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

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_remote.py

@@ -1,23 +1,24 @@
 # Licensed under a 3-clause BSD style license - see LICENSE.rst
-from pathlib import Path
 import os
+from pathlib import Path
+import re
 import sys
+
+from numpy.testing import assert_allclose
 import pytest
 import requests
 from requests import ReadTimeout
-from numpy.testing import assert_allclose
 
+from astropy.coordinates import SkyCoord
+from astroquery.exceptions import InvalidQueryError
 from astropy.table import Table
 from astropy.units import arcsec, arcmin
 
-from astropy.coordinates import SkyCoord
-
 try:
     from regions import CircleSkyRegion
 except ImportError:
     pass
 
-from astroquery.exceptions import InvalidQueryError
 from ...xmatch import XMatch
 
 
@@ -126,9 +127,14 @@ def test_xmatch_query_with_cone_area(self, xmatch):
     def test_xmatch_invalid_query(self, xmatch):
         input_table = Table.read(DATA_DIR / "posList.csv", format="ascii.csv")
         # columns in input table and kwargs are not matching
-
-        with pytest.raises(InvalidQueryError) as err:
+        with pytest.raises(InvalidQueryError,
+                           match='Column name "DEC" not found in table metadata'):
             xmatch.query(input_table, cat2='vizier:II/246/out', max_distance=5 * arcsec,
                          colRA1='ra', colDec1='DEC')
 
-            assert 'Column name "DEC" not found in table metadata' in str(err)
+    def test_table_not_available(self, xmatch):
+        cat1 = "vizier:J/A+A/331/81/table2"
+        cat2 = "blabla"
+        # reproduces #1464
+        with pytest.raises(ValueError, match=f"'{re.escape(cat1)}' is not available *"):
+            xmatch.query_async(cat1=cat1, cat2=cat2, max_distance=5 * arcsec)