Most real detectors have nonlinear Intensity Transfer Functions (ITF), e.g. photographic plates, the effects of the dead time at high count rates in photon counting systems and the low light level nonlinearity of Image Dissector Scanners (see M. Rosa, The Messenger, 39, 15, 1985). If the ITF is known analytically, the command COMPUTE/IMAGE will be sufficient for the correction of the raw data. For example, the paired pulse overlap (dead-time) correction of photoelectric equipment could be corrected for by COMPUTE/IMA OUT = IN/(1+TAU*IN), where TAU is the known time constant of the counting electronics an IN must be in units of a count rate rather than total counts. If the ITF is defined in tabular form, the command ITF/IMAGE can be used to obtain the ITF correction for each image element (pixel value) by interpolation in an ITF table; this command assumes a uniform transfer function over the image field.
CCDs are generally more nearly linear than are most other detectors used in astronomy. However, especially in particular pixels or regions, CCDs are clearly non-linear and/or suffer deviating signal zero points. Procedures which may be useful for the treatment of such deficiencies are (partly) described in Appendix B.
Problems related to background estimation and more sophisticated flat-field corrections generally are very instrument dependent. Therefore, it is not possible to give one standard recipe here; check the various instrumental appendices for more specific advice.