Read World Cooordinate System parameters from a .wcs file, such as those written by SAOImageDS9.

Install

pip install git+https://github.com/will-henney/wcsfile.git

It needs to be installed from github since it has not been submitted to PyPI yet.

How to use

Use wcsfile.read() to read a WCS file into a python dict.

import wcsfile
wcsdict = wcsfile.read("testdata/mosaic-1996-HH204-align-robberto.wcs")
wcsdict

{'DATE-OBS': '21/03/95',
 'EQUINOX': 2000,
 'CTYPE1': 'RA---TAN',
 'CRPIX1': 388.39887,
 'CRVAL1': 83.8433551,
 'CDELT1': 0.0,
 'CTYPE2': 'DEC--TAN',
 'CRPIX2': 567.14165,
 'CRVAL2': -5.4192964,
 'CDELT2': 0.0,
 'CD1_1': -2.76664e-05,
 'CD1_2': 3.804223e-08,
 'CD2_1': 4.913188e-09,
 'CD2_2': 2.764793e-05}

The dict can be used to initialize a FITS header object.

from astropy.io import fits

hdr = fits.Header(wcsdict)
hdr

DATE-OBS= '21/03/95'                                                            
EQUINOX =                 2000                                                  
CTYPE1  = 'RA---TAN'                                                            
CRPIX1  =            388.39887                                                  
CRVAL1  =           83.8433551                                                  
CDELT1  =                  0.0                                                  
CTYPE2  = 'DEC--TAN'                                                            
CRPIX2  =            567.14165                                                  
CRVAL2  =           -5.4192964                                                  
CDELT2  =                  0.0                                                  
CD1_1   =         -2.76664E-05                                                  
CD1_2   =         3.804223E-08                                                  
CD2_1   =         4.913188E-09                                                  
CD2_2   =         2.764793E-05                                                  

Or it can be used to initialize a WCS object.

from astropy.wcs import WCS

w = WCS(wcsdict)
w

WARNING: FITSFixedWarning: 'datfix' made the change 'Set MJD-OBS to 49797.000000 from DATE-OBS.
Changed DATE-OBS from '21/03/95' to '1995-03-21''. [astropy.wcs.wcs]

WCS Keywords

Number of WCS axes: 2
CTYPE : 'RA---TAN'  'DEC--TAN'  
CRVAL : 83.8433551  -5.4192964  
CRPIX : 388.39887  567.14165  
CD1_1 CD1_2  : -2.76664e-05  3.804223e-08  
CD2_1 CD2_2  : 4.913188e-09  2.764793e-05  
NAXIS : 0  0

assert w.wcs.lattyp == "DEC"

Python Library Dependencies

• toml
• astropy (for usage examples and tests only)