[0.25.2] test_fitswcs_imaging fails with Astropy 7.1.0

[olebole]

In the current Debian unstable environment (Astropy 7.1.0, wcslib 8.4, numpy 2.2.4, asdf-wcs-schemas 0.5.0) test_fitswcs_imaging of gwcs 0.25.2 fails:

    def test_fitswcs_imaging(fits_wcs_imaging_simple):
        """Test simple FITS type imaging WCS."""
        gwcs, astwcs = fits_wcs_imaging_simple
        forward_transform = gwcs.forward_transform
        crpix = forward_transform.crpix
        crval = forward_transform.crval
        ra, dec = gwcs(*crpix)
        ast_ra, ast_dec = astwcs.wcs_pix2world(crpix[0], crpix[1], 1)
        assert_allclose((ra, dec), crval)
        if crval[1] in (90, -90):
>           assert_allclose((ast_ra, ast_dec), crval)
E           AssertionError: 
E           Not equal to tolerance rtol=1e-07, atol=0
E           
E           Mismatched elements: 1 / 2 (50%)
E           Max absolute difference among violations: 180.
E           Max relative difference among violations: 32.14285714
E            ACTUAL: array([185.6, -90. ])
E            DESIRED: array([  5.6, -90. ])

gwcs/tests/test_wcs.py:1909: AssertionError

This is new test code that was introduced with #616.

[nden]

Hmm. Thanks for the report!

[nden]

@olebole Do these tests use a system installed wcslib or the one bundled with astropy?

[olebole]

I use a system installed wcslib 8.4.

[olebole]

The failure line looks a bit like a copy&paste without a needed adjustment (both branches have identical code):

gwcs/gwcs/tests/test_wcs.py

Lines 1908 to 1911 in a680872

	if crval[1] in (90, -90):
	assert_allclose((ast_ra, ast_dec), crval)
	else:
	assert_allclose((ast_ra, ast_dec), crval)

[nden]

Yes, it looks like a commit was missed.

But I don't think that's the reason for the failure. It's passing locally and on GA.

[nden]

I was able to reproduce this result on a RHEL9 system with a system installed wcslib.

[nden]

The failure occurs when astropy is built from source using a system level wcslib. System built wcslib gives different results from the one bundled with astropy only when crval1=-90.
I suspect the failure is due to differences in the wcslib build process (system vs bundled with astropy). A minimal reproducible example is

from astropy import wcs
w = wcs.WCS(naxis-2)
w.wcs.ctype = ['RA---TAN', 'DEC--TAN']
w.wcs.crval = [5.6, -90]
w.wcs.crpix = [6, 6]
w.all_pix2sky(6, 6, 1)

The result is ra=185.6, dec=-90 with system built wcslib, while it's (5.6, -90) with the bundled wcslib.

One thing to note is that the internally computed euler angles are different, the last one being sin(180) but not equal to 0 in the bundled wcslib which fails a comparison internally in wcslib/

print(w.wcs.cel.euler)
[ 5.6000000e+00  1.8000000e+02  1.8000000e+02 -1.0000000e+00 1.2246468e-16] 

I will leave this open but not spend more time on it, since it's not strictly a gwcs issue, and will skip the test for now.