2.5 ISOPHOT Measurement Concept

A PHT measurement comprises all detector readouts telemetered following a `start measurement' command with the instrument in a fixed configuration. A change of instrument configuration can be a change of detector, a change of aperture, a change of filter, a change of polariser, or a switching-on or -off of the FCS. These changes can only occur between measurements and not during a measurement. The measurement time is the total time necessary to perform a measurement. This time is different from the on-source integration time, which excludes the time when the background is measured by the chopper in case of a chopped measurement.

Data collected while the spacecraft was performing a raster map belong to the same measurement. The integration time per raster point, or the raster point dwell time and the time for the microslews between the raster points are included in the measurement time.

Apart from the sky measurements, the observations provide a number of special internal measurements:

• FCS measurement. All AOTs except PHT40 include at least one FCS measurement, see also Chapter 5, Sections 5.2.4 and 5.7.2.
• Dark instrument configuration measurement. This measurement type is obtained with AOTs PHT05, PHT25, and PHT40 and is used to subtract dark signals (in case of PHT05 and PHT25) or to identify possible detector memory artifacts from a preceding measurement of a bright source (in case of PHT40).
• Cold FCS measurement. This measurement is used to assess the zero signal level of an accompanying FCS measurement by applying no heating power to the FCS. It is also referred to as `FCS straylight' measurement because parasitic light from the sky into the instrument (ISOPHOT had no shutter) or from the instrument itself can be measured. The absolute photometry AOTs PHT05 and PHT25 can contain such a measurement.