Merge pull request #15659 from eerovaher/deduplicate-separation

Ensure `BaseCoordinateFrame` separation methods work with `SkyCoord` input

astropy/coordinates/baseframe.py

@@ -1779,7 +1779,7 @@ def separation(self, other):
 
         Parameters
         ----------
-        other : `~astropy.coordinates.BaseCoordinateFrame`
+        other : `~astropy.coordinates.BaseCoordinateFrame` or `~astropy.coordinates.SkyCoord`
             The coordinate to get the separation to.
 
         Returns
@@ -1798,8 +1798,11 @@ def separation(self, other):
         from .angles import Angle, angular_separation
 
         self_unit_sph = self.represent_as(r.UnitSphericalRepresentation)
-        other_transformed = other.transform_to(self)
-        other_unit_sph = other_transformed.represent_as(r.UnitSphericalRepresentation)
+        other_unit_sph = (
+            getattr(other, "frame", other)
+            .transform_to(self)
+            .represent_as(r.UnitSphericalRepresentation)
+        )
 
         # Get the separation as a Quantity, convert to Angle in degrees
         sep = angular_separation(
@@ -1814,7 +1817,7 @@ def separation_3d(self, other):
 
         Parameters
         ----------
-        other : `~astropy.coordinates.BaseCoordinateFrame`
+        other : `~astropy.coordinates.BaseCoordinateFrame` or `~astropy.coordinates.SkyCoord`
             The coordinate system to get the distance to.
 
         Returns
@@ -1835,7 +1838,7 @@ def separation_3d(self, other):
             )
 
         # do this first just in case the conversion somehow creates a distance
-        other_in_self_system = other.transform_to(self)
+        other_in_self_system = getattr(other, "frame", other).transform_to(self)
 
         if issubclass(other_in_self_system.__class__, r.UnitSphericalRepresentation):
             raise ValueError(

astropy/coordinates/tests/test_frames.py

@@ -736,6 +736,21 @@ def test_sep():
         i7.separation_3d(i3)
 
 
+@pytest.mark.parametrize(
+    "method,expectation",
+    [
+        pytest.param("separation", 0.69815121 * u.deg, id="separation"),
+        pytest.param("separation_3d", 0.12184962 * u.pc, id="separation_3d"),
+    ],
+)
+def test_seps_with_skycoord(method, expectation):
+    coords = (1 * u.deg, 2 * u.deg, 10 * u.pc)
+    assert_allclose(
+        getattr(FK5(*coords), method)(SkyCoord(*coords, frame=FK5, equinox="B1950")),
+        expectation,
+    )
+
+
 def test_time_inputs():
     """
     Test validation and conversion of inputs for equinox and obstime attributes.

docs/changes/coordinates/15659.bugfix.rst

@@ -0,0 +1,4 @@
+Previously passing a ``SkyCoord`` instance to the ``BaseCoordinateFrame``
+``separation()`` or ``separation_3d()`` methods could produce wrong results,
+depending on what additional frame attributes were defined on the ``SkyCoord``,
+but now ``SkyCoord`` input can be used safely.