The ISO imaging photo-polarimeter, ISOPHOT, was composed of three subsystems optimized for specific photometric modes. These were: (1) PHT-P, a multi-band, multi-aperture photometer with three single detectors covering the wavelength range 3 ^m to 120 ^m from the near-infrared to the far-infrared; (2) PHT-C, two photometric far-IR cameras for the wavelength range 50 ^m to 240 ^m; and (3) PHT-S, two grating spectrophotometers, operated simultaneously, for the wavelength ranges of about 2.5 ^m to 5 ^m and 6 ^m to 12 ^m. In addition, ISOPHOT was equipped with two sets of three polarizers, one set for the PHT-P detector group and one for the PHT-C detectors, covering the whole wavelength region from 3 ^m to 240 ^m, although during the mission, polarization observations were actually only ever done at 25 ^m and 170 ^m. The instrument design incorporated a number of design features to maximize the performance of the instrument including the fact that a range of apertures (5-180") could be selected to match the point spread function at each selected filter wavelength and thus optimize source-to-background contrast.
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