This paper is an analytical study of the scan noise in image scanning systems and its effects upon the sensor. The specific source of scan noise to be considered is that generated by the detectors as they pass over the diffraction pattern of an off-axis source. The supports of a Cassegrain optical system are the diffracting apertures of concern in the present study. Two types of support structures will be compared for their applicability to system trade-offs. In addition, the three principal types of scan noise are classified according to their effects upon the signal processing. The relative importance of each noise type if analyzed, in detail, from the standpoint of frequency filtering, while incorporating several real world system complexities into the general analytical solution.