A noiseless kilohertz frame rate imaging detector based on microchannel plates read out with the Medipix2 CMOS chip - A new wavefront sensor for adaptive.

A noiseless kilohertz frame rateA noiseless kilohertz frame rateimaging detector based onimaging detector based on

microchannel plates read outmicrochannel plates read outwith the Medipix2 CMOS chipwith the Medipix2 CMOS chip

--A new wavefront sensorA new wavefront sensor

for adaptive opticsfor adaptive optics

Bettina Mikulec, A.G. Clark, D. Ferrère, D. La Marra University of Geneva

J.B. McPhate, O.H.W. Siegmund, A.S. Tremsin, J.V. Vallerga

Space Science Laboratory, University of California

J. Clément, C. Ponchut, J.-M. Rigal ESRF Grenoble

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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

Point focus blur

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

Palomar Hale 5m telescope

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

currently 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

- integrated into a vacuum tube

ITT Industries

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Tube FabricationTube Fabrication

GaAs photo-cathode

Medipix2chip

MCP pair

• Design finished• Ceramics chip carrier (=

tube backend) received from fabrication

• Bonding tests underway

34 mm

12

.7 m

m

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The Medipix2 Photon Counting ChipThe Medipix2 Photon Counting Chip

256 x 256 pixel array (pixel size 55 µm square)

Amplify, discriminate, gate and count in 14-bit counter per pixel (count rate: ~1 MHz/pixel = 0.33 GHz/mm2)

Readout is digital -> noiseless and fast (266 µs / frame)

3-side buttable -> 512 x 512 pixel array

singlepixel:

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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• A standard UV Hg pen-ray lamp with collimator

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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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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

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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• Measured dynamic range: ~1 cps to 500M cps• 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 commercial firm (with bi-alkali and GaAs photo-cathode)

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

continuous frame rate with a 2x2 chip arrangement

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

• Final test at a telescope

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Backup Slides!

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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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Parallel Readout BoardParallel Readout Board

• Successful review of the new parallel readout board PRIAM in May

– Five 32-bit parallel input ports to read out up to 5 Medipix2 chips in <290 s (clock 100 MHz)

– XILINX FPGA for data arrangement, optional flat field and dead time correction as well as data reduction (e.g. spot coordinates)

– Provides all control signals and voltages

– 4 bi-directional 1.6 Gbit/s links total data output time 660 s

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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

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Rear Field (V)

Spot Area (pixel)

Gain 25k

Gain 50k

Gain 100k

Gain 200k

Gain 400k

Rear Field = 1600V

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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

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0.1 1

CsBr

KI

Energy (keV)

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EUV and FUVEUV and FUV

0

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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)

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

0.1

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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)

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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

are needed to see this picture.

(… with silicon detector)