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.