diff --git a/notebooks/LPD/LPD_Correct_Fast.ipynb b/notebooks/LPD/LPD_Correct_Fast.ipynb
index d8c61f6f46e997a1dc6371fed261fc8208f9a142..cf6bac5aaf10d37fb8e763ace7026b2627f1d437 100644
--- a/notebooks/LPD/LPD_Correct_Fast.ipynb
+++ b/notebooks/LPD/LPD_Correct_Fast.ipynb
@@ -455,8 +455,7 @@
     "            outp_source.create_compressed_key('image.mask', data=out_mask)\n",
     "            \n",
     "            if output_source != input_source:\n",
-    "                outp_file[f'/INSTRUMENT/{inp_source_name}'] = h5py.SoftLink(f'/INSTRUMENT/{outp_source_name}')\n",
-    "                outp_file[f'/INDEX/{inp_source_name}'] = h5py.SoftLink(f'/INDEX/{outp_source_name}')\n",
+    "                outp_file.create_legacy_source(inp_source_name, outp_source_name)\n",
     "\n",
     "    write_time = perf_counter() - start\n",
     "    \n",
diff --git a/src/cal_tools/agipdlib.py b/src/cal_tools/agipdlib.py
index f52125bbb9d41df63b10e94929d1669d348e6dda..66128cc39383201018b9be85e3e141f9416c40ff 100644
--- a/src/cal_tools/agipdlib.py
+++ b/src/cal_tools/agipdlib.py
@@ -801,10 +801,7 @@ class AgipdCorrections:
             image_grp = agipd_src.require_group("image")
 
             # backward compatibility BEGIN
-            outfile[f"INDEX/{agipd_base}"] = h5py.SoftLink(
-                f"/INDEX/{agipd_corr_source}")
-            outfile[f"INSTRUMENT/{agipd_base}"] = h5py.SoftLink(
-                f"/INSTRUMENT/{agipd_corr_source}")
+            outfile.create_legacy_source(agipd_base, agipd_corr_source)
             # backward compatibility END
 
             # Set up all the datasets before filling them. This puts the
diff --git a/src/cal_tools/files.py b/src/cal_tools/files.py
index 9d9f4827f728436578bc98ead87926bdd742d22a..8468ba3693d821e8c70d134461e6d6c98c07bf0a 100644
--- a/src/cal_tools/files.py
+++ b/src/cal_tools/files.py
@@ -257,6 +257,34 @@ class DataFile(h5py.File):
         return InstrumentSource(self.create_group(f'INSTRUMENT/{source}').id,
                                 source)
 
+    def create_legacy_source(self, legacy_source, target_source):
+        """Create a legacy source.
+
+        A legacy source allows to access an instrument source under a
+        different name, primarily for the purpose of backwards
+        compatibility. It inserts soft links under the legacy name in
+        the INSTRUMENT and INDEX groups, pointing to the respective
+        groups of this source.
+
+        Args:
+            legacy_source (str): Legacy source.
+            target_source (str or InstrumentSource): Target source.
+
+        Returns:
+            None
+        """
+
+        if isinstance(target_source, InstrumentSource):
+            target_source = target_source.name[1:].partition('/')[2]
+        elif not isinstance(target_source, str):
+            raise ValueError('target_source must be str or InstrumentSource')
+
+        self.file[f'/INSTRUMENT/{legacy_source}'] = h5py.SoftLink(
+            f'/INSTRUMENT/{target_source}')
+        self.file[f'/INDEX/{legacy_source}'] = h5py.SoftLink(
+            f'/INDEX/{target_source}')
+        self.__instrument_sources.add(legacy_source)
+
     def require_control_source(self, source):
         """Return group for a control source, creating it if it doesn’t exist.