Due to pointing uncertainties, it may happen that one image appears to
be slightly shifted relative to another image of the same field,
taken in another wavelength range or by another instrument. It is
possible (and often quite useful) to shift one image before
superimposing it onto another image where the astrometry is more
accurate. This can be done either by an automatic method based on the
correlation between the images (using the /SHIFT keyword), or manually
by specifying directly the desired offset
(OFFSET=[x_offset,y_offset]). Note that only one image is shifted
with respect to the reference image (see section 3.5 about the REF
keyword).
Example:
[ ISOCONT, 'm51_dss.fits', iso_struct_raster, /SHIFT ]
Create a color image of m51_dss.fits with contour levels of iso_struct_raster. The non-reference image (iso_struct_raster in this example) is shifted in order to correspond better to the reference image, using an automatic correlation method (/SHIFT). The offset can be introduced manually in pixel units (OFFSET=[-1,+2] for -1/+2 pixels in the X/Y directions respectively). Note that the scaling factor determined by ISOCONT is applied to OFFSET. Note also that the default reference image is IMAGE_1, and IMAGE_2 is shifted with respect to IMAGE_1. This can be changed by setting REF=2 (see Fig. 11).