Astros: A Sub-Arcsec CCD Star Tracker

In early March 1986 the first Astro** mission will be launched to observe Halley's comet and a variety of other astronomical targets. Operating from the Shuttle bay, this payload consists of three large ultraviolet telescopes and a smaller wide field camera. An important part of the payload will be the first of a new generation of star trackers using CCDs as the star sensing element. The ASTROS tracker provides extremely precise measurements of star image coordinates as inputs to the Image Motion Compensation (IMC) system used to stabilize the science instrument focal planes. These coordinates, which reflect true star image motion to an accuracy of 0.2 arcsec, are required over a field of view of 2.2 x 2.5 degrees. This paper describes the design and application of ASTROS, with emphasis on performance test results acquired with a prototype system. Algorithms and software will be described in a later paper. Performance tests on real and simulated stars have consistently demonstrated 1/100 pixel accuracy and a noise equivalent angle of 1/300 pixel. These performance levels provide dramatic improvements, both in tracking accuracy and stability, relative to image dissector designs with comparable fields-of-view. These improvements, combined with other advantages inherent in a CCD-based approach are expected to lead to widespread application of this technology on future missions.