From version 4.0 on, the parameter badclean is removed from the parameter list, and the pixels in the neighbourhood of bad pixels, CCD borders, and offset columns are treated according to the flag selections of the input image. The flag selections are read from the DSS keywords of the input image.
From the instrument maps, exposure maps are constructed which may be either output in detector or in sky coordinates (this is the default; see parameter withdetcoords). Note that the input image has to be of the same coordinate type (detector/sky) as the required output image. In the case of sky coordinates, the attitude file generated by the task atthkgen is rebinned. A new attitude bin is started when the change in attitude exceeds the required positional accuracy (parameter attrebin). The integration time is calculated from the good time intervals valid for each chip and is corrected for subsequent time selections performed by the user. The exposure falling in each time bin is obtained from the exposure extension of the input dataset.
An attitude histogram is created from the rebinned attitude file, and for each attitude bin, the corresponding exposure values are finally projected onto the sky and 'accumulated' into the respective sky image pixels. The resulting exposure maps will thus contain the exposure (in units of seconds) for a particular EPIC sky image.
The following filters are read from the data subspace keywords of the input images and are taken into account for the calculation of the exposure maps: TIME range filters, GTI filters, CCDNR range filters, FLAG range filters, FLAG bit-mask filters. The task tries to determine the exposure time by looking for GTI extensions in the input image of the form STDGTInn, STDGTIn, STDGT, or GTI. If no GTI extension is found, the exposure time is taken from the EXPOSURE keyword. No merging of multiple gti is performed in the usedss=false case but the first set of GTI extensions in the sequence above is used. Note that the task expects the gti information in an extension of the input image whereas the BADPIX and EXPOSURE extensions are read from the input event file. The usedss=false setting is mainly useful for the processing of non-SAS-derived input datasets. In the case of SAS-derived images it is strongly advised that the parameter writedss of task evselect is set to true. For EPIC MOS input images, the effective frame time is taken from the column TIMEDEL of the EXPOSURE extensions in order to correct for mode dependent dead-times. In the case of EPIC PN, the keyword TIMEDEL is used. Since the value of TIMEDEL incorporates mode dependent corrections for out-of-time events, a keyword OOTCORR=true is written to EPN exposure maps in order to avoid double correction by emldetect. The keyword OOTFRAC contains the ratio between the keywords TIMEDEL/FRAMETIM.
Task eexpmap supports the calculation of several exposure maps in different energy bands in one run of the task. The exposure maps are used in the EPIC detection chain by the tasks emask, esplinemap, eboxdetect, emldetect, ewavelet, and esensmap.
