diff --git a/py/desiutil/annotate.py b/py/desiutil/annotate.py
index a201402..2e377a6 100644
--- a/py/desiutil/annotate.py
+++ b/py/desiutil/annotate.py
@@ -11,6 +11,7 @@
 import os
 import sys
 from argparse import ArgumentParser
+from warnings import warn
 import yaml
 from astropy.io import fits
 from astropy.table import Table, QTable
@@ -19,6 +20,12 @@
 from .log import get_logger, DEBUG
 
 
+class FITSUnitWarning(UserWarning):
+    """Warnings related to invalid FITS units.
+    """
+    pass
+
+
 def find_column_name(columns, prefix=('unit', )):
     """Find the column that contains unit descriptions, or comments.
 
@@ -95,11 +102,10 @@ def validate_unit(unit, error=False):
     Raises
     ------
     :exc:`ValueError`
-        If `error` is set and the unit can't be parsed.
+        If `error` is set and `unit` can't be parsed.
     """
     if unit is None:
         return None
-    log = get_logger()
     acceptable_units = ('maggie', 'maggy', 'mgy',
                         'electron/Angstrom',
                         'log(Angstrom)')
@@ -112,10 +118,9 @@ def validate_unit(unit, error=False):
             return bad_unit
         else:
             if error:
-                log.critical(str(e))
                 raise
             else:
-                log.warning(str(e))
+                warn(m, FITSUnitWarning)
     return None
 
 
diff --git a/py/desiutil/test/test_annotate.py b/py/desiutil/test/test_annotate.py
index c562a2d..0b478dd 100644
--- a/py/desiutil/test/test_annotate.py
+++ b/py/desiutil/test/test_annotate.py
@@ -10,7 +10,7 @@
 import numpy as np
 from astropy.io import fits
 from astropy.table import Table, QTable
-from ..annotate import (annotate_fits, annotate_table, find_column_name,
+from ..annotate import (FITSUnitWarning, annotate_fits, annotate_table, find_column_name,
                         find_key_name, validate_unit, load_csv_units,
                         load_yml_units, _options)
 
@@ -97,8 +97,7 @@ def test_find_key_name(self):
             k = find_key_name({'RA': 'deg', 'DEC': 'deg', 'COLUMN': None}, prefix=('comment', 'description'))
         self.assertEqual(e.exception.args[0], "No key matching 'comment' found!")
 
-    @patch('desiutil.annotate.get_logger')
-    def test_validate_unit(self, mock_log):
+    def test_validate_unit(self):
         """Test function to validate units.
         """
         m = "'ergs' did not parse as fits unit: At col 0, Unit 'ergs' not supported by the FITS standard. Did you mean erg?"
@@ -109,9 +108,13 @@ def test_validate_unit(self, mock_log):
         self.assertIsNone(c)
         c = validate_unit('erg')
         self.assertIsNone(c)
-        c = validate_unit('ergs', error=False)
+        with self.assertWarns(FITSUnitWarning) as w:
+            c = validate_unit('ergs', error=False)
+        self.assertEqual(str(w.warning), m + l)
         self.assertIsNone(c)
-        c = validate_unit('1/deg^2')
+        with self.assertWarns(FITSUnitWarning) as w:
+            c = validate_unit('1/deg^2')
+        self.assertEqual(str(w.warning), d + l)
         self.assertIsNone(c)
         c = validate_unit('nanomaggies', error=True)
         self.assertEqual(c, "'nanomaggies'")
@@ -121,8 +124,6 @@ def test_validate_unit(self, mock_log):
         with self.assertRaises(ValueError) as e:
             c = validate_unit('1/nanomaggy^2', error=True)
         self.assertEqual(str(e.exception), f + l)
-        mock_log().warning.assert_has_calls([call(m + l), call(d + l)])
-        mock_log().critical.assert_has_calls([call(m + l), call(f + l)])
 
     @patch('desiutil.annotate.get_logger')
     def test_load_csv_units(self, mock_log):