Open astrobatty opened 3 years ago
Interesting, this is almost certainly an aliasing issue. A simple fix might be to apply a low-pass filter. Can't come up with an example right now, but if you fft, shift it, zero the corners, then shift back, it might fix this.
I tried to set the corners to zero, but as the information is in the high frequencies the result is a blurred image. Another try was zero-padding the original image, but the result didn't change.
just setting the corners to zero shouldn't result in blurring. But some sort of tapering function is needed to prevent this ringing.
I set more than 1-1 value to zero. So, if I set only the corners to zero, then this is the result.
(Edit: changed the figure to a better example.)
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.
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.
Hi,
I'm trying to shift some CCD FITS images using
chi2_shift
thenshiftnd
. The x and y offsets between two images are calculated without any problem. However, when I try to shift my images withshiftnd
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