An ISO observation is a combination of spacecraft and instrument operations. This section describes the available spacecraft observing modes and the overheads involved. An overhead is the time to prepare the satellite, and the instrument, for a new observation or measurement before photons can be collected from the source. The instrument specific overheads are described in the corresponding sections of the OBSERVER'S MANUALS for the instruments. All overheads are considered to be part of the observation, and therefore the time for these have to be added to the exposure time for the observation. The total time is the target dedicated time, which should appear in the list of observations of the proposal.
In addition to the spacecraft observing modes the satellite construction constrains observations. The instruments are fixed with respect to the satellite axes (Fig. 2). Therefore the satellite orientation determines how e.g. apertures are projected on the sky. This may be relevant when the aperture has a rectangular shape, when an array is used or when internal chopping is required. The instrument specific OBSERVER'S MANUALS provide detailed information about how to cope with the satellite orientation constraints.