The S/N calculations for scans and maps are essentially the same as for extended sources in the PHT-P and PHT-C photometry. The integration times are specified for one position and the field of view of one pixel. For calculation of the total exposure time of a map the repeated coverage of one celestial position (i. e. the solid angle of one pixel on the sky), expressed by a coverage factor, has to be taken into account. The coverage factor is defined as the total number of samplings of one individual celestial position with anyone pixel of the C100 or C200 array. Due to the PHT32 sampling procedure partial coverage of a celestial position by only a fraction of a detector pixel may occur, which also contribute to the coverage factor.
The observer should use the following recipe, which is also illustrated in the example in Sect. 6.6.5. For each filter determine the integration time in the same way as for normal photometry of an extended source. Divide this integration time by the coverage factor for the selected detector array and oversampling factor (see Tab. 12). This yields the integration time per chopper plateau, which should be greater than or equal to 1 sec. Multiply this time by the number of chopper plateaux per spacecraft raster pointing and by the number of raster pointings. Finally, add the time needed for all intermediate microslews (see ISO Observer's Manual for calculation of the slew time). This yields the total time to perform a raster map in one filter.
Note that different coverage factors exist for linear scans and maps, respectively. Furthermore, the coverage factor only applies to the fully sampled part of the map or scan. The outer regions have less coverage, and the minimum numbers of raster positions to achieve complete coverage in the inner regions are listed in Tab. 12. Maps smaller than these minimum values or the outer regions of larger maps have at least the coverage of a linear scan. Thus, the coverage factor for linear scans can be used as a conservative value for integration time estimates.
The integration time does not vary between individual points of a scan or map.