% sky

coverage withlaser guide staradaptive optics!

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Laser Guide Star ParallaxLaser Guide Star Parallax

d

L

589.2 nm

• “Star” more of a streak• Shape changes over pupil• Can use pulsed laser to limit

spatial extent• Requires gated detector

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Advantages of Multi-Pixel Sampling of Shack Advantages of Multi-Pixel Sampling of Shack Hartmann SpotsHartmann Spots

Linear response off-nullInsensitive to input widthMore sensitive to readout noise

2 x 22 x 2 5 x 55 x 5

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Deformable MirrorsDeformable Mirrors

• Range from 13 to > 900 actuators (degrees of freedom)

Xinetics~ 50 mm

~ 300mm

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Position Error (550 Events/Spot) Position Error (550 Events/Spot)

0

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50

-20 -15 -10 -5 0 5 10 15 20

Centroid difference (microns)

Number of centroids

rms = 2.0 µm

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Medipix readout of semiconductor Medipix readout of semiconductor arraysarrays

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X-ray of FishX-ray of Fish

QuickTime™ and aCinepak decompressor

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(… with silicon detector)