Both ROSAT PSPC and Einstein IPC have regions of their fields masked by the window support structure. In addition, vignetting results in a significantly lower effective exposure time toward the edge of the field.
An exposure map is provided for each ROSAT field (e.g., rp110590_mex.imh).
The simplest method of applying an exposure correction is to divide the data by the exposure map yielding a count rate image. The image must first be blocked to the same size as the exposure map, e.g., for ROSAT PSPC.
xi> imcalc input file (`-' for STDIN) or imcalc commands () : - # interactive mode IMC > ctrate.imh = ``xdata$rp110590.qp[bl=30]'' / ``xdata$rp110590_mex.imh'' IMC > quit
Alternatively, when using imcnts to extract the counts in a region, the same region can be applied to the exposure map to determine an average exposure time.
Note: The ROSAT exposure map has pixels i.e., a 128128 image. See To Resample Calibration Images for an example of re-gridding this map to match the image data files.