Editing Infrared Space Observatory (section)

== Optical telescope ==

The ISO telescope was mounted on the [[Centre (geometry)|center line]] of the dewar, near the bottom-side of the toroidal helium tank. It was of the [[Ritchey-Chretien telescope|Ritchey-Chrétien]] type with an effective [[entrance pupil]] of 60&nbsp;cm, a [[Focal length|focal length ratio]] of 15 and a resulting focal length of 900&nbsp;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 and 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 [[Percentage|percent]] of the sky was available to ISO.

A [[Pyramid (geometry)|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.

[[File:Iso lsw flight spare.jpg|thumb|[[Flight spare]] for the LWS instrument in ISO]]