A small scan angle-dependent background systematic in non-standard OSSE observations

We have used the large set of OSSE data acquired during AGN observations and high galactic latitude mapping observations to study scan-angle dependent background. A small scan-angle dependent background is observed in continuum bands and is adequately characterized by a sinusoidal function of scan angle. The effect is not significant for observations which use the standard OSSE background offsets or for gamma ray lines like the 511 keV positron annihilation line. It can be significant for non-standard observations of sources near the OSSE sensitivity limit which use large or asymmetrical background offsets or one-sided offsets.