[gnuastro-devel] [bug #50073] ImageWarp 'rotate' does not correctly propagate WCS pixel scale information into header

[Lee Kelvin]

Follow-up Comment #7, bug #50073 (project gnuastro):

Thank you for all this extra information! What bizarre behaviour! How curious
that MJD-OBS should interfere with the plotting of images in DS9...? I do not
understand why this should be the case.

Firstly, I followed your suggestion and upgraded my version of DS9. I am now
running DS9 version 7.5. This did make a difference (bringing the before and
after images into closer agreement), but did not completely fix the issue.

On your suggestion, I upgraded my WCSLIB to version 5.15. I have recompiled
GNU Astro and I can now confirm that the CDELTn header keywords print all
significant figures and does also print MJD-OBS into the header.

As an aside, task #14315 <https://savannah.gnu.org/task/index.php?14315> may
help with issues such as this in the future, by propagating all FITS header
keywords into any resultant output files.

I attempted to blink between the images and, once more, found a small offset.
The offset now is exceptionally small (single-pixel level), and I would not
have noticed it had we not started this bug hunt with a much larger offset.

I took a close look at the header to see if anything else was going on. I
found a few things:

* MJD-OBS is missing 1 significant figure when compared with the input file
* the header keyword RADECSYS has been changed to RADESYS (missing the C)
* A LATPOLE/LONPOLE have been assumed (rightly so?)

However, on a whim, I tried one more thing... I used DS9 to blink between
exactly the same input image! Lo and behold, I found an offset! The same
single-pixel level offset I mentioned above. So, I can conclude that the
initial bug has now been fixed by upgrading to the latest version of WCSLIB
and DS9, and this continuing mis-match must be a DS9 issue.

Thank you for your help with this bug, much appreciated.

Best, 
Lee Kelvin


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