Requirements sometimes exist for the measurement and/or detection of weak optical sources in the vicinity of bright sources. Capability of observing weak optical sources is dependent on the system sensitivity which is limited by the detector D*, processing electronics noise, mechanically coupled scan or jitter noise, thermal effects on the detector sensitivity, optical emissions of sensor housing and imaging optics, and the point spread function. The point spread function is mainly governed by the optical element size, shape, and aberrations. When a strong source is in the proximity of the weak signal, the wings or roll offs of the point spread function play an important role in the system detection capability. Two factors dominate the roll offs, namely diffraction and scattering of the optical train. This paper discussed the point spread function requirements or conversely, the out-of-field rejection requirements needed to suppress the unwanted sources, small or large in angular extent, to a level tolerable to the system. Stray light sources are discussed and suppression designs considered.