doc: make the error message state the two possible causes for failure

either the catalog is not available (no coordinates or typo in the name) or the user has to specify colRa and colDec

Author: ManonMarchand

CHANGES.rst

@@ -286,6 +286,9 @@ xmatch
 - Fix xmatch query for two local tables. The second table was written over the first one,
   resulting in a confusing "missing cat1" error. [#3116]
 
+- Make the error message clearer about VizieR tables not available for
+  crossmatching [#3168]
+
 
 0.4.7 (2024-03-08)
 ==================

astroquery/xmatch/core.py

@@ -146,7 +146,12 @@ def _prepare_sending_table(self, cat_index, payload, kwargs, cat, colRA, colDec)
 
         if not self.is_table_available(cat):
             if ((colRA is None) or (colDec is None)):
-                raise ValueError('Specify the name of the RA/Dec columns in the input table.')
+                raise ValueError(
+                    f"'{cat}' is not available on the XMatch server.If you are "
+                    "using a VizieR table name, note that only tables with "
+                    "coordinates are available on the XMatch server. If you are "
+                    f"using a local table, the arguments 'colRA{cat_index}' and "
+                    f"'colDec{cat_index}' must be provided.")
             # if `cat1` is not a VizieR table,
             # it is assumed it's either a URL or an uploaded table
             payload['colRA{0}'.format(cat_index)] = colRA

astroquery/xmatch/tests/test_xmatch.py

@@ -1,5 +1,6 @@
 # Licensed under a 3-clause BSD style license - see LICENSE.rst
 from pathlib import Path
+import re
 
 import requests
 import pytest
@@ -114,3 +115,13 @@ def test_two_local_tables():
                              max_distance=1 * arcsec,
                              get_query_payload=True)
     assert 'cat1' in payload[1]["files"] and 'cat2' in payload[1]["files"]
+
+
+def test_table_not_available(monkeypatch):
+    xm = XMatch()
+    monkeypatch.setattr(xm, '_request', request_mockreturn)
+    cat1 = "vizier:J/A+A/331/81/table2"
+    cat2 = "blabla"
+    # reproduces #1464
+    with pytest.raises(ValueError, match=f"'{re.escape(cat1)}' is not available *"):
+        xm.query_async(cat1=cat1, cat2=cat2, max_distance=5 * arcsec)