Why features on WCS projected subplot are in the wrong place in matplotlib?
I have a fits file about an astronomical object. I can plot it like this:
from astropy.io import fits
from astropy.wcs import WCS
hdul = fits.open(fitsfilename)[0]
wcs = WCS(hdul.header)
fig = plt.figure(figsize=(12,12))
fig.add_subplot(111, projection=wcs)
plt.imshow(hdul.data)
This works, and produces a nice pic:
I would like to add some additional features to this plot, which doesn't work. For example lets try to add a circle to 119°, -67°30'. I expand the code by:
plt.scatter([119],[-67.5],c='r',s=500)
What I get is:
Which is really not what we wanted, the circle is around 118°5', -67°5', not where it is supposed to be (119°, -67°30').
What am I getting wrong, or what is a good fix to this issue?
Note: when I run wcs = WCS(hdul.header), I get a warning:
WARNING: VerifyWarning: Verification reported errors: [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'A_2_0' is not FITS standard (invalid value string: '3.29341755408e-05'). Fixed 'A_2_0' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Note: astropy.io.fits uses zero-based indexing. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'A_1_1' is not FITS standard (invalid value string: '1.51709339878e-05'). Fixed 'A_1_1' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'A_0_2' is not FITS standard (invalid value string: '5.17973753556e-06'). Fixed 'A_0_2' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'B_2_0' is not FITS standard (invalid value string: '2.97627426087e-06'). Fixed 'B_2_0' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'B_1_1' is not FITS standard (invalid value string: '2.71948126373e-05'). Fixed 'B_1_1' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'B_0_2' is not FITS standard (invalid value string: '1.66848449653e-05'). Fixed 'B_0_2' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'AP_1_0' is not FITS standard (invalid value string: '1.79541533196e-06'). Fixed 'AP_1_0' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'AP_0_1' is not FITS standard (invalid value string: '9.20624843151e-07'). Fixed 'AP_0_1' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'AP_2_0' is not FITS standard (invalid value string: '-3.29292923201e-05'). Fixed 'AP_2_0' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'AP_1_1' is not FITS standard (invalid value string: '-1.51738446887e-05'). Fixed 'AP_1_1' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'AP_0_2' is not FITS standard (invalid value string: '-5.18321445978e-06'). Fixed 'AP_0_2' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'BP_1_0' is not FITS standard (invalid value string: '8.99029048217e-07'). Fixed 'BP_1_0' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'BP_0_1' is not FITS standard (invalid value string: '1.15967736014e-06'). Fixed 'BP_0_1' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'BP_2_0' is not FITS standard (invalid value string: '-2.97837492348e-06'). Fixed 'BP_2_0' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'BP_1_1' is not FITS standard (invalid value string: '-2.71998518336e-05'). Fixed 'BP_1_1' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'BP_0_2' is not FITS standard (invalid value string: '-1.66872388359e-05'). Fixed 'BP_0_2' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'WCSR_PRJ' is not FITS standard (invalid value string: '3.6679e-07'). Fixed 'WCSR_PRJ' card to meet the FITS standard. [astropy.io.fits.verify] WARNING: VerifyWarning: Card 'WCSR_PIX' is not FITS standard (invalid value string: '8.2565e-05'). Fixed 'WCSR_PIX' card to meet the FITS standard. [astropy.io.fits.verify]
So this might be relevant; question of how to fix it still holds.
To plot in world coordinates, you need to specify the transform, e.g.:
ax = fig.gca()
ax.scatter([34], [3.2], transform=ax.get_transform('world'))
Generally, you can ignore those FITS header warnings, since none of the FITS header cards there are related to the WCS (afaik).