The In-Flight Performance of the Zebra Daytime Star Sensor

The low energy balloon-borne gamma-ray telescope ZEBRA is capable of producing images of the sky with a resolution of 1° and a point source location accuracy of a few arcminutes. In order to fully exploit the sensitivity of this instrument, a platform stabilised in three axes to this precision has been developed, which however has an absolute pointing accuracy of 30′-1°. Therefore there is a need to have on board a system which will allow post facto reconstruction of the absolute pointing direction of the telescope as a function of time, for observations in both day- and night-time conditions. An independently steerable starsensor has been constructed, based on a CCD imaging system, for this purpose. The results obtained with this device during the first flight of the ZEBRA telescope in May 1989 from the NASA balloon base in Palestine, Texas are presented.