Straylight comparison of SIRTF designs with different aperture stop locations

The APART/PADE program is used to examine whether the SIRTF aperture stop can be moved back to the primary mirror without jeopardizing the requirement for natural background-limited observations. The straylight performance of the baseline design, which has the aperture stop at the secondary mirror, is compared to that of a revised design with the stop at the primary mirror. Both designs include reimaging optics and a LYOT stop, as well as a chopping secondary mirror. The straylight due to off-axis sources and telescope thermal emission is investigated at 50 and 550 microns. It is found that in SIRTF, where every instrument will include reimaging optics and a LYOT stop, the aperture stop may be moved to the primary mirror with modest degradation of the straylight performance.