Background maps are provided by the standard processing for all ROSAT and Einstein instruments. For the PSPC and IPC these are in three standard bands: soft, hard, and broad (Table 6.2).
|Table 6.2 Standard Bands|
|After Dec 92|
|Instrument||Band||Energy||PI Bins||Energy||PI Bins|
In order to make use of a background map, the data must first be blocked and PI-filtered appropriately.
The background templates for Einstein IPC data are obtained in a fixed energy band (Table 6.2), and have a spatial resolution that is 1/2 that of the corresponding QPOE file (i.e., 16"/pixel). While the PROS tasks can take into account the different pixel size in the source and background files with the physical coordinate system (cf. Using the PROS software), the energy filters must be explicitly applied. The background data can generally be excluded from the computation of the errors (the *rbkmap files have been rescaled to match the background level of the source field, see Scientific Specification of the Data Analysis System for the Einstein Observatory (HEAO-2) Imaging Proportional Counter, by F.R. Harnden, D.G. Fabricant, D.E. Harris and J. Schwarz.), and the statistical error associated with them is negligible.
N.B.: ROSAT PSPC background maps in SASS 5-6 or higher are generated by smoothing a source-subtracted, exposure corrected image (see the ROSAT Data Products Guide). Goodness of fit depends on the amount of data left after source subtraction. Maps based upon short exposure images, those with extended sources or strong gradients or very crowded fields are likely to contain spurious structure. Background maps from earlier SASS versions were smoothed without correcting for the ribs and contain structure dominated by the ribs, these are not useful.