diff --git a/quartical/apps/backup.py b/quartical/apps/backup.py
index fcb44a69..1a15ea8c 100644
--- a/quartical/apps/backup.py
+++ b/quartical/apps/backup.py
@@ -1,5 +1,6 @@
 import argparse
 from math import prod, ceil
+from quartical.data_handling.selection import filter_xds_list
 from daskms import xds_from_storage_ms, xds_to_storage_table
 from daskms.experimental.zarr import xds_to_zarr, xds_from_zarr
 from daskms.fsspec_store import DaskMSStore
@@ -10,8 +11,9 @@
 def backup():
     parser = argparse.ArgumentParser(
         description='Backup any Measurement Set column to zarr. Backups will '
-                    'be labelled automatically using the current datetime, '
-                    'the Measurement Set name and the column name.'
+                    'be labelled using a combination of the passed in label '
+                    '(defaults to datetime), the Measurement Set name and '
+                    'the column name.'
     )
 
     parser.add_argument(
@@ -33,19 +35,34 @@ def backup():
         type=str,
         help='Name of column to be backed up.'
     )
+    parser.add_argument(
+        '--label',
+        type=str,
+        help='An explicit label to include in the backup name. Defaults to '
+             'datetime at which the backup was created. Full name will be '
+             'given by [label]-[msname]-[column].bkp.qc.'
+    )
     parser.add_argument(
         '--nthread',
         type=int,
         default=1,
         help='Number of threads to use.'
     )
+    parser.add_argument(
+        '--field-id',
+        type=int,
+        help='Field ID to back up.'
+    )
 
     args = parser.parse_args()
 
     ms_name = args.ms_path.full_path.rsplit("/", 1)[1]
     column_name = args.column_name
 
-    timestamp = time.strftime("%Y%m%d-%H%M%S")
+    if args.label:
+        label = args.label
+    else:
+        label = time.strftime("%Y%m%d-%H%M%S")
 
     # This call exists purely to get the relevant shape and dtype info.
     data_xds_list = xds_from_storage_ms(
@@ -55,8 +72,11 @@ def backup():
         group_cols=("FIELD_ID", "DATA_DESC_ID", "SCAN_NUMBER"),
     )
 
+    # Use existing functionality. TODO: Improve and expose DDID selection.
+    xdso = filter_xds_list(data_xds_list, args.field_id)
+
     # Compute appropriate