skycoord obstime and equinox support

.github/workflows/ci.yml

@@ -78,7 +78,7 @@ jobs:
 
       - name: install dependencies
         run: |
-          python -m pip install ".[test]"
+          python -m pip install . --group test
 
       - name: install astropy-xarray
         run: python -m pip install --no-deps .

CHANGELOG.md

@@ -5,6 +5,11 @@
 
 # What's New
 
+## **Unreleased**
+
+- Added supported for {py:attr}`astropy.coordinates.SkyCoord.obstime`.
+- Added supported for {py:attr}`astropy.coordinates.SkyCoord.equinox`.
+
 ## 0.2.1 (18 Apr 2026)
 
 - Added support for `astropy==7.2.0` sky coordinates.

astropy_xarray/coordinates/frame.py

@@ -176,11 +176,6 @@ def load_frame(frame_dict: dict, with_data: bool = False) -> BaseCoordinateFrame
             frame = HCRS(
                 obstime=load_optional_object(Time, frame_dict["obstime"]), **kwargs
             )
-        case "itrs":
-            frame = ITRS(
-                location=load_optional_earthlocation(frame_dict["location"]),
-                **kwargs,
-            )
         case "altaz":
             frame = AltAz(
                 obstime=load_optional_object(Time, frame_dict["obstime"]),
@@ -274,7 +269,7 @@ def load_representation(
     frame_name: str | None,
     data: dict[str, np.ndarray],
 ) -> BaseRepresentation:
-    RepresentationClass = representation.REPRESENTATION_CLASSES.get(representation_type)
+    RepresentationClass = representation.REPRESENTATION_CLASSES[representation_type]
     DifferentialClass = representation.DIFFERENTIAL_CLASSES.get(differential_type)
 
     if frame_name is None:

astropy_xarray/coordinates/sky_coord.py

@@ -5,8 +5,10 @@
 import numpy as np
 import xarray as xr
 from astropy.coordinates import SkyCoord
+from astropy.time import Time
 from astropy.utils import ShapedLikeNDArray
 
+from astropy_xarray.coordinates.core import dump_time, load_optional_object
 from astropy_xarray.coordinates.frame import dump_frame, load_frame, load_representation
 
 _ArrayLike = list | np.ndarray | ShapedLikeNDArray
@@ -102,11 +104,15 @@ def skycoord_to_dataset(
         quantified dataset.
     """
     return xr.Dataset(
-        coords=coords if coords is not None else None,
+        coords=coords,
         data_vars=_skycoord_to_dataarrays(skycoord, coords),
-        attrs=dict(
-            frame=dump_frame(skycoord.frame),
-        ),
+        attrs={"frame": dump_frame(skycoord.frame)}
+        | {
+            attr: dump_time(value)
+            for attr in ("obstime", "equinox")
+            if (value := getattr(skycoord, attr)) is not None
+            and not hasattr(skycoord.frame, attr)
+        },
     )
 
 
@@ -128,5 +134,15 @@ def dataset_to_skycoord(ds: xr.Dataset) -> SkyCoord:
         {k: v.data for k, v in dsq.data_vars.items()},
     )
     frame.representation_type = ds.attrs["frame"]["representation_type"]
-    frame.differential_type = ds.attrs["frame"]["differential_type"]
-    return SkyCoord(frame)
+    frame.differential_type = ds.attrs["frame"].get("differential_type")
+
+    kwargs = {
+        attr: value
+        for attr in ("obstime", "equinox")
+        if not hasattr(frame, attr)
+        and (value := load_optional_object(Time, ds.attrs.get(attr))) is not None
+    }
+    return SkyCoord(
+        frame,
+        **kwargs,
+    )

docs/examples/skycoord.ipynb

@@ -40,6 +40,8 @@
     "        representation_type=\"unitspherical\",\n",
     "        differential_type=\"unitsphericalcoslat\",\n",
     "    ),\n",
+    "    obstime=\"J2000\",\n",
+    "    equinox=\"J2000\",\n",
     ")\n",
     "display(sky_direction)\n",
     "skycoord_to_dataset(\n",
@@ -77,6 +79,7 @@
     "        representation_type=\"spherical\",\n",
     "        differential_type=\"unitsphericalcoslat\",\n",
     "    ),\n",
+    "    obstime=\"J2000\",\n",
     ")\n",
     "display(sky_position)\n",
     "display(\n",
@@ -110,7 +113,7 @@
     "# frame specific info stored as dataset attribute\n",
     "assert load_frame(ds.attrs[\"frame\"]) == sky_position.replicate_without_data()\n",
     "\n",
-    "# dataset\n",
+    "# restored skycoord matches orginal\n",
     "result = dataset_to_skycoord(ds)\n",
     "display(result)\n",
     "np.testing.assert_array_equal(result.ra, sky_position.ra)\n",

pyproject.toml

@@ -27,7 +27,7 @@ requires-python = ">=3.10"
 dependencies = [
   "numpy>=1.23,<2.3.0",
   "xarray>=2022.06.0,<=2025.4.0",
-  "astropy>=6.1.0",
+  "astropy>=6.1.0,<8.0.0",
   "pandas>=2.3.0,<3.0.0",
 ]
 dynamic = ["version"]
@@ -38,7 +38,7 @@ documentation = "https://astropy-xarray.readthedocs.io/en/stable"
 repository = "https://github.com/calgray/astropy-xarray"
 issues = "https://github.com/calgray/astropy-xarray/issues"
 
