Coordinates and WCS

The sunpy.coordinates sub-package provides a mapping between FITS-WCS CTYPE convention and the coordinate frames as defined in sunpy.coordinates. This is used via the astropy.wcs.utils.wcs_to_celestial_frame function, with which the sunpy frames are registered upon being imported. This list is used by for example, wcsaxes to convert from astropy.wcs.WCS objects to coordinate frames.

The sunpy.map.GenericMap class creates astropy.wcs.WCS objects as amap.wcs, however, it adds some extra attributes to the WCS object to be able to fully specify the coordinate frame. It adds heliographic_observer and rsun.

If you want to obtain a un-realized coordinate frame corresponding to a GenericMap object you can do the following:

>>> import sunpy.map
>>> from sunpy.data.sample import AIA_171_IMAGE  

>>> amap = sunpy.map.Map(AIA_171_IMAGE)  
>>> amap.observer_coordinate  
<SkyCoord (HeliographicStonyhurst: obstime=2011-06-07T06:33:02.770, rsun=696000.0 km): (lon, lat, radius) in (deg, deg, m)
    (-0.00406308, 0.04787238, 1.51846026e+11)>

which is equivalent to:

>>> from astropy.wcs.utils import wcs_to_celestial_frame 

>>> wcs_to_celestial_frame(amap.wcs)  
<Helioprojective Frame (obstime=2011-06-07T06:33:02.770, rsun=696000.0 km, observer=<HeliographicStonyhurst Coordinate (obstime=2011-06-07T06:33:02.770, rsun=696000.0 km): (lon, lat, radius) in (deg, deg, m)
    (-0.00406308, 0.04787238, 1.51846026e+11)>)>