group_id again. central_wavelength for flat, arc and slitillum only, …

…not targets.

gemini_instruments/gmos/adclass.py

@@ -689,7 +689,6 @@ def group_id(self):
                                          'read_mode', 'amp_read_area']
         if 'SPECT' in tags:
             unique_id_descriptor_list_all.append('disperser')
-            unique_id_descriptor_list_all.append('central_wavelength')
 
         # List to format descriptor calls using 'pretty=True' parameter
         call_pretty_version_list = ['filter_name', 'disperser']
@@ -703,8 +702,11 @@ def group_id(self):
             id_descriptor_list = ['exposure_time']
         elif 'IMAGE' in tags and ('FLAT' in tags or 'TWILIGHT' in tags):
             id_descriptor_list = ['filter_name']
-        elif 'SPECT' in tags and ('FLAT' in tags or 'SLITILLUM' in tags):
-            id_descriptor_list = ['filter_name']
+        elif 'SPECT' in tags and ('FLAT' in tags or 'SLITILLUM' in tags
+                or 'ARC' in tags):
+            # unclear if STANDARD requires central_wavelength
+            # depends on the effects of grating efficiency as f(cwave)
+            id_descriptor_list = ['filter_name', 'central_wavelength']
         else:
             id_descriptor_list = ['observation_id', 'filter_name']