diff --git a/src/xradio/image/_util/_casacore/xds_to_casacore.py b/src/xradio/image/_util/_casacore/xds_to_casacore.py
index 82bc9abf..7ffd442f 100644
--- a/src/xradio/image/_util/_casacore/xds_to_casacore.py
+++ b/src/xradio/image/_util/_casacore/xds_to_casacore.py
@@ -40,9 +40,48 @@ def _compute_direction_dict(xds: xr.Dataset) -> dict:
         [xds.sizes[dim] for dim in ("l", "m")], dtype=np.int32
     )
     direction["_image_axes"] = np.array([2, 3], dtype=np.int32)
-    direction["system"] = xds_dir["reference_direction"]["attrs"]["equinox"].upper()
+    if "equinox" in xds_dir["reference_direction"]["attrs"]:
+        direction["system"] = xds_dir["reference_direction"]["attrs"]["equinox"].upper()
+        # Allow J2000.0 for backward compatibility; it will be normalized below.
+        allowed_systems = {
+            "FK5",
+            "FK4",
+            "ICRS",
+            "GALACTIC",
+            "J2000",
+            "B1950",
+            "J2000.0",
+        }
+        if direction["system"] not in allowed_systems:
+            raise RuntimeError(
+                f"Unsupported direction equinox '{direction['system']}'. "
+                "Supported systems are: FK5, FK4, ICRS, GALACTIC, J2000, B1950."
+            )
+    elif "frame" in xds_dir["reference_direction"]["attrs"]:
+        frame = xds_dir["reference_direction"]["attrs"]["frame"].upper()
+        frame_to_system = {
+            "FK5": "J2000",
+            "FK4": "B1950",
+            "ICRS": "ICRS",
+            "GALACTIC": "GALACTIC",
+            "J2000": "J2000",
+            "B1950": "B1950",
+        }
+        if frame not in frame_to_system:
+            raise RuntimeError(
+                f"Unsupported direction coordinate system frame '{frame}'. "
+                "Supported frames are: FK5, FK4, ICRS, GALACTIC, J2000, B1950."
+            )
+        direction["system"] = frame_to_system[frame]
+    else:
+        raise RuntimeError(
+            "Cannot determine direction coordinate system frame. "
+            f"direction metadata is {xds_dir}"
+        )
     if direction["system"] == "J2000.0":
         direction["system"] = "J2000"
+    elif direction["system"] == "B1950.0":
+        direction["system"] = "B1950"
     direction["projection"] = xds_dir["projection"]
     direction["projection_parameters"] = xds_dir["projection_parameters"]
     direction["units"] = [
diff --git a/tests/unit/image/test_image.py b/tests/unit/image/test_image.py
index 56b57032..5bc18bcb 100644
--- a/tests/unit/image/test_image.py
+++ b/tests/unit/image/test_image.py
@@ -57,6 +57,88 @@ def clean_path_logic(text: str) -> str:
     return text
 
 
+def _make_test_sky_xds_for_casa_coord_write():
+    xds = make_empty_sky_image(
+        phase_center=np.array([0.2, -0.5]),
+        image_size=np.array([4, 4]),
+        cell_size=np.array([1e-4, 1e-4]),
+        frequency_coords=np.array([1.4e9]),
+        pol_coords=np.array(["I"]),
+        time_coords=np.array([51544.0]),
+        do_sky_coords=False,
+        direction_reference="fk5",
+    )
+    xds["SKY"] = xr.DataArray(
+        np.zeros((1, 1, 1, 4, 4), dtype=np.float32),
+        dims=["time", "frequency", "polarization", "l", "m"],
+    )
+    xds["SKY"].attrs["type"] = "sky"
+    xds.attrs["type"] = "image_dataset"
+    xds.attrs["data_groups"] = {"base": {"sky": "SKY"}}
+    return xds
+
+
+def test_write_image_uses_reference_direction_frame_without_equinox(tmp_path):
+    xds = _make_test_sky_xds_for_casa_coord_write()
+    ref_dir_attrs = xds.attrs["coordinate_system_info"]["reference_direction"]["attrs"]
+    del ref_dir_attrs["equinox"]
+    outname = tmp_path / "frame_only.im"
+
+    write_image(xds, str(outname), "casa")
+
+    with open_image_ro(str(outname)) as im:
+        direction = im.coordinates().dict()["direction0"]
+
+    assert direction["system"] == "J2000"
+    assert direction["conversionSystem"] == "J2000"
+
+
+def test_write_image_requires_reference_direction_frame_or_equinox(tmp_path):
+    xds = _make_test_sky_xds_for_casa_coord_write()
+    ref_dir_attrs = xds.attrs["coordinate_system_info"]["reference_direction"]["attrs"]
+    del ref_dir_attrs["equinox"]
+    del ref_dir_attrs["frame"]
+    outname = tmp_path / "missing_frame.im"
+
+    with pytest.raises(
+        RuntimeError,
+        match=r"Cannot determine direction coordinate system frame",
+    ):
+        write_image(xds, str(outname), "casa")
+
+
+def test_write_image_uses_fk4_frame_to_set_b1950(tmp_path):
+    xds = _make_test_sky_xds_for_casa_coord_write()
+    ref_dir_attrs = xds.attrs["coordinate_system_info"]["reference_direction"]["attrs"]
+    # Ensure the frame is FK4 and that we rely on the frame (not equinox) mapping
+    ref_dir_attrs["frame"] = "FK4"
+    ref_dir_attrs.pop("equinox", None)
+    outname = tmp_path / "fk4_frame.im"
+
+    write_image(xds, str(outname), "casa")
+
+    with open_image_ro(str(outname)) as im:
+        direction = im.coordinates().dict()["direction0"]
+
+    assert direction["system"] == "B1950"
+    assert direction["conversionSystem"] == "B1950"
+
+
+def test_write_image_raises_for_unsupported_reference_direction_frame(tmp_path):
+    xds = _make_test_sky_xds_for_casa_coord_write()
+    ref_dir_attrs = xds.attrs["coordinate_system_info"]["reference_direction"]["attrs"]
+    # Set an unrecognized frame and remove equinox so that frame mapping is required
+    ref_dir_attrs["frame"] = "UNSUPPORTED"
+    ref_dir_attrs.pop("equinox", None)
+    outname = tmp_path / "unsupported_frame.im"
+
+    with pytest.raises(
+        RuntimeError,
+        match=r"Unsupported direction coordinate system frame",
+    ):
+        write_image(xds, str(outname), "casa")
+
+
 def test_load_visibility_normalization_block_squeezes_spatial_axes(tmp_path):
     imagename = tmp_path / "synthetic.sumwt"
     data = np.arange(8, dtype=np.float32).reshape(4, 2, 1, 1)