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WCS.dropaxis() does not update "NAXIS" keyword

See original GitHub issue

When working with WCS.dropaxis() I’ve encountered the following issue:

from astropy.io import fits
from astropy.wcs import WCS

# Define an example WCS reference header
header = fits.Header()
header['WCSAXES'] = 3
header['NAXIS'] = 3
header['NAXIS1'] = 100
header['NAXIS2'] = 100
header['NAXIS3'] = 3
header['CRPIX1'] = 100.5
header['CRPIX2'] = 100.5
header['CRPIX3'] = 0.5
header['CDELT1'] = -0.02
header['CDELT2'] = 0.02  
header['CDELT3'] = 0.2
header['CTYPE1'] = 'RA---CAR'
header['CTYPE2'] = 'DEC--CAR'
header['CTYPE3'] = 'ENER-LOG'
header['CRVAL1'] = 0
header['CRVAL2'] = 0
header['CRVAL3'] = 1

wcs = WCS(header)
print(repr(wcs))

Returns:

WCS Keywords

Number of WCS axes: 3
CTYPE : 'RA---CAR'  'DEC--CAR'  'EN-LOG'  
CRVAL : 0.0  0.0  1.0  
CRPIX : 100.5  100.5  0.5  
PC1_1 PC1_2 PC1_3  : 1.0  0.0  0.0  
PC2_1 PC2_2 PC2_3  : 0.0  1.0  0.0  
PC3_1 PC3_2 PC3_3  : 0.0  0.0  1.0  
CDELT : -0.02  0.02  0.2  
NAXIS : 100  100  3

If I now drop the third axis using wcs.dropaxis():

wcs = wcs.dropaxis(2)
print(repr(wcs))
print(wcs._naxis)

The axis is dropped correctly, however it is still present in the "NAXIS" keyword of the repr:

WCS Keywords

Number of WCS axes: 2
CTYPE : 'RA---CAR'  'DEC--CAR'  
CRVAL : 0.0  0.0  
CRPIX : 100.5  100.5  
PC1_1 PC1_2  : 1.0  0.0  
PC2_1 PC2_2  : 0.0  1.0  
CDELT : -0.02  0.02  
NAXIS : 100  100  3
[100, 100, 3]

Issue Analytics

State:
Created 4 years ago
Comments:5 (5 by maintainers)

Top GitHub Comments

1reaction

pllimcommented, Aug 15, 2022

I don’t think we have any planned 4.x releases no more, so it is okay if you cannot test on 4.x. Thanks!

1reaction

cmarmocommented, Aug 14, 2022

Hello, I have checked the snippet of code with the development version

import astropy
from astropy.io import fits
from astropy.wcs import WCS

print(astropy.__version__)

# Define an example WCS reference header
header = fits.Header()
header['WCSAXES'] = 3
header['NAXIS'] = 3
header['NAXIS1'] = 100
header['NAXIS2'] = 100
header['NAXIS3'] = 3
header['CRPIX1'] = 100.5
header['CRPIX2'] = 100.5
header['CRPIX3'] = 0.5
header['CDELT1'] = -0.02
header['CDELT2'] = 0.02  
header['CDELT3'] = 0.2
header['CTYPE1'] = 'RA---CAR'
header['CTYPE2'] = 'DEC--CAR'
header['CTYPE3'] = 'ENER-LOG'
header['CRVAL1'] = 0
header['CRVAL2'] = 0
header['CRVAL3'] = 1

wcs = WCS(header)
print("\nRepr wcs:\n", repr(wcs))

wcs = wcs.dropaxis(2)
print("\nRepr wcs:\n", repr(wcs))
print("\nwcs._naxis:\n", wcs._naxis)

And the output is

5.2.dev341+gf4615bfaf

Repr wcs:
 WCS Keywords

Number of WCS axes: 3
CTYPE : 'RA---CAR'  'DEC--CAR'  'ENER-LOG'  
CRVAL : 0.0  0.0  1.0  
CRPIX : 100.5  100.5  0.5  
PC1_1 PC1_2 PC1_3  : 1.0  0.0  0.0  
PC2_1 PC2_2 PC2_3  : 0.0  1.0  0.0  
PC3_1 PC3_2 PC3_3  : 0.0  0.0  1.0  
CDELT : -0.02  0.02  0.2  
NAXIS : 100  100  3

Repr wcs:
 WCS Keywords

Number of WCS axes: 2
CTYPE : 'RA---CAR'  'DEC--CAR'  
CRVAL : 0.0  0.0  
CRPIX : 100.5  100.5  
PC1_1 PC1_2  : 1.0  0.0  
PC2_1 PC2_2  : 0.0  1.0  
CDELT : -0.02  0.02  
NAXIS : 100  100

wcs._naxis:
 [100, 100]

It looks to me that the problem is now solved, but I don’t know the code enough to understand when and where this was fixed.