Selection of the observing mode

The observer is recommended to consider the following:

• ``Complex'' or ``smooth'' background?
  If the variations of the background on scales of typical source-background separations is comparable to the expected source flux, the observer should choose the mapping AOTs for faint source observations. Else the less time consuming observing modes (staring or chopping) are recommended.
  Background variations consist of two parts, large scale systematic trends, which can be checked from the IRAS maps, and CC. The cirrus confusion noise can be calculated from the numbers given in the Addendum to the ISOPHOT Observer's Manual or using the IPAC software package IRSKY.
• Staring or chopping?
  Both, chopping and staring mode, show similar noise values. Chopping, however, may suffer from loss of signal caused by transient drifts within the chopper plateaux (Section 2.2.1) and signal variation due to the inhomogeneity of the FOV (Section 2.2.3). For both effects correction algorithms should be available in the future, but a staring sequence will always give a more direct measurement avoiding the uncertainties introduced by the correction parameters which are determined separately. Staring mode is recommended if the observer wants to achieve a good photometric calibration of a possible detection or upper limit. Chopping mode can be selected in the case the observer needs quick information whether the source can be detected or not or if the available observing time is very limited. Chopping is also expected to correct for at least the low frequency signal variations of the C100 detector.
• Raster or oversampled maps?
  Raster maps are obtained faster than P32 oversampled maps. The oversampling on the other hand provides higher spatial resolution and redundancey for a better elimination of background variations. Dedicated mapping observations are currently designed to quantify the differences between these two observing modes concerning the detection limit.