The ISO scientific payload consisted of four instruments: a camera (ISOCAM); an imaging photopolarimeter (ISOPHOT); a long wavelength spectrometer (LWS); and a short wavelength spectrometer (SWS). Each instrument was built by an international consortium of scientific institutes and industry, headed by a Principal Investigator (PI), using national funding. Although developed separately, the four instruments were designed to form a complete, complementary and versatile common-user package.
The total payload provided photometric, imaging and polari-metric capabilities at various spectral and spatial resolutions from 2.5 to 240 μm, and spectroscopic capabilities at medium and high resolutions from 2.5 to 195 μm. The instruments were mounted behind the primary mirror. Each one occupied an 80 degree segment of the cylindrical volume available. The 20 arc minute total unvignetted field of view of the telescope was distributed radially to the four instruments by a pyramid mirror. Each experiment received a 3 arc minute unvignetted field, centred on an axis at an angle of 8.5 arc minutes to the main optical axis, therefore the instruments viewed separate areas of the sky. Switching between them involved repointing the satellite. In principle, only one instrument was operational in prime mode at a time.