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Fix PC to CD matrix computation in HSTWCS #228

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Fix PC to CD matrix computation in HSTWCS #228

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5 changes: 5 additions & 0 deletions CHANGES.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -8,10 +8,15 @@ Unreleased

- Return default response when request to the GSC service failed. [#227]

- Fix a bug in ``stwcs.wcsutil.hstwcs.HSTWCS.pc2cd()`` that would result in
incorrect CD matrix when the WCS has a PC matrix. [#228]


1.7.5 (2025-07-14)
------------------
- Work around an issue with the AStrometry DB which returns NaNs in certain cases. [#221]


1.7.4 (2025-01-07)
------------------

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26 changes: 26 additions & 0 deletions stwcs/tests/test_hstwcs.py
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@@ -1,5 +1,6 @@
from astropy.io import fits
from stwcs.wcsutil import hstwcs, HSTWCS
import numpy as np


def test_radesys():
Expand All @@ -22,3 +23,28 @@ def test_prihdu_with_extver_no_extname():
extname = HSTWCS(hdulist).extname
assert extname == ('PRIMARY', 7)
assert hdulist[extname] is hdulist[0]


def test_pc2cd():
e = np.exp(1)

@nden nden Aug 29, 2025

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why not use np.e?

hdulist = fits.HDUList([
fits.PrimaryHDU(header=fits.Header([('extver', 1)])),
fits.ImageHDU(header=fits.Header([
('pc1_1', 0.1),
('pc1_2', 0.08),
('pc2_1', -0.05),
('pc2_2', 0.7),
('cdelt1', e),
('cdelt2', np.pi),
('crpix1', 512.0),
('crpix2', 512.0),
('crval1', 150.0),
('crval2', 2.0),
('ctype1', 'RA---TAN'),
('ctype2', 'DEC--TAN'),
]))
])
wcs = HSTWCS(hdulist, ext=1)
assert wcs.wcs.has_cd()
cd = np.array([[0.1 * e, 0.08 * e], [-0.05 * np.pi, 0.7 * np.pi]])
assert np.allclose(wcs.wcs.cd, cd)
2 changes: 1 addition & 1 deletion stwcs/wcsutil/hstwcs.py
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Expand Up @@ -473,7 +473,7 @@ def _idc2hdr(self):

def pc2cd(self):
if self.wcs.has_pc():
self.wcs.cd = self.wcs.pc * self.wcs.cdelt[1]
self.wcs.cd = self.pixel_scale_matrix

@nden nden Aug 29, 2025

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This will work for HST files because they don't have a PC matrix. However, the example in the test generates an incorrect file because it has now a CD, a PC, and CDELT all defined.

I'm not sure offhand what the correct action here is. I need to look into how this is used and whether it should be deleted.

@mcara mcara Jan 11, 2026

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the test generates an incorrect file because it has now a CD, a PC, and CDELT all defined

@nden Which test?


@deprecated_renamed_argument('accuracy', 'tolerance', '1.6.1', arg_in_kwargs=True)
def all_world2pix(self, *args, **kwargs):
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