Staring Raster and Mapping Observations

For the staring raster and mapping modes the exposure times need not be a power of two in seconds as is the case for single pointing observations. The measurements are performed in a way that for one filter setting a continuous exposure is started at the first spacecraft raster position. This exposure lasts for the performance of the complete raster or mapping observation including all intervening microslews. Since a microslew cannot be guaranteed to last for an integer multiple of 2 seconds (see ISO Observer's Manual for calculation of microslew duration) the constraint of having powers of two in seconds for the exposure time was not pertained for raster staring and mapping observations. This means that individual integration ramps can start or end when the spacecraft is slewing. For data reduction the data taken when the spacecraft is dwelling on a raster position can be distinguished from those taken during the microslews by checking the status of the so-called On-Target Flag which will be always set, if the satellite has achieved a stable pointing on a raster position (within an error cone of a few arcseconds).

For staring raster and mapping observations there are lower and upper limits for exposure time values per raster position. The lower limit is given by a minimum spacecraft dwell time per raster position which is 10 seconds. The upper limits are represented as powers of two in seconds. Note that obeying the maximum exposure time limit given in Tab. 6 does not guarantee the feasibility of a mapping observation as this also depends on the total number of selected raster positions and hence the total performance time of the map. One contiguous map should not last for longer than 20000 seconds (5.5 hours). A special calculation for the exposure time per raster position is needed for the PHT-C array observing mode, which performs oversampled mapping, where a certain sky position will be viewed several times from different raster positions, and the measurement per raster position comprises 12 (for C100) or 6 (for C200) chopper positions on adjacent sky pointings. A detailed description how to assess the exposure times in this mode is given in the cookbook (Section 6.6). Table 6 gives a summary of the minimum and maximum exposure times per raster point for staring raster and mapping observing modes.