Calculates differential and integral Point Spread Function, PSF. This accumulates all the counts within annuli of increasing radii. Each annulus increases by one pixel. Rebin the image to change the step size. The results are written to a QDP format file, and are also plotted using QDP. The differential PSF is given as a surface brightness (counts/s/sq arc min) and the integral as the Encircled Energy Function, EEF. The expected values for a pointlike source are also written to the QDP file and displayed. An accurate estimate of the image background is required, and is obtained using the background command. The cursor can be used to specify the source centroid, and the outer ring psf/cursor. The psf command can be used to extract the surface brightness profile of extended sources, for further analysis outside XIMAGE.
In most cases the PSF is independent of energy, but for the ROSAT PSPC this is not the case. In this case extract/psf_pha is used to accumulate a spectrum for the source (use circle to define the region). This spectrum is written to a file called rosat_pspc.psf, which is then used by psf to calculate the PSF weighted for spectrum. If no spectrum is accumulated, then a PSF at 1 keV is used. To speed up detect and sosta the 1keV average is used.
For a detector that is unknown to XIMAGE, a PSF can be created, using psf/calfile. This writes a PSF to the local directory with the name psf.dat. This file is then used by detect and sosta for subsequent analysis.
The following is an example of how to make a PSF for the ROSAT PSPC. This procedure is more complex because the PSF is energy dependent, so a spectrum must first be accumulated.
For ROSAT PSPC only:
read/rfits/xpix=5000/ypix=5000 rp500012.fits ! make an image from the
! events file.
disp ! display it
circle ! define a source region
extract/psf_pha ! extract a spectrum
For all instruments (for non-PSPC first read in the image):
back/opt ! optimum background psf/cur/back=1.0e-03 ! calculate the psf