Straylight in ISOPHOT?

Design measures and preflight tests were the first steps taken to reduce and check for instrumental straylight. Requirements for ISO derived from the natural sky brightness were the basis for relevant preflight studies involving ISO and ISOPHOT. Inspections of the cryostat identified several IR photon leaks. Their elimination was verified by a "Dark background test". In flight, the extended profiles of bright point-like sources were measured to study potential near-field straylight. During solar eclipses,the straylight of the Sun and the Moon was searched for. Finally we checked for a potential unwanted signal of the Earth when it fully illuminated the inner sunshade. Surprisingly, the 170 mum Serendipity Survey contributed to our knowledge of instrumental straylight. To summarise, straylight was a negligible contribution to most measurements, but it can be a small correction factor under rare observing conditions near the detection limit.