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Detectors for LGS WF sensing on an ELT: characteristics, special features, limitations, technology

Abstract 143

Submitted by Johann KOLB


J. Kolb, M. Downing




The wave front sensing of Laser Guide Star (LGS) light with a Shack-Hartmann sensor (SHWFS) on an ELT poses the problem of finding the appropriate detector. In the case of a 42-meter diameter ELT and the side launching of the LGS, an 84 x 84 SHWFS requires a 1680 x 1680 pixels detector (assuming 20 x 20 pixels per sub-aperture) that should be able to run at up to 800 Hz and deliver images with a Read-Out Noise (RON) better than 3 electrons per pixel and per frame. Also one can add as requirements high Quantum Efficiency (QE), low latency, charge diffusion and dark current, and excellent cosmetic quality. Such a device doesn’t exist yet, although the CCD technology is already stretched to its limits in order to deliver detectors for the soon-to-come next generation of AO systems (e2v CCD220 for the VLT AO Facility and SPHERE). For two years, ESO has been gathering the requirements for AO detectors for a European ELT (E-ELT), and pushing the development of a technological solution that fulfils those requirements. The most promising is a CMOS Imager which will provide excellent performance while at the same time offering flexibility and reliability. In this paper I will present the main specifications of the needed detector and how they were originated, then give an overview of the detector development program at ESO, and describe the properties of CMOS technology. Finally I will discuss the practical advantages of using a CMOS detector in an ELT AO system.

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