diff --git a/CAL/CalibrationCommon/test/t_holography_dataset_class.py b/CAL/CalibrationCommon/test/t_holography_dataset_class.py
index d1f05ac140d1f6744603e5209720ff3c29ca1505..707d364e9804003bb363e4b0937abfdf874649fd 100755
--- a/CAL/CalibrationCommon/test/t_holography_dataset_class.py
+++ b/CAL/CalibrationCommon/test/t_holography_dataset_class.py
@@ -7,6 +7,8 @@ from lofar.calibration.common.datacontainers import HolographyObservation
 
 
 logger = logging.getLogger('t_holography_dataset_class')
+# READ doc/Holography_Data_Set.md!  It contains the location from which the
+# test data must be downloaded.
 path_to_test_data = '/data/test/HolographyObservation'
 path_to_test_data = '/home/thomas/workspace.astron/data/holography'