Multiwavelength active optics Shack-Hartmann sensor for seeing and turbulence outer scale monitoring

摘要

Real-time seeing and outer scale estimation at the location of the focus of atelescope is fundamental for the adaptive optics systems dimensioning andperformance prediction, as well as for the operational aspects of instruments.This study attempts to take advantage of multiwavelength long exposure imagesto instantaneously and simultaneously derive the turbulence outer scale andseeing from the full-width at half-maximum (FWHM) of seeing-limited imagestaken at the focus of a telescope. These atmospheric parameters are commonlymeasured in most observatories by different methods located away from thetelescope platform, and thus differing from the effective estimates at thefocus of a telescope, mainly because of differences in pointing orientation,height above the ground, or local seeing bias (dome contribution). Longexposure images can either directly be provided by any multiwavelengthscientific imager or spectrograph, or alternatively from a modified activeoptics Shack-Hartmann sensor (AOSH). From measuring simultaneously the AOSHsensor spot point spread function FWHMs at different wavelengths, one canestimate the instantaneous outer scale in addition to seeing. Although AOSHsensors are specified to measure not spot sizes but slopes, real-time r0 and L0measurements from spot FWHMs can be obtained at the critical location wherethey are needed with major advantages over scientific instrument images:insensitivity to the telescope field stabilization, and being continuouslyavailable. Assuming an alternative optical design allowing simultaneousmultiwavelength images, AOSH sensor gathers all the advantages for real-timeseeing and outer scale monitoring. With the substantial interest in the designof extremely large telescopes, such a system could have a considerableimportance.