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Bettina Mikulec, Allan Clark University of Geneva

A novel high resolution, high frame rate detector based on a microchannel plate read out with the Medipix2 counting CMOS pixel chip. Bettina Mikulec, Allan Clark University of Geneva John Vallerga, Jason McPhate, Anton Tremsin, Oswald Siegmund

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Bettina Mikulec, Allan Clark University of Geneva

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  1. A novel high resolution, high frame rate detectorbased on amicrochannel plate read out with the Medipix2 counting CMOS pixel chip Bettina Mikulec, Allan Clark University of Geneva John Vallerga, Jason McPhate, Anton Tremsin, Oswald Siegmund Space Science Laboratory, University of California

  2. Introduction* • 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 Synchrotron Radiation Workshop, Rome, 22 October 2004 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

  3. Point focus  Light rays affected by turbulence Parallel light rays  blur Adaptive Optics proposal for a new WFS - Optical Medipix tube Synchrotron Radiation Workshop, Rome, 22 October 2004

  4. Adaptive Optics Synchrotron Radiation Workshop, Rome, 22 October 2004 Example for the enormous improvements using AO (Lick Observatory).

  5. Adaptive Optics • 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. Synchrotron Radiation Workshop, Rome, 22 October 2004 Shack-Hartmann wavefront sensor

  6. Wavefront Sensor Requirements • High QE for dimmer guide stars (~80% optical QE) • 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. • 1 kHz frame rate (light integration, readout, calculations, send out 5000 signals and ready for new frame) corresponding to the timescale of the atmospheric turbulences • Very low readout noise (< 3e-) • Gate the detector in 2-4 s range for operation with laser guide stars Synchrotron Radiation Workshop, Rome, 22 October 2004 • Large pixel array, high frame rate and no readout noise • not simultaneously achievable with CCDs!

  7.   2 µm pores on 3 µm centers (Burle Industries) Proposal 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 • Noiseless chip readout Synchrotron Radiation Workshop, Rome, 22 October 2004

  8. The Medipix2 Photon Counting Chip • 0.25 m CMOS technology (33M transistors/chip) • square pixel size of 55 µm • 256 x 256 pixels • sensitive to positive or negative input charge (free choice of different detector materials) • pixel-by-pixel detector leakage current compensation • window in energy • discriminators designed to be linear over a large range • 14-bit counter per pixel • count rate: ~1 MHz/pixel (0.33 GHz/mm2) • 3-side buttable • serial or parallel I/O (min. readout time of full matrix 266 µs) Synchrotron Radiation Workshop, Rome, 22 October 2004

  9. Measurement 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) Synchrotron Radiation Workshop, Rome, 22 October 2004

  10. Feasibility Tests • Event size function of MCP gain, rear field, MCP-Medipix distance and Medipix threshold 06 April 2004 single photon events Synchrotron Radiation Workshop, Rome, 22 October 2004 gain 106, rear field 427 V gain 50k, rear field 980 V It works!

  11. Flood 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) Ratio = flood1 / flood2. Histogram of ratio consistent with counting statistics (rms 0.02) Synchrotron Radiation Workshop, Rome, 22 October 2004

  12. Resolution • The Air Force test pattern was used to demonstrate the imaging properties of the detector, in particular the resolution. increase shutter time Synchrotron Radiation Workshop, Rome, 22 October 2004 100 s exposure; the spots correspond to individual photon events. 1 s exposure. Group 3-2 visible (~9 lp/mm corresponding to the Nyquist limit of 55 m pixels)

  13. Event 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. centroiding Synchrotron Radiation Workshop, Rome, 22 October 2004 Group 4-2 starts to be resolved (17.95 lp/mm; 55.7 m corresponding to ~28 m pixels). Could be useful for low rate imaging applications!

  14. UV Photon Counting Movie Synchrotron Radiation Workshop, Rome, 22 October 2004 Air Force resolution mask, 100 ms exposures

  15. Electron 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. Gain ~60k, rear field 1600 V Medipix threshold ~38 ke- Synchrotron Radiation Workshop, Rome, 22 October 2004 63Ni, 67 keV max. ~300 counts/pixel 204Tl, 764 keV max. ~100 counts/pixel

  16. Conclusions • New detector concept proven to work! • Systematic tests varying different detector parameters underway • 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 Synchrotron Radiation Workshop, Rome, 22 October 2004

  17. Future Plans • Tube fabricationat SSL Berkeley and at commercial firm; finalize ceramic chip carrier design • Specific parallel readout board to be 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 telescope SSL received 3-year NOAO grant, 2 more years to go… • Are there other applications for such a detector? • Beam monitor for hadron therapy, readout for Cherenkov counter, detector for X-ray microscopes at synchrotron…??? Synchrotron Radiation Workshop, Rome, 22 October 2004

  18. Backup Slides! Synchrotron Radiation Workshop, Rome, 22 October 2004

  19. The Setup at SSL - Photos Synchrotron Radiation Workshop, Rome, 22 October 2004

  20. Soft X-Ray Photocathodes Synchrotron Radiation Workshop, Rome, 22 October 2004

  21. EUV and FUV Synchrotron Radiation Workshop, Rome, 22 October 2004

  22. GaN UV Photocathodes, 1000- 4000Å Synchrotron Radiation Workshop, Rome, 22 October 2004

  23. h Isoplanatic Angle (0) & Sky Coverage Bright stars + 0 =1% sky coverage Synchrotron Radiation Workshop, Rome, 22 October 2004 Telescope Primary mirror

  24. Laser Guide Stars Can achieve>70% skycoverage withlaserguidestaradaptive optics! Synchrotron Radiation Workshop, Rome, 22 October 2004

  25. 589.2 nm L d Laser Guide Star Parallax • “Star” more of a streak • Shape changes over pupil • Can use pulsed laser to limit spatial extent • Requires gated detector Synchrotron Radiation Workshop, Rome, 22 October 2004

  26. Advantages of Multi-Pixel Sampling of Shack Hartmann Spots Synchrotron Radiation Workshop, Rome, 22 October 2004 5 x 5 2 x 2 • Linear response off-null • Insensitive to input width • More sensitive to readout noise

  27. Deformable Mirrors • Range from 13 to > 900 actuators (degrees of freedom) ~ 300mm Synchrotron Radiation Workshop, Rome, 22 October 2004 ~ 50 mm Xinetics

  28. Next Generation of Large Telescopes (proposed) 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 Synchrotron Radiation Workshop, Rome, 22 October 2004

  29. (G. Molinari, CERN) collection of secondary electrons (SE) emitted by 0.1 – 0.4 mm aluminum (Al-Al2O3-Al) foils Electron Detection • One example for different application than AO: Beam monitor for hadron therapy Synchrotron Radiation Workshop, Rome, 22 October 2004 slide from W. Dulinsky, presented at IWORID2004 Glasgow; SUCIMA project.

  30. Lamp Pinhole Detector Sub-Pixel Spatial Linearity Synchrotron Radiation Workshop, Rome, 22 October 2004

  31. Average Movement of 700 Spots 1 pixel Synchrotron Radiation Workshop, Rome, 22 October 2004

  32. Position Error (550 Events/Spot) rms = 2.0 µm Synchrotron Radiation Workshop, Rome, 22 October 2004

  33. Spot Size Synchrotron Radiation Workshop, Rome, 22 October 2004 Spot area versus Medipix2 low threshold. Spot area versus rear field.

  34. X-ray of Fish (… with silicon detector) Synchrotron Radiation Workshop, Rome, 22 October 2004

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