Once-asked Questions

Q: My images have some crazy WCS that your code doesn’t understand. What do I do?

My code looks like this:

from astrometry.util.util import Tan
from tractor import FitsWcs

wcs = FitsWcs(Tan('myimage.fits'))

and it’s failing like:


What do I do?


One option is to create a Tan object yourself and populate it with the required parameters:

t = Tan()
t.set_crpix(24, 530)
t.set_crval(234.66, 47.8765)
t.set_cd(1., 0., 0., 1.)
t.set_imagesize(1024, 1024)
wcs = FitsWcs(t)

And you’ll probably actually do that by opening your image file and parsing its crazy header cards, converting them to TAN as understood by our code:

import pyfits

hdr = pyfits.open('myimage.fits')[0].header

t = Tan()
t.set_crpix(hdr.get('CRPIX1'), hdr.get('CRPIX2'))
t.set_crval(hdr.get('CRVAL1'), hdr.get('CRVAL2'))
cd1 = hdr.get('CDELT1')
cd2 = hdr.get('CDELT2')
# assume your images have no rotation...
t.set_cd(cd1, 0., 0., cd2)
t.set_imagesize(1024, 1024)
wcs = FitsWcs(t)