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A High Resolution, High Frame Rate Detector

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tocA High Resolution, High Frame Rate Detector Based on a MicrochanBettina Mikulec, Member, IEEE, John V. Vallerga, Jason B. McPhatI. I NTRODUCTIONFig.€1. Schematic diagram of an AO system. The distorted wavefroFig.€2. Shack Hartmann wavefront sensing method: An input plane A. Wavefront Sensor Requirements for Future Large TelescopesII. A P HOTON C OUNTING MCP D ETECTOR AS WFSFig.€3. Schematic diagram of a Medipix2 pixel cell. The charge cA. The Medipix2 Photon Counting ASICIII. M EASUREMENT R ESULTSA. Measurements With UV PhotonsFig.€4. Flood images taken with a UV pen-ray lamp. The right imaFig.€5. Detail of the image of an Air Force test pattern illumin1) Simulating a Shack Hartmann WFS: As described before a Shack Fig.€6. Image obtained with a uniformly spaced pinhole array of Fig.€7. X- and y-shifts for the 700 spots at each lamp location B. First Measurements With ElectronsFig. 8. Flood images taken with a (a) $^{63}{\rm Ni}$ and a (b) IV. C ONCLUSIONR. K. Tyson, Introduction to Adaptive Optics . Bellingham, WA: SR. Angel et al. . (2000, July) A Roadmap for the Development of J. Vallerga, J. McPhate, B. Mikulec, A. Tremsin, A. Clark, and OO. H. W. Siegmund, A. S. Tremsin, and J. V. Vallerga, Advanced MJ. Vallerga and J. McPhate, Optimization of the readout electronX. Llopart, M. Campbell, R. Dinapoli, D. San Segundo, and E. PerB. Mikulec, Development of segmented semiconductor arrays for quH. van der Graaf et al., The readout of a GEM or Micromegas-equiR. Bellazzini et al., Reading a GEM with a VLSI pixel ASIC used J. Vallerga, J. McPhate, B. Mikulec, A. Tremsin, A. Clark, and OM. I. Babenkov, V. S. Zhdanov, and S. A. Starodubov, Chevron of L. Badano et al., SLIM, an innovative non-destructive beam monitIEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 4, AUGUST 2005 1021A High Resolution, High Frame Rate DetectorBased on a Microchannel Plate Readout Withthe Medipix2 Counting CMOS Pixel ChipBettina Mikulec, Member, IEEE, John V. Vallerga, Jason B. McPhate, Anton S. Tremsin, Member, IEEE,Oswald H. W. Siegmund, and Allan G. ClarkAbstract—The future of ground-based optical astronomy lieswith advancements in adaptive optics (AO) to overcome the lim-itations that the atmosphere places on high resolution imaging.A key technology for AO systems on future very large telescopesare the wavefront sensors (WFS) which detect the optical phaseerror and send corrections to deformable mirrors. Telescopeswith30 m diameters will require WFS detectors that have largepixel formats (512512), low noise( 3epixel)and veryhigh frame rates (1 kHz). These requirements have led to theidea of a bare CMOS active pixel device (the Medipix2 chip)functioning in counting mode as an anode with noiseless readoutfor a microchannel plate (MCP) detector and at 1 kHz continuousframe rate. First measurement results obtained with this noveldetector are presented both for UV photons and beta particles.Index Terms—Adaptive optics, CMOS pixel chip, high framerate, microchannel plate detector, photon counting, wavefrontsensor.I. INTRODUCTIONGROUND-BASED astronomy in the optical and infraredspectral regions suffers from distortions of the incomingwavefronts by the atmosphere. Turbulence combined withpressure and more importantly with temperature gradients yieldslightly different refractive indices of the patches of air that thewavefronts traverse. Consequently, the resulting phase errorsgive rise to a blurred image at the telescope focal plane. Ideally,the angular resolution of a telescope would only be limited bythe diffraction limit of the primary mirror that is proportionalto. But even at good observing sites the coherence lengthin the atmospheric layers is only around 20 cm; thus it wouldnot help to use primary mirrors with a diameterlarger thanthat (except for the increased light collection).It is therefore essential for large telescopes to use a systemthat compensates for atmospheric distortions. Such a system iscalled an adaptive optics (AO) system. It adjusts the phase ofthe incoming wavefront using deformable mirrors to providecompensation for the phase distortion. The three main parts ofan AO system are shown in Fig. 1: the deformable mirror, theManuscript received November 15, 2004; revised April 7, 2005. The work ofJ. Vallerga, J. McPhate, A. Tremsin, and O. Siegmund was supported by AURAthrough the NSF under AURA cooperative agreement AST-0 132 798-SPO6(AST-0 336 888).B. Mikulec and A. G. Clark are with the University of Geneva, CH-1211Geneva, Switzerland (e-mail: [email protected]).J. V. Vallerga, J. B. McPhate, A. S. Tremsin, and O. H. W. Siegmund are withthe Space Science Laboratory, University of California, Berkeley, CA 94720USA.Digital Object Identifier 10.1109/TNS.2005.852694Fig. 1. Schematic diagram of an AO system. The distorted wavefront iscorrected by a deformable mirror. A beamsplitter sends the light from a brightguide star to the WFS that measures the distortion. From this information thecontrol computer calculates the new values for the correction and sends signalsto the actuators that modify the deformable mirror such that it cancels thedistortions.wavefront sensor (WFS) and the control electronics. The systemneeds a bright reference star whose light traverses approximatelythe same path through the atmosphere as the light from the objectto be studied. Since bright reference stars are not usually situatedclose to the object being studied, powerful lasers projected intothe sky are used as artificial guide stars. Laser guide stars relyon the light from a laser being backscattered to the telescope,and thus sampling the atmosphere below the scattering location.Rayleigh guide stars use Rayleigh scattering in the atmosphere,while sodium guide stars use the resonant scattering of sodiumatoms present in a thin layer at90 km height [1]. The distortedwavefront from distant objects and these beacons are collectedby the telescope, and the collimated beam is reflected by a de-formable mirror. The beamsplitter (most current ground-basedAO astronomy uses optical light for wavefront sensing whilepassingtheinfraredto thescience camera)dividesupthe lightintothe scientificpath with a high-resolution camera at its end and intothe wavefront sensing path. There the WFS measures the phasedistribution in the optical beam. Its signals are subsequently usedto calculate in a control computer the necessary compensation,which is applied to the deformable mirror via so-called actuators.This


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