Merge pull request #7598 from meeseeksmachine/auto-backport-of-pr-759…

…4-on-5.1

Backport PR #7594 on branch 5.1 (Enable use of  SunPyBaseCoordinateFrame outside sunpy)

changelog/7594.bugfix.rst

@@ -0,0 +1 @@
+Fixed a bug that interfered with :func:`astropy.wcs.utils.celestial_frame_to_wcs` when working with a custom subclass of :class:`~sunpy.coordinates.frames.SunPyBaseCoordinateFrame`.

sunpy/coordinates/tests/test_wcs_utils.py

@@ -15,6 +15,7 @@
     HeliographicCarrington,
     HeliographicStonyhurst,
     Helioprojective,
+    SunPyBaseCoordinateFrame,
 )
 from sunpy.coordinates.wcs_utils import (
     _set_wcs_aux_obs_coord,
@@ -438,3 +439,11 @@ def test_observer_hgln_crln_priority():
     # Note: don't test whether crlt or hglt is used---according to
     # _set_wcs_aux_obs_coord, those are expected to always be the same and so
     # the same one is always used
+
+
+def test_sunpybaseframe_external():
+    class MyFrame(SunPyBaseCoordinateFrame):
+        pass
+
+    out = solar_frame_to_wcs_mapping(MyFrame())
+    assert out is None

sunpy/coordinates/wcs_utils.py

@@ -250,6 +250,9 @@ def solar_frame_to_wcs_mapping(frame, projection='TAN'):
             xcoord = 'HGLN' + '-' + projection
             ycoord = 'HGLT' + '-' + projection
             wcs.wcs.cunit = ['deg', 'deg']
+        else:
+            # A subclass not supported by the core library
+            return None
 
     else:
         return None