The PoGOLite star tracker system

PoGOLite is a balloon-borne experiment which will study polarised soft gamma-ray emission from astrophysical targets in the 25keV – 80keV energy range by applying well-type phoswich detector technology [1]. Polarised gamma-rays are expected from a wide variety of sources including rotation-powered pulsars, accreting black holes and neutron stars, and jet-dominated active galaxies. Polarization measurements provide a powerful probe of the gamma-ray emission mechanism and the distribution of magnetic and radiation fields around the source. The polarization is determined using Compton scattering and photoelectric absorption in an array of 217 plastic scintillators. The PoGOLite polarimeter has a field of view of 2.4° x 2.4° and must be kept aligned to objects of interest on the sky within 5% of the FOV to secure a minimum detectable polarization MDP=10% for a 200 mCrab source. This alignment can be obtained by mean of different attitude sensors: a DGPS (Differential Global Positioning System), two star trackers and gyroscopes. The most accurate sensor is the star tracker which is the focus of this paper. Preliminary studies [2] and calibration have been made which pave the way to an autonomous star tracking device which together with the other attitude control devices will reconstruct the pointing solution.