Infrared detectors of high sensitivity usually show non-linearities, memory effects, time-dependent effects and peculiar responses to high energy particle impacts. All instruments and detectors are calibrated prior to launch on the ground. The results of these calibrations are partly used in the OBSERVER'S MANUALS for the instruments to enable e.g.\ observing time calculations. However, these measurements are not sufficient to specify the instrumental response in orbit. Therefore, calibration will be carried out during the operations as well.
The in-orbit instrument calibration procedures can be subdivided into two classes: `Calibration measurements', which form an intrinsic part of an AOT and are performed, using internal calibration sources, within every observation using this particular AOT, and `Calibration observations', which are performed at regular intervals by the ISO Science Operations Team and generally employ celestial calibration sources. The necessary calibration measurements are described in the corresponding AOT documentation included in the instrument specific OBSERVER'S MANUALS. The observers do not have to take into account the calibration observations in their proposals. From the observers' point of view the instruments are calibrated independently of their particular observations. If this assumption is not valid for an AOT, then the AOT has been constructed in such a way that the necessary calibration measurements will be included.
In-orbit calibrations involve a whole range of measurements such as flat-fields, read-out noise, dark current, flux and spectral calibrations and many others. Photometric and wavelength calibration will be performed against astronomical standard sources of known continuum and line emission. It is expected that knowledge of the in-orbit behaviour of the instruments and detectors improves in time. This may result in calibration updates. These updates will be made available to observers who already received their data (Sect. 4.9).
An extensive preparatory programme of ground-based observations has been conducted to provide a sufficient number of astronomical standard sources to support ISO calibrations. Continuum measurements of selected stellar sources are fed back into stellar atmosphere models to determine the expected fluxes at wavelengths up to 50 m with 10% accuracy. These results will be available before the launch of ISO. The long wavelength flux calibration is based on asteroids and planets.
Wavelength calibrations of the spectrometers will be done internally, in the case of SWS, and by observations of fine structure lines originating in planetary nebulae in the case of LWS. The wavelength calibration will be accurate to below one resolution element, i.e. better than where is the wavelength and R the resolving power.
For CAM and PHT the overall expected accuracy of photometric calibration is about 15% of the flux level in a specific wavelength range. For SWS and LWS, which are designed to detect line emission, the line fluxes are expected to be accurate to about 30%.