To overlay images from different missions, e.g. ROSAT ontop of ASCA, the images must be rebinned as they are read, so that the scale size of each pixel is the same number of arc sec per pixel. To do a contour overlay then do the following sequence:
read/fits/size=800 inst1.fits ! read the 1st instrument image
save_image ! save the image
display ! display the 1st image
read/fits/rebin=30 inst2.fits ! read the 2nd instrument image, rebinning to
! get the same plate scale
smooth ! smooth the 2nd image
cont/overlay ! make a contour plot and overlay it
! onthe current image.
grid 0.5 0.5 ! draw a skygrid