These AOTs are used to obtain a so-called sparse map with PHT-P or PHT-C. A sparse map is a sequence of measurements irregularly distributed on the sky. Instead of defining step sizes the observer specifies the coordinates for each point in the map.
The AOTs perform multi-filter photometry with one detector, i. e. either P1, P2, or P3 in the case of PHT-P (AOT PHT17), or either C100 or C200 in the case of PHT-C (AOT PHT37). A sequence of up to 30 positions is measured, which can be irregularly distributed, to create a sparse map. The sequence is only allowed in staring mode, therefore at least one measurement of the sequence has to provide the background level. The size of the sparse map is confined to 1.5 x 1.5 square degrees. Unlike a multi-filter map any position within this field can be selected. Thus, this AOT is optimised for observing complex sources with different brightness levels, and for sources with complex background environment where several background references shall be taken. The detector remains switched-on during the complete sequence of measurements, thus increasing the signal stability and reducing the times for overheads. Furthermore, the dynamic range of the detector can be adjusted to the brightness at each individual position.
Calibration measurements with the internal fine calibration source (FCS) are performed on the first and the last position of the sparse map. These will be used for calibrating the measurements taken at all intermediate positions. If reference measurements are required at each position of the map, a sequence or a concatenated chain of multi-filter single pointing photometric observations (AOTs PHT03 and PHT22) must be performed instead (see ISO Observer's Manual for concatenated observations).
Similar to maps, the measurements at the individual points of the sparse map are concatenated, which means that either all measurements of the sparse map will be carried out or none. The filter sequence must be the same for all measurement points in the sparse map, and for PHT17 the same aperture(s) must be used for all points.
Technical note: The AOTs PHT17 and PHT37 do not reset the ISOPHOT instrument, and the detectors remain switched-on after the observation. Therefore, these AOTs must be executed in a sequence with PHT18 and PHT19 (in the case of PHT17) and PHT38 and PHT39 (in the case of PHT37). Thus, the first observation must be performed with PHT17 or PHT37, then all the following points of the sparse map except the last one must be measured with PHT18 or PHT38, respectively. The last observation and the switch-off of the detectors must be performed with PHT19 or PHT39, respectively.