Calculating stellar magnitude to enhance guide star catalogs

Abstract

When compiling guide star catalogs formodern star trackers, the use of instrument stellar magnitudes, rather than standard photometric star catalogs, can help avoid ‘blind spots’ in the sky, where the instrument’s field of view has an insufficient number of stars to enable satellite orientation. The calculation of instrument stellar magnitude is a necessary step in developing guide star catalogs for star trackers that use silicon photosensors, such as star tracker 348K (see Figures 1 and 2). Using standard photometric star catalogs—as used in older trackers—may lead either to the tracking of too many stars that a guide star catalog lacks and that will be regarded by the instrument as pseudo-stars, or to stars emerging in the catalog that will never be tracked by the instrument. Working out a guide star catalog requires that one transfer stellar magnitudes to the star tracker’s passband. This allows optimization of a tracker’s characteristics, namely, weight dimension characteristics of the optical system, the amount of memory used by the instrument calculating unit, and the time of calculations required for orientation purposes. Star tracker 348K, developed by JSC NPP Geofizika-Cosmos, is an autonomous multiple-head star tracker with a 19 degree field of view. Each optical head comprises an active pixel sensor (APS)-based camera with a Peltier cooler. Using advanced field and spectral correction and radiation-hard glass, each optical head has three variants of baffles: 40, 30, and 25 degrees. A unique feature of 348K is an internal optical reference mechanism in the optical head. This provides in-orbit self-calibration of bias shift and estimation of object defocus. Using its tracker hardware and software, 348K can reach 11 arcseconds (three sigma) accuracy of attitude determination during its 150,000 hour lifetime. Figure 1. Star tracker 348K.