Measurement Results• Detector concept works!

• Flood fields show MCP fixed pattern noise that divides out

• Spatial resolution consistent with theory (Nyqvist sampling of 55 m pixels)

• Parameters can be tuned to optimize spot size

Detector Concept1. High-QE photocathode (GaAs) converts incoming

photons.

2. Two microchannel plates (MCP) in chevron configuration amplify photo-electron (gain between several thousands to millions).

3. Charge cloud gets detected by the Medipix2 pixel circuits. If detected charge > threshold --> pixel counter gets incremented.

4. Noiseless chip readout after programmable shutter time.

Medipix2* photon counting pixel readout ASIC:

• 256 x 256 pixels, 55 m square

• window discriminator, 14-bit counter per pixel

• 3-side buttable (512 x 512 arrays), serial (LVDS) or parallel readout (32-bit CMOS bus; 266 s @ 100 MHz)

• ~500 transistors/pixel; 0.25 m CMOS technology• Developed within the framework of the Medipix Collaboration; http://medipix

.web.cern.ch/MEDIPIX/

Tube Fabrication for WFS

‘PRIAM’ Parallel Readout Board

• Five 32-bit parallel input ports to read out max. 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 readout time 660 s

Motivation• Adaptive Optics are indispensable for new generation ground-based telescopes!

• Detectors for wave-front sensors (WFS) require large pixel arrays (512 x 512), noise <3e- per pixel, high quantum efficiency (QE) and kHz frame rates*

• Current CCDs have high QE, but must trade noise performance and array size for speed

* Angel, R. et al. ‘A Roadmap for the Development of Astronomical Adaptive Optics’, July 6, 2000; http://www.noao.edu/dir/ao/

A Noiseless Kilohertz Frame Rate Imaging Detector Based on Microchannel Plates Read out

with the Medipix2 CMOS Pixel Chip

Point-like objects get blurredby turbulences in the atmosphere.

WFS measure the atmospheric turbulences, send this informationto deformable mirrors that compensate online for the distortions.

Shack-Hartman correction methodusing an equally spaced lenslet array.

AO reveals a previously undetected star in the Orion cluster. Thesurface plot shows the dramatic increase in intensity and sharpness.

take 2 indepen-dent uniform illuminations(flood fields) histogram

Flood field (500 Mcps). Ratio = flood1 / flood2.Histogram of ratio is consistentwith counting statistics.

increaseshutter time

Test pattern; 100 s exposure.The spots correspond to individualphoton events.

Test pattern; 1 s exposure.Group 3-2 visible~9 lp/mm.

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.

A.G. Clark, D. Ferrère, D. La Marra, A.G. Clark, D. Ferrère, D. La Marra, B. MikulecB. Mikulec - University of Geneva, Switzerland- University of Geneva, SwitzerlandJ.B. McPhate, O.H.W. Siegmund, A.S. Tremsin, J.V. Vallerga J.B. McPhate, O.H.W. Siegmund, A.S. Tremsin, J.V. Vallerga - SSL Berkeley, USA- SSL Berkeley, USAJ. Clement, C. Ponchut, J.-M. Rigal J. Clement, C. Ponchut, J.-M. Rigal - ESRF Grenoble, France- ESRF Grenoble, France

GaAs photo-cathode

MCP pairMedipix2chip