The ISO telescope is a 60cm diameter Richey-Chrétien system with
an overall focal ratio of f/15. The optical quality of the primary and
secondary mirrors provides diffraction limited performance in
wavelengths beyond 5 
m. Note, however, that the pointing
performance (Sect. 2.1.3) limits diffraction-limited use of
the system to wavelengths longer than about 10 m. Stringent
control of straylight is accomplished by a sunshade, the cassegrain and
main baffles and additional light-tight shields around instruments, as well
as by imposition of viewing constraints (cf. Sect.2.2.2.
Fig. 3 shows the configuration of the ISO optical
subsystem.
A part of the 20 arcmin unvignetted field of view of the telescope is distributed into the entrance pupil of each instrument by a pyramidal mirror located on the telescope axis. The 3 arcmin field of view of each instrument is at an angle of 8.5 degrees from the telescope optical axis. All four instrument fields of view are centered around this axis. Since the four instruments view separate areas of the sky, switching an astronomical target between instruments requires a repointing of the satellite. Fig. 4 shows schematically how the individual apertures are used by the instruments.
Figure 4: Focal plane map of all apertures of the four ISO instruments. Each of the four instruments has a 3' unvignetted field of view. LWS accepts the central $1.65'$ of its allocated field. The three possible SWS entrance apertures are shown. The four pixel fields of view (1.5'', 3'', 6'' and 12'') for CAM are shown. The two larger fields of view extend to the vignetted region. For PHT the photometer and the spectrophotometer apertures are shown. The 100 micron camera fills the unvignetted field of view while the 200 micron camera is somewhat larger.
Figure 3: Telescope design