FITSRegister : Semi-Automatic Image Alignment for FITS Files
Sky Coyote, 23 Jan 2007

FITSRegister is a Macintosh OS X program for aligning a set of astronomical or other images which are saved in the FITS (Flexible Image Transport System) format. For more info about the FITS format, see

Each FITSRegister document contains 2 monochrome images of the same size. Each image can be scaled and displayed with independent settings. Images can be flipped or have their values inverted, and can be viewed at any zoomed-in factor. The second image can be shifted relative to the first in X and Y, at both integer and fractional pixel values (using FFT sinc interpolation). The images can be animated in a looped sequence for easier detection of alignment errors. A circle, and vertical and horizontal lines can be overlayed on both images as an alignment guide. A subregion of the frame can be used to automatically adjust the brightness and contrast to be the same in both images. In addition, both a direct (normalized parametric shift & multiply) and FFT cross-correlation can be computed between the subregions in each image, at both integer and fractional pixel resolutions. A 6-parameter quadratic surface can also be fit to the resulting correlation surface. Once aligned, the second image can be saved as another FITS file. Using FITSRegister, one can quickly and easily align several images for stacking, further processing, or motion detection of features between images.

FITSRegister requires a G4, G5, or Intel Macintosh computer running OS X 10.3.9, 10.4.8, or greater. FITSRegister is free for all non-commercial users.

To download FITSRegister, click here:

----------> FITSRegister Online User's Manual and Tutorial <----------

Latest updates (details in linked pages):

01/23/07: Fixed display bug which did not apply {DX, DY} shift unless image 2 was showing.
12/29/06: Incorporation of sub-pixel estimation into main program.
12/22/06: Sub-pixel estimation of cross-correlation peaks by iterative image interpolation.
12/07/06: Sub-pixel estimation of cross-correlation peaks by quadratic surface fitting.
11/30/06: Additional work on cross-correlations.
11/04/06: Cross-correlations on subsets of two images.

FITSRegister was created by:

Software development, manual, and website by Sky Coyote.

This software was partially funded by a grant from the NASA AIS program and additional funding from the NASA Infrared Telescope Facility.

Please send comments, requests, wish-lists, etc..., to Dr. Eliot Young:

Please send bug reports and website errors to:

İSky Coyote and Dr. Eliot Young, 2006-2007