Wavefront sensing and reconstruction using a twisted nematic liquid crystal device.

摘要

In this dissertation a closed-loop adaptive-optics system is introduced using a twisted nematic liquid-crystal television (LCTV) as an “adaptive” Shack-Hartmann wave front sensor (SHWS). Unlike a conventional SHWS, this sensor uses three innovative techniques as it tracks the Hartmann spot movements while monitoring the input irradiance of each subaperture. (1) Adaptive lenslet generation: the system writes an adaptive lenslet array directly onto the LCTV so that each subaperture generates a focal spot at the focal length of the lenslet array. The focal spots move around if turbulence exists in the system, therefore the locations of the distorted focal spots are computed using a centroid algorithm and used to correct the local tilted wave fronts. (2) Zonal fitting: using the centroid shift data from all of the subapertures of the lenslet array, the incident wave front is estimated and a continuous phase screen is reconstructed using a zonal fitting method. (3) Subaperture area adaptation: the adaptive Shack-Hartmann wave front sensor detects and responds to significant variations in subaperture irradiance by automatically changing the number of subapertures in the lenslet array to match the input irradiance at each lenslet. Therefore, when this flexible SHWS is used under conditions of low irradiance on the telescope (i.e., when scintillation is present), the number of subapertures is decreased in order to increase the area of spatial integration and thus collect more photons per subaperture. Existing systems increase the integration time to collect more photons, thereby enabling continued (albeit somewhat degraded) telescope operation under poor seeing (low irradiance) conditions. A comparison of the two methods (increased integration time versus increased integration area using larger subapertures) shows that an increased integration area (subaperture size) causes the residual wave front error to increase by about 1/3 as much as an equivalent increase in integration time (i.e., to collect the same number of photons).