-[project.optional-dependencies]
+[dependency-groups]
 test = [
   "pytest>=8.0",
   "pytest-cov",

tests/test_sky_coord.py

@@ -49,6 +49,7 @@
     dataset_to_skycoord,
     skycoord_to_dataset,
 )
+from astropy_xarray.coordinates.core import dump_time
 from astropy_xarray.coordinates.sky_coord import (
     _skycoord_differential_component_names,
     _skycoord_representation_component_names,
@@ -346,3 +347,100 @@ def test_skycoord_roundtrip(
     assert actual.is_equivalent_frame(expected)
     assert ds.coords.equals(xr.Coordinates(dict(coords)))
     np.testing.assert_array_equal(actual, expected)
+
+
+def test_skycoord_obstime_empty():
+    empty = SkyCoord(ra=[[0.1], [0.2]] * u.deg, dec=[[0.5], [0.7]] * u.deg)
+    assert empty.obstime is None
+    assert "obstime" not in skycoord_to_dataset(empty).attrs
+    ds = skycoord_to_dataset(empty)
+    assert "obstime" not in ds.attrs
+    assert dataset_to_skycoord(ds).obstime is None
+
+
+def test_skycoord_obstime_optional():
+    value = SkyCoord(
+        ra=[[0.1], [0.2]] * u.deg, dec=[[0.5], [0.7]] * u.deg, obstime="J2000"
+    )
+    assert value.obstime == Time("J2000")
+    ds = skycoord_to_dataset(value)
+    assert ds.attrs["obstime"] == dump_time(Time("J2000"))
+    assert dataset_to_skycoord(ds).obstime == Time("J2000")
+
+
+def test_skycoord_obstime_frame_default():
+    value = SkyCoord(
+        ra=[[0.1], [0.2]] * u.deg,
+        dec=[[0.5], [0.7]] * u.deg,
+        frame=FK4(),
+    )
+    assert value.obstime == Time("B1950")
+    ds = skycoord_to_dataset(value)
+    assert "obstime" not in ds.attrs
+    assert dataset_to_skycoord(ds).obstime == Time("B1950")
+
+
+def test_skycoord_obstime_frame_override():
+    _ = SkyCoord(
+        ra=[[0.1], [0.2]] * u.deg,
+        dec=[[0.5], [0.7]] * u.deg,
+        frame=FK5(),
+        obstime="J2000",
+    )
+    with pytest.raises(ValueError):
+        _ = SkyCoord(
+            ra=[[0.1], [0.2]] * u.deg,
+            dec=[[0.5], [0.7]] * u.deg,
+            frame=FK4(),
+            obstime="J2000",
+        )
+
+
+def test_skycoord_equinox_empty():
+    empty = SkyCoord(ra=[[0.1], [0.2]] * u.deg, dec=[[0.5], [0.7]] * u.deg)
+    assert empty.equinox is None
+    assert "equinox" not in skycoord_to_dataset(empty).attrs
+    ds = skycoord_to_dataset(empty)
+    assert "equinox" not in ds.attrs
+    assert dataset_to_skycoord(ds).equinox is None
+
+
+def test_skycoord_equinox_optional():
+    value = SkyCoord(
+        ra=[[0.1], [0.2]] * u.deg, dec=[[0.5], [0.7]] * u.deg, equinox="J2000"
+    )
+    assert value.equinox == Time("J2000")
+    assert skycoord_to_dataset(value).attrs["equinox"] == dump_time(Time("J2000"))
+    ds = skycoord_to_dataset(value)
+    assert ds.attrs["equinox"] == dump_time(Time("J2000"))
+    assert dataset_to_skycoord(ds).equinox == Time("J2000")
+
+
+def test_skycoord_equinox_frame_default():
+    value = SkyCoord(
+        ra=[[0.1], [0.2]] * u.deg,
+        dec=[[0.5], [0.7]] * u.deg,
+        frame=FK4(),
+    )
+    assert value.equinox == Time("B1950")
+    assert "equinox" not in skycoord_to_dataset(value).attrs
+    ds = skycoord_to_dataset(value)
+    assert "equinox" not in ds.attrs
+    assert dataset_to_skycoord(ds).equinox == Time("B1950")
+
+
+def test_skycoord_equinox_frame_override():
+    with pytest.raises(ValueError):
+        _ = SkyCoord(
+            ra=[[0.1], [0.2]] * u.deg,
+            dec=[[0.5], [0.7]] * u.deg,
+            frame=FK5(),
+            equinox="J2000",
+        )
+    with pytest.raises(ValueError):
+        _ = SkyCoord(
+            ra=[[0.1], [0.2]] * u.deg,
+            dec=[[0.5], [0.7]] * u.deg,
+            frame=FK4(),
+            equinox="J2000",
+        )