Transform between coordinate frames

Both SkyCoord and BaseCoordinateFrame instances have a transform_to method. This can be used to transform the frame to any other frame, either implemented in sunpy or in Astropy (see also Transforming between Systems). An example of transforming the center of the solar disk to Carrington coordinates is:

>>> import astropy.units as u
>>> from astropy.coordinates import SkyCoord

>>> from sunpy.coordinates import frames

>>> coord = SkyCoord(0*u.arcsec, 0*u.arcsec, frame=frames.Helioprojective, obstime="2017-07-26",
...              observer="earth")
>>> coord
<SkyCoord (Helioprojective: obstime=2017-07-26T00:00:00.000, rsun=695700.0 km, observer=<HeliographicStonyhurst Coordinate for 'earth'>): (Tx, Ty) in arcsec
    (0., 0.)>
>>> coord.transform_to(frames.HeliographicCarrington)
<SkyCoord (HeliographicCarrington: obstime=2017-07-26T00:00:00.000, rsun=695700.0 km, observer=<HeliographicStonyhurst Coordinate for 'earth'>): (lon, lat, radius) in (deg, deg, AU)
    (283.95956776, 5.31701821, 0.00465047)>

It is also possible to transform to any coordinate system implemented in Astropy. This can be used to find the position of the solar limb in AltAz equatorial coordinates:

>>> from astropy.coordinates import EarthLocation, AltAz

>>> time = '2017-07-11 15:00'
>>> greenbelt = EarthLocation(lat=39.0044*u.deg, lon=-76.8758*u.deg)
>>> greenbelt_frame = AltAz(obstime=time, location=greenbelt)
>>> west_limb = SkyCoord(900*u.arcsec, 0*u.arcsec, frame=frames.Helioprojective,
...                      observer=greenbelt.get_itrs(greenbelt_frame.obstime), obstime=time)  
>>> west_limb.transform_to(greenbelt_frame)  
<SkyCoord (AltAz: obstime=2017-07-11 15:00:00.000, location=(1126916.53031967, -4833386.58391627, 3992696.62211575) m, pressure=0.0 hPa, temperature=0.0 deg_C, relative_humidity=0.0, obswl=1.0 micron): (az, alt, distance) in (deg, deg, m)
    (111.40782056, 57.1660434, 1.51859559e+11)>