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Exposure Map

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 $60^{\prime\prime}\times
60^{\prime\prime}$ pixels i.e., a 128$\times$128 image. See To Resample Calibration Images for an example of re-gridding this map to match the image data files.