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Fix swapped pixel coordinates in skycoord_to_pixel and pixel_to_skycoord - #20473

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fix-skycoord-pixel-axis-order
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Fix swapped pixel coordinates in skycoord_to_pixel and pixel_to_skycoord#20473
astrofrog wants to merge 2 commits into
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fix-skycoord-pixel-axis-order

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@astrofrog astrofrog commented Sep 23, 2026 •
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Description

This fixes a long-standing bug which caused pixel coordinates returned from skycoord_to_pixel and passed to pixel_to_skycoord to be assumed to be in the wrong order if the original WCS has world coordinates in lat/lon or dec/ra

However we can't simply change this straight away since the existing code has been in use for 10 years, so I've added a deprecation mechanism:

In [1]: from astropy.coordinates import SkyCoord
   ...: from astropy.wcs.utils import pixel_to_skycoord, skycoord_to_pixel
   ...: from astropy.wcs import WCS
   ...: wcs = WCS(naxis=2)
   ...: wcs.wcs.ctype = 'DEC--TAN', 'RA---TAN'
   ...: wcs.wcs.cdelt = -2, 1
   ...: coord = SkyCoord.from_name('M31')

In [2]: skycoord_to_pixel(coord, wcs)
   ...: 
WARNING: AstropyDeprecationWarning: The WCS has latitude before longitude, and skycoord_to_pixel currently
 treats pixel coordinates as being swapped (with x corresponding to longitude and y to latitude) rather than in the
 native order of the WCS pixel axes. In a future version, the native order will be used. Pass native_pixel_order=True 
to opt in to the new behavior now, or native_pixel_order=False to keep the current behavior. [astropy.wcs.utils]
Out[2]: (array(9.8103131), array(-26.58367674))

In [3]: skycoord_to_pixel(coord, wcs, native_pixel_order=False)
   ...: 
Out[3]: (array(9.8103131), array(-26.58367674))

In [4]: skycoord_to_pixel(coord, wcs, native_pixel_order=True)
   ...: 
Out[4]: (array(-26.58367674), array(9.8103131))

Fixes #4976
Fixes #10468

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This was coded up partially by hand and partially by Claude. I have reviewed and agree with all the changes.

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@astrofrog
astrofrog requested a review from mcara as a code owner September 23, 2026 16:10
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@astrofrog astrofrog added this to the v8.1.0 milestone Sep 23, 2026
@astrofrog astrofrog changed the title Fix swapped pixel coordinates in skycoord_to_pixel and pixel_to_skycoord Fix swapped pixel coordinates in skycoord_to_pixel and pixel_to_skycoord Sep 23, 2026
@astrofrog
astrofrog requested a review from Cadair September 23, 2026 16:13
@astrofrog astrofrog added the Bug label Sep 23, 2026
@pllim pllim added the API change PRs and issues that change an existing API, possibly requiring a deprecation period label Sep 23, 2026
@pllim

pllim commented Sep 23, 2026

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tsk tsk Tom R, this isn't from a fork...

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@mcara mcara Awaiting requested review from mcara mcara is a code owner
@Cadair Cadair Awaiting requested review from Cadair

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Successfully merging this pull request may close these issues.

WCS does not deal with flipped ctype FITS header astropy.wcs.utils.skycoord_to_pixel() does not work as expected

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