Daylight pointing calibration for optical telescope systems

Abstract

Daytime operation of optical telescope systems introduces severe perturbations, stemming from rapidly varying ambient temperatures, solar radiation causing non-uniform thermal expansion, and increased atmospheric turbulence, all of which degrade optical alignment, pointing precision and imaging quality. However, select applications such as satellite laser ranging, free-space optical communication and to some extent space debris observation necessitate operation during dawn and even daylight conditions. Compensating the effects on the pointing precision is an important first step towards reliable daylight operation and requires a robust, time-efficient pointing calibration procedure. This paper describes the development of a bright-to-dim single-star-centering algorithm for robust recalibration of a telescope pointing model in daylight conditions. The chosen setup consists of a 700nm longpass filter in combination with a back-illuminated CMOS camera, resulting in a limiting magnitude of 7.0 for an 80-cm Ritchey-Chr´etien AltAz telescope. The telescope system obtains a pointing error measurement by sorting visible stars by apparent magnitude and measuring the difference between their recorded and cataloged position, in descending order of magnitude. With this approach, the algorithm is most robust against temporary changes in visibility. Using a grid of 6 stars to update the pointing error terms, which are the most susceptible to hourly drift, a pointing error of 4.1” RMS is achieved immediately after recalibration in daytime operation. An adaptive update schedule is presented to maintain a 5” RMS static pointing error throughout the day.