Download program from main page
The sub-pixel algorithm demonstrated last time has been incorporated into FITSRegister. All recent updates are now consolidated, and both PPC and Intel versions of the program are available from the main page. The cross-correlation window has been changed to the following:
Select the resolution you want from the pop-up menu. If you select 1 pixel, only one cross-correlation will be performed, and the result displayed to the nearest integer. If you choose a resolution less than 1 pixel, additional iterations of cross-correlation will be performed, each one yielding an additional bit of information about the peak location (e.g. 1, 1/2, 1/4, 1/8, etc... pixels of accuracy). This will in general change both the location of the peak and also its value. The plot and the table, however, will still only display the integer cross-correlation surface, since due to the iterative algorithm there is only a small subset of the entire surface available at each step.
Caution should be exercised in performing high-resolution cross-correlations of more than the smallest image subsets (selection rectangle area), as both subsets of the images will be interpolated to very large arrays during the iterations. For example, a cross-correlation of 25x25 and 75x75 subimages to 1/64 pixel accuracy eventually produces 1537x1537 and 1541x1541 arrays, even through the displayed result is only 51x51. Larger subimages produce correspondingly larger arrays, as the number of pixels of each subimage is multiplied by almost 4 at each iteration.
İSky Coyote and Dr. Eliot Young, 2006