Diffraction spikes show artifacts after shifting by shiftnd

Hi,

I’m trying to shift some CCD FITS images using chi2_shift then shiftnd. The x and y offsets between two images are calculated without any problem. However, when I try to shift my images with shiftnd the new images show some artifacts. There are diffraction spikes around the bright(ish) targets, and these features are shifted in a very strange way. Here is a plot, where the upper panel shows the original image and the bottom shows the shifted.

I can see a periodicity with a period of 1 pixel in the spikes which is introduced by shiftnd. This affects the photometry in a bad way. My only solution is to use integer offsets, but this means +/- 0.5 pixel uncertainty, which is greater than I can accept. Has anyone encountered this issue? Any solution?

Thanks

[astrobatty]

I checked all the stars and all of these problematic cases have at least one saturated pixel, that is the problem. Thanks for the suggestions.

[keflavich]

I think this is coming from a couple of super-bright, maybe saturated pixels? If they are genuinely saturated, try replacing them with the local median, then re-shifting. If they’re not saturated and the star really is that bright, I’m afraid you have some dynamic range issues and I’m not 100% sure how to deal with them.