Optically sensitive Medipix2 detector for adaptive optics wavefront sensing

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Abstract

A new hybrid optical detector is described that has many of the attributes desired for the next generation adaptive optics (AO) wavefront sensors. The detector consists of a proximity focused microchannel plate (MCP) read out by multi-pixel application specific integrated circuit (ASIC) chips developed at CERN (“Medipix2”) with individual pixels that amplify, discriminate and count input events. The detector has 256×256 pixels, zero readout noise (photon counting), can be read out at 1 kHz frame rates and is abutable on 3 sides. The Medipix2 readout chips can be electronically shuttered down to a temporal window of a few microseconds with an accuracy of 10 ns. When used in a Shack–Hartmann style wavefront sensor, a detector with 4 Medipix chips should be able to centroid approximately 5000 spots using 7×7 pixel sub-apertures resulting in very linear, off-null error correction terms. The quantum efficiency depends on the optical photocathode chosen for the bandpass of interest.

Introduction

Ground-based astronomy in the optical and infrared has the distinct disadvantage of observing through the atmosphere. Though mostly transparent over much of the bandpass, the atmosphere is a constantly changing and dynamic medium. The index of refraction of air is a function of density and temperature and the vertical spatial profile of these parameters change with time due to windshear induced turbulence. Hence, a plane wave of light from a distant star will be distorted across the constant phase wavefront, leading to blurry images in the focal plane of a telescope or twinkling stars to the human eye.

Ideally, the angular resolution of a telescope would be limited only by the diffraction limit of the primary mirror, which improves linearly with diameter. The typical ∼1 arc second blurring caused by the atmosphere corresponds to the diffraction limit of a 20 cm diameter mirror in the optical bandpass. Therefore, making a ground-based telescope larger than 20 cm does not improve its angular resolution (though it does improve its light gathering power). Space-based telescopes, like the Hubble Space Telescope, do not suffer from atmospheric distortion, but they are expensive to build, launch and operate, and therefore difficult to make much larger.

In the past two decades, techniques have been developed to remove the effects of the atmosphere on the light from distant sources. Adaptive optics is the method of using fast deformable mirrors [1] to conjugate and therefore cancel any phase errors introduced in the light path between the object of interest and its image at the focal plane (Fig. 1). Before correcting the phase of the light, it must be measured using a wavefront sensor that samples the wavefront across the pupil. One method (among many) is the Shack–Hartmann sensor [1] (Fig. 2). The wavefront is sampled by a lenslet array creating individual images focused on an imaging array. If the light is perfectly collimated (e.g. a plane wave from a distant star), the images would be spots of light at the regular spacing of the lenslet array. If the plane wave is distorted, the centroids of the spots would spatially shift depending on the local slope of the wavefront. By measuring the centroids of these spots in real time, one can determine the wavefront error and feed this error signal back to the deformable mirror to “close the loop” and correct all time variable wavefront distortions.

Each centroid determination measures the slope of the constant phase wavefront at that particular location on the wavefront at that particular time. Larger telescopes with larger pupils use deformable mirrors with more actuators and hence more phase measurements, therefore, requiring detectors with many pixels. The atmosphere's variability on most nights necessitates wavefront sampling rates on the order of 100–1000 Hz. The number of photons from the guide stars used as the reference beacon are almost always limited in number since bright stars are rare, so the wavefront sensor detector should have high-quantum efficiency and low-readout noise to improve the signal to noise ratio of the centroid determinations.

A recent white paper on the AO instrumentation needs for future large (>30 m diameter) telescopes entitled “A Roadmap for the Development of Astronomical Adaptive Optics” [2] specified that wavefront sensor detectors should have:

  • Quantum efficiencies>80%.

  • Pixel formats of 512×512.

  • Frame rates of 1 kHz or faster.

  • Readout noise less than 3 electrons rms.

The last three are not simultaneously achievable with the current generation of CCDs. Larger detectors require faster clocking rates to read out at fast frame rates. But higher clocking speeds increase the readout noise because of the increase in bandwidth required of the amplifiers. Below, we describe a detector that can meet the last three of the above requirements, but with a quantum efficiency approaching 40%.

Section snippets

Photon counting MCP detectors

Photon counting detectors, unlike charge integrating arrays like CCDs, register each detected photon as one count and so have no “readout noise”. A variety of readout anodes are used in imaging microchannel plate (MCP) detectors (Fig. 3) to report the location and time of arrival of every detected photon. Imaging MCP detectors can have large area (100×100 mm), high spatial resolution (20 μm FWHM), low background dark count, and event timing resolution less than 1 ns [3]. Their QE is determined by

Medipix2 as a readout anode for MCPs

Fortunately, we soon discovered that such a device already existed, being designed and constructed by the Microelectronics Group at CERN for the multi-national MEDIPIX collaboration (http://www.cern.ch/medipix).

Our novel detector scheme should achieve the first three of the specific goals listed above with a QE approaching 40%. The detector (Fig. 5) is an MCP image tube with a GaAs photocathode and a new pixelated CMOS readout chip called the “Medipix2” [4], [5]. Photons interacting with the

Conclusions

We have described our initial results with an MCP detector read out with a Medipix2 ASIC. The next step for its use as an AO detector is to integrated the individual components into a vacuum tube with a high efficiency optical photocathode such as GaAs. Other applications that can make use of a windowless detector (such as X-ray, far ultraviolet and electron imaging) could adopt this technology immediately by appropriate choice of photocathode.

Acknowledgments

The authors wish to thank the Medipix Collaboration for the Medipix2 chips, readout hardware and software and for valuable advice. The material presented here is based upon work supported by AURA through the NSF under AURA cooperative agreement # AST-0132798-SPO#6(AST-0336888).

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