Color index computation for the NASA Standard Fixed Head Star Tracker

When a Fixed Head Star Tracker (FHST) is tracking a star, the stellar magnitude as measured by the detector is telemetered. The value of the stellar magnitude depends on the visual magnitude of that star, the sensitivity of the tracker, the spectral type of that star, and the spectral type of the star with which the FHST is calibrated. The difference between the stellar magnitude and the visual magnitude is called the color index of that star. The method of computing the color index for any star, while using a NASA Standard FHST, is presented. The color indices computed by this method agree closely with flight experience.