Optical Telescope
The ISO telescope was mounted on the center line of the dewar, near the bottom-side of the torodial helium tank. It was of the Ritchey-Chrétien type with an effective entrance pupil of 60 cm, a focal length ratio of 15 and a resulting focal length of 900 cm. Very strict control over straylight, particularly that from bright infrared sources outside the telescope's field of view, was necessary to ensure the guaranteed sensitivity of the scientific instruments. A combination of light-tight shields, baffles inside the telescope and the sunshade on top of the cryostat accomplished full protection against straylight. Furthermore, ISO was constrained from observing too close to the Sun, Earth and Moon; all major sources of infrared radiation. ISO always pointed between 60 to 120 degrees away from the Sun and it never pointed closer than 77 degrees to Earth, 24 degrees to the Moon or closer than 7 degrees to Jupiter. These restrictions meant that at any given time only about 15 percent of the sky was available to ISO.
A pyramid-shaped mirror behind the primary mirror of the telescope distributed the infrared light to the four instruments, providing each of them with a 3 arc-minute section of the 20 arc-minute field of view of the telescope. Thus, pointing of a different instrument to the same cosmic object meant repointing the entire ISO satellite.
Read more about this topic: Infrared Space Observatory
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