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A detector map is used to provide a grid of positions over which to calculate and flux weight the ARF. The default detector map is 'flat' which has a weighting of 1.0 for each detector map pixel and is rectangular having sides which just encompass the region specified in the DSS. Only detector map pixels falling within the source region are included in the ARF calculation, e.g. for a circular source box, the pixels in the corners of the detector map will be ignored. The default size of the map is 5 by 5 pixels. This may not be fine enough for some regions, e.g. a narrow annulus at a large off-axis angle would not be well represented by the default map. In these cases more map pixels can be introduced by specifying e.g. detxbins=10, detybins=10 on the command line.
arfgen supports 3 types of detector map, selectable through the detmaptype parameter:
This is the default internal representation where all pixel values are set to 1.
Given that all the pixels are of the same value, the function of the flat detector map is essentially to describe the spatial grid points which the task should consider during the computation of the ARF.
The map bounds
are by default matched to the extent of the
selected regions, but can be set explicitly by
setting withdetbounds 
true and then
choosing values for the parameters
detxoffset, detyoffset.
The map is centred on the source position.
The number of pixels can be set using
the parameters detxbins and detybins.
This is an internal map based on a model of the PSF taken from the CAL. This is the most accurate for point sources where the majority of the flux falls at the centre of the source region. In practise it is unlikely to produce a very different ARF to the 'flat' map, except at large off-axis angles where the calibration parameters can change rapidly.
The extent and coarseness of the detector map may be specified on the command line similarly to that of the 'flat' map.
Given that the PSF is energy dependent, the energy at which the PSF model is taken may also be specified by the user, through the parameter psfenergy.
This is a user-defined detector map. This allows the user to have control over the spatial distribution of the source which is particularly useful for extended sources.
This must be in the format of an image array of a dataset, and that image array must contain WCS information that describes the mapping onto DET coordinates. Such maps can be generated in xmmselect given an event list that contains a column DETX, DETY, by selecting those columns in the main GUI dialog of that task, clicking the 'Image' button and then specifying the preferred image binning in the subsequent evselect dialog. Alternatively, it could be generated directly from evselect - section 5.4 provides an example of doing this.
A detector map of this form can be passed to the task through the detmaparray parameter.
Note that in order for arfgen to generate an output consistent with the data, the detector map must enclose the regions over which the spectrum is accumulated; one or more warnings are raised if the extent of the selected regions exceeds the bounds of the detector map along either the X and/or Y axes.
NB: The detector map may not be specified in X/Y coordinates in the current release.