CCD Star Sensor for Fine Pointing Control of Spaceborne Telescopes

A star sensor for measuring small pointing errors in astronomical telescopes is described. By using solid-state imaging arrays (CCD's) to track guide-star images, this design realizes a number of performance advantages relative to more conventional approaches. Specific topics include star image position measurement, CCD sensitivity including color effects, and performance simulation results. Experimental results for CCD's operating at temperatures below 100 K are also summarized. Two telescope configurations are considered to illustrate the range of possible applications of this technology: 1) a large orbiting telescope for general astronomy requiring a multi-CCD sensor, and 2) a Space^Shuttle-based infrared observatory operating at 20 K. Although the requirements and configurations of these applications are widely different, the CCD approach offers substantial advantages to both systems. INE pointing control of large astronomical telescopes in space requires star trackers of extreme accuracy and stability. Pointing errors as small as a few percent of a star image diameter must be measured and corrected to prevent the resulting image motion from significantly degrading image quality. In this paper, we describe a general fine pointing sensor (FPS) design approach which uses charge coupled device (CCD) imaging arrays to track focal plane star images. Since CCD's provide accurate positional information regardless of where the image falls within the field-of-view, the proposed sensor design can accommodate wide variations in guide-star geometry without moving parts. This advantage, coupled with the fact that several stars can be tracked simultaneously (using one or several CCD's), makes the CCD approach a strong candidate for application to a variety of future space-based astronomical telescopes now being planned by NASA. CCD imaging arrays have several qualities which make them attractive for star tracking. A natural coordinate system is defined by the array of photosensitiv e elements which are not dependent on electromagnet ic deflection fields (as in the case for vidicons and image dissectors). Other advantages include high sensitivity, low operating voltage and the ability to survive exposure to strong light. The quantum efficiency of these detectors, combined with their efficient use of signal integration time (all elements integrate simultaneously), makes the noise performance of CCD trackers generally competitive with other approaches limited only by quantum noise (photon counting). In many cases, further noise reductions may be achieved by tracking several stars, simultaneously, on a single CCD.! One potential limitation of using CCD's for star tracking is the limited number of sensing elements (pixels) available with present arrays. Currently, the maximum array size is in the