For thermal and power reasons, but also to prevent straylight from reaching the instruments, there are a number of constraints and restrictions on the allowed pointing directions for ISO. The restrictions may be overridden if necessary but the constraints may not be violated. To prevent radiation from the Sun from falling onto the cryostat and the service module, ISO will always point only in a direction between 60 and 120 degrees away from the Sun (the Sun constraint). The Earth, the Moon and Jupiter are strong infrared emitters. Thus, ISO may not point closer than 77 degrees to the Earth limb (the Earth constraint), not closer than 24 degrees to the Moon (the Moon restriction). Jupiter should be kept away from the optical axis by at least 7 degrees (the Jupiter restriction); unless, of course, Jupiter or one of its moons are the target of an observation. The constraints and restrictions are included in Fig. 6.
In addition to the constraints and restrictions due to celestial sources, there are also technical limitations to the pointing. The spacecraft cannot be rotated around its x-axis (Fig. 2). This eliminates the possibilities to orientate rectangular entrance apertures arbitrarily on the sky or to align scans and maps with respect to celestial coordinates (Sect. 13.3).