A region file can be made which defines the region enclosed within a specific intensity level contour. First the image must be smoothed:
cl> xray xr> xspatial xs> imsmooth ``xdata$snr.qp(400:600,400:600)'' snr gaus 5 # output is snr_smo.imh xs> isoreg xnr_smo snr 0.2 # output is snr_iso.pl xs> pllist snr_iso.pl # shows the mask.