Name
       xcorr2d - Find offsets between frames of a cube.

Synopsis
       xcorr2d [options] <in>

Description
       xcorr2d  is used to detect offsets between frames of a cube.  This com-
       mand is useful for jittered data sets, where the offsets between frames
       have unfortunately not been saved together with the data files, or can-
       not easily be derived from the FITS headers.

       The algorithm makes  use  of  an  FFT-based  cross-correlation  between
       frames to identify the offsets. This search is normally precise to +/-2
       pixels in X and Y.

       Several assumptions are made about the input data  set.   Input  images
       must  be  larger  than  512x512,  otherwise there would probably not be
       enough information for the  cross-correlation  method  to  work.  Input
       images should only differ in shift, not in scale, not in rotation. They
       should also not present too much offset or gain difference.

       The algorithm expects all images to be compared to the first one in the
       cube.  If  you  want  to  use  another reference, use the -r/--refplane
       option.

Options
       -r or --refplane num
              Specify which plane is to be taken as a reference.   This  plane
              will have offset (0,0) by definition.

Algorithm
       Cross-correlation  is  performed on subsampled images in Fourier space,
       so the results cannot be more precise than +/-2 pixels.

Files
       The input must be a cube. A cube can either be a FITS file with several
       planes  (NAXIS==3  and  NAXIS3>1)  or a list of frame names given in an
       ASCII file.