Adaptive Optics for Ground-based Telescopes Using Noiseless Kilohertz Frame Rate Imaging Detector

1.   Point-like objects get blurred by turbulences in the atmosphere. WFS measure the atmospheric turbulences, send this information to deformable mirrors that compensate online for the distortions.  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. take 2 indepen- dent uniform illuminations (flood fields) histogram Histogram of ratio is consistent with counting statistics. Flood field (500 Mcps). Ratio = flood1 / flood2. increase shutter time Group 3-2 visible ~9 lp/mm. Test pattern; 100 s exposure. The spots correspond to individual photon events. Test pattern; 1 s exposure. Spot area versus rear field. Spot area versus Medipix2 low threshold. 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 - University of Geneva, Switzerland J.B. McPhate, O.H.W. Siegmund, A.S. Tremsin, J.V. Vallerga - SSL Berkeley, USA J. Clement, C. Ponchut, J.-M. Rigal - ESRF Grenoble, France • 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/ • Detector Concept • High-QE photocathode (GaAs) converts incoming photons. • Two microchannel plates (MCP) in chevron configuration amplify photo-electron (gain between several thousands to millions). • Charge cloud gets detected by the Medipix2 pixel circuits. If detected charge > threshold --> pixel counter gets incremented. • 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 • 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 GaAs photo- cathode MCP pair Medipix2 chip