Syntax: arc2arc (FITS file) (2nd FITS file)
                        [-desat] [-plot] [pxsh=]
    Find pixel shift between two reduced arclamp spec/images.
    Find pixel shift of #2 relative to #1.
    Copy wavelength scale from #1 to #2.
    It is assumed that the setups are similar (+/- 20 pixels.)
 -desat : Blank out saturated and bleeding columns.
 -plot  : Show correlation in PGDISP (XDISP) window.
 pxsh=  : Specify pixel shift explicitly (bypass calculation.)
          Use "pxsh=0" to copy the wavelength scale.