Absolute fluxes are most accurately determined when the internal fine calibration source (FCS) is used. The source is observed against the FCS and not against the sky background as is usually done. This mode permits the determination of the zero-point, or dark current, of the detector. The calibration source is switched off for this measurement.
This mode is mainly designed for the measurement of the absolute total sky brightness (e. g. Zodiacal Light, Galactic emission). For point sources this mode does not provide any information on the sky background for subtraction of the background contribution. For these cases chopped measurements against the sky, or an additional measurement on a background reference position are better suited.
Absolute photometric measurements can be performed on a one- or two-dimensional spacecraft raster.