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Optically sensitive Medipix2 detector for adaptive optics in very large telescopes John Vallerga, Jason McPhate, Anton Tremsin and Oswald Siegmund Space Sciences Laboratory, University of California, Berkeley Bettina Mikulec and Allan Clark University of Geneva IWORID 2004 Glasgow - J. Vallerga Adaptive optics tutorial* Turbulence in earth’s atmosphere makes stars twinkle More importantly, turbulence spreads out light; makes it a blob rather than a point *Adapted from AO lectures of Claire Max, Astro 289C, UC Santa Cruz Even the largest ground-based astronomical telescopes have no better resolution than a 20 cm telescope! IWORID 2004 Glasgow - J. Vallerga Optical consequences of turbulence • Temperature fluctuations in small patches of air cause changes in index of refraction (like many little lenses) • Light rays are refracted many times (by small amounts) • When they reach telescope they are no longer parallel • Hence rays can’t be focused to a point: Point focus Parallel light rays blur Light rays affected by turbulence IWORID 2004 Glasgow - J. Vallerga How a deformable mirror works (idealization) BEFORE Incoming Wave with Aberration AFTER Deformable Mirror IWORID 2004 Glasgow - J. Vallerga Corrected Wavefront Adaptive optics increases peak intensity of a point source Lick Observatory No AO With AO Intensity With AO No AO IWORID 2004 Glasgow - J. Vallerga Schematic of adaptive optics system Feedback loop: next cycle corrects the (small) errors of the last cycle Optical Medipix tube goes here IWORID 2004 Glasgow - J. Vallerga Lick adaptive optics system at 3m Shane Telescope DM Wavefront sensor Off-axis parabola mirror IWORID 2004 Glasgow - J. Vallerga IRCAL infrared camera How to measure turbulent distortions (one method among many) “Shack-Hartman” WFS IWORID 2004 Glasgow - J. Vallerga The new generation: adaptive optics on 8-10 m telescopes Summit of Mauna Kea volcano in Hawaii: Subaru 2 Kecks ESO VLT Gemini South Gemini North And at other places: MMT, VLT, LBT, Gemini South IWORID 2004 Glasgow - J. Vallerga Neptune in infra-red light (1.65 microns) With Keck adaptive optics 2.3 arc sec Without adaptive optics May 24, 1999 June 27, 1999 IWORID 2004 Glasgow - J. Vallerga VLT NAOS AO first light Cluster NGC 3603: IR AO on 8m ground-based telescope achieves same resolution as HST at 1/3 the wavelength Hubble Space Telescope WFPC2, = 800 nm NAOS AO on VLT = 2.3 microns IWORID 2004 Glasgow - J. Vallerga Faint companions around bright stars Two images from Palomar of a brown dwarf companion to GL 105 Credit: David Golimowski IWORID 2004 Glasgow - J. Vallerga End Tutorial Vision Science End Tutorial IWORID 2004 Glasgow - J. Vallerga 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 IWORID 2004 Glasgow - J. Vallerga WFS detector requirements • High optical QE for dimmer guide stars • Lots of pixels - eventually 512 x 512 • Very low readout noise • kHz frame rates The last three are not simultaneously achievable with the current generation of CCDs IWORID 2004 Glasgow - J. Vallerga MCP Detectors at SSL Berkeley COS FUV for Hubble (200 x 10 mm windowless) 25 mm Optical Tube GALEX 68 mm NUV Tube (in orbit) IWORID 2004 Glasgow - J. Vallerga Imaging, Photon Counting Detectors Photocathode converts photon to electron MCP(s) amplify electron by 104 to 108 Rear field accelerates electrons to anode Patterned anode measures charge centroid IWORID 2004 Glasgow - J. Vallerga Photocathode type determines wavelength response • Soft x-ray to near IR GaAs Photocathodes (GenIII) IWORID 2004 Glasgow - J. Vallerga Wavefront Sensor Event Rates (example for big telescope) • 5000 centroids • Kilohertz feedback rates (atmospheric timescale) • 1000 detected events per spot for sub-pixel centroiding 5000 x 1000 x 1000 = 5 Gigahertz counting rate! • Requires integrating detector IWORID 2004 Glasgow - J. Vallerga Our concept • An optical imaging tube using: – GaAs photocathode – Microchannel plate to amplify a single photoelectron by 104 – Bare Medipix2 to count these events per pixel Photocathode Photon e- Q = 104e- Pij = Pij + 1 Window IWORID 2004 Glasgow - J. Vallerga MCP Medipix2 Vacuum Tube Design IWORID 2004 Glasgow - J. Vallerga Vacuum Tube Design IWORID 2004 Glasgow - J. Vallerga Vacuum Tube Design IWORID 2004 Glasgow - J. Vallerga Vacuum Tube Design IWORID 2004 Glasgow - J. Vallerga First test detector • Demountable detector • Simple lab vacuum, no photocathode • Windowless – UV sensitive IWORID 2004 Glasgow - J. Vallerga Initial Results It Works! First light! Lower gain, higher rear field IWORID 2004 Glasgow - J. Vallerga MCP event spot area 200V Rear Field = 1600V 20 Mean Spot Area (pixel) 18 16 G=20k, Area G=20k, Area G=50k, Area G=50k, Area 14 G=100k, Area G=100k, Area G=200k, Area G=200k, Area 12 10 8 6 4 2 0 0 5 10 15 20 25 30 - Lower Lower Threshold Threshold (ke (ke ) IWORID 2004 Glasgow - J. Vallerga 35 40 MCP charge cloud size 1600V rear field 160 Normalized Charge 140 200K 100K 50K 20K 120 100 80 60 40 20 0 0 20 40 60 80 100 R (microns) IWORID 2004 Glasgow - J. Vallerga 120 140 Spatial Resolution 100 µs 1s IWORID 2004 Glasgow - J. Vallerga Group 3-2 visible 9 lp/mm = 55µm (Nyquist limit) Interesting tangent - sub pixel resolution •Use single spot events and calculate centroids •Accumulate event x,y list •2-d histogram on finer pitch 9 lp/mm IWORID 2004 Glasgow - J. Vallerga Interesting tangent - sub pixel resolution • Calculate centroids of each event • Accumulate event x,y list • 2-d histogram on finer pitch 16 lp/mm IWORID 2004 Glasgow - J. Vallerga Flat Field MCP deadspots Hexagonal multifiber boundaries 1200 cts/bin - 500Mcps IWORID 2004 Glasgow - J. Vallerga Flat Field (cont) Ratio Flat1/Flat2 Histogram of Ratio consistent with counting statistics (2% rms) IWORID 2004 Glasgow - J. Vallerga Future Work (3 yr. NOAO grant) • Optimize MCP-Medipix2 interface design • Design and build tube with Medipix2 and GaAs • Develop parallel readout with European collaborators • Develop FPGA to reduce output bandwidth – 5 million centroids/s vs. 262 million pixels/s. • Test at AO laboratory at CFAO, U.C. Santa Cruz • Test at telescope IWORID 2004 Glasgow - J. Vallerga Acknowledgements This work was funded by an AODP grant managed by NOAO and funded by NSF Thanks to the Medipix Collaboration: • Univ. of Barcelona • University of Napoli • University of Cagliari • NIKHEF • CEA • University of Pisa • CERN • University of Auvergne • University of Freiburg • Medical Research Council • University of Glasgow • Czech Technical University • Czech Academy of Sciences • ESRF • Mid-Sweden University • University of Erlangen-Nurnberg IWORID 2004 Glasgow - J. Vallerga Soft X-Ray Photocathodes Quantum Detection Efficiency (%) 100 Cs Br KI 80 60 40 20 0 0.1 Energy (keV) IWORID 2004 Glasgow - J. Vallerga 1 EUV and FUV CsI 1985 vs 1999 0.7 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° 0.6 0.5 QDE 0.4 0.3 0.2 0.1 0 0 500 1000 1500 Wavelength (Å) IWORID 2004 Glasgow - J. Vallerga 2000 GaN UV Photocathodes, 1000- 4000Å IWORID 2004 Glasgow - J. Vallerga Isoplanatic Angle (0) & Sky Coverage h IWORID 2004 Glasgow - J. Vallerga Telescope Primary mirror Can achieve >70% sky coverage with laser guide star adaptive optics! IWORID 2004 Glasgow - J. Vallerga Laser Guide Star Parallax • “Star” more of a streak • Shape changes over pupil • Can use pulsed laser to limit spatial extent • Requires gated detector 589.2 nm L d IWORID 2004 Glasgow - J. Vallerga Deformable mirrors come in many sizes • Range from 13 to > 900 actuators (degrees of freedom) ~ 300mm ~ 50 mm Xinetics IWORID 2004 Glasgow - J. Vallerga 30 m telescope capability IWORID 2004 Glasgow - J. Vallerga