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A Noiseless Kilohertz Frame Rate Imaging Detector Based on Microchannel Plates Read out with the Medipix2 CMOS Pixel Chip A.G. Clark, D. Ferrère, D. La Marra, B. Mikulec J.B. McPhate, O.H.W. Siegmund, A.S. Tremsin, J.V. Vallerga J. Clement, C. Ponchut, J.-M. Rigal - University of Geneva, Switzerland - SSL Berkeley, USA - ESRF Grenoble, France Motivation • Adaptive Optics are indispensable for new generation groundbased 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 Detector Concept 1. 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. Photocathode Photon e- Q = 104e- Pij = Pij + 1 Window Medip ix2 MCP Point-like objects get blurred WFS measure the atmospheric turbulences, send this information by turbulences in the atmosphere. to deformable mirrors that compensate online for the distortions. Medipix2* photon counting pixel readout ASIC: Shack-Hartman correction method using an equally spaced lenslet array. AO reveals a previously undetected star in the Orion cluster. The surface plot shows the dramatic increase in intensity and sharpness. * Angel, R. et al. ‘A Roadmap for the Development of Astronomical Adaptive Optics’, July 6, 2000; http://www.noao.edu/dir/ao/ Tube Fabrication for WFS • 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/ Measurement Results • Detector concept works! • Flood fields show MCP fixed pattern noise that divides out GaAs photoMedipix2 cathode chip take 2 independent uniform illuminations (flood fields) MCP pair ‘PRIAM’ Parallel Readout Board Flood field (500 Mcps). histogram Histogram of ratio is consistent with counting statistics. Ratio = flood1 / flood2. • Spatial resolution consistent with theory (Nyqvist sampling of 55 m pixels) increase shutter time • provides all control signals and voltages Rear Field = 1600V Spot Area vs Rear Field 20 40 Gain 25k 35 Gain 50k 18 Mean Spot Area (pixel) • XILINX FPGA for data arrangement, optional flat field and dead time correction as well as data reduction (e.g. spot coordinates) Test pattern; 1 s exposure. • Parameters can be tuned to optimize spot size Gain 100k 30 Spot Area (pixel) • Five 32-bit parallel input ports to read out max. 5 Medipix2 chips in <290 s (clock 100 MHz) Test pattern; 100 s exposure. The spots correspond to individual photon events. Gain 200k 25 Gain 400k 20 15 10 5 • 4 bi-directional 1.6 Gbit/s links total readout time 660 s Group 3-2 visible ~9 lp/mm. 16 G=20k, Area 14 G=50k, Area 12 G=100k, Area 10 G=200k, Area 8 6 4 2 0 0 200 400 600 800 1000 1200 1400 Rear Field (V) Spot area versus rear field. 1600 0 0 5 10 15 20 25 30 35 40 Lower Threshold (ke -) Spot area versus Medipix2 low threshold.