*makevig, vigdata* - Apply Vignetting Correction to an image

First generate a vignetting mask appropriate for an energy of 1.0 keV ($\sim$central energy in the ROSAT PSPC) for the image xdata$rp110590.qp:

task: makevig

input file: xdata$snr.qp

output file snr_vig.pl

  xr> xspatial 
  xs> makevig "xdata$rp110590.qp[bl=30]" arlac " " 1.              # generates 
                                      # output mask arlac_vig.pl at energy 1.0 keV

Now apply vignetting correction to the image generating a vignetting corrected output image and error array:

task: vigdata

input files: xdata$snr.qp, snr_vig.pl

output files: vigsnr.imh, vigsnr_err.imh

  xs> ximage                                                         # load package 
  xi> vigdata                                  	            # vignetting correction 
  Input image file :  xdata$110590.qp[bl=30]          # From the original QPOE file
  Output vignetted image file : arlac                       # give *.imh extension! 
  Vignetting corrections mask : arlac_vig.pl              # All extensions explicit 
  fraction of image which is non-vign bkgd (x.xx format): 0.05 
                              # 5% of the image counts are non-vignetted background 
                                                       #  and will not be corrected 
                                                          # The format is important 
  Input error image file : xdata$rp110590.qp[bl=30]         # The error is taken as 
                                          # square root of n from the original data 
  Output vignetted error file : <cr>                              # =vigsnr_err.imh

This method of applying vignetting assumes that background counts dominate the image. In the ROSAT PSPC the non-vignetted (charged particle) background is a minor contribution to the background with a typical rate of $4\times 10^{-6}$ cts s^-1arcmin^-2keV^-1(see Plucinsky et al., Ap.J., 418, 519, 1993 for a detailed discussion).