High-performance FIR bandpass filters for the ISOPHOT instrument

The far-infrared filters for the satellite subexperiment ISOPHOT have to fulfil a number of specific requirements. This is a report about the developed filter concept which was used to match all specifications. In a wide wavelength region this concept is capable of defining broad or narrow bandpasses in a flexible way. The filters which have been constructed are robust and survive multiple cryogenic cycles. They combine high passband transmission with extreme stopband rejection. Details are given for some representative ISOPHOT FIR-filters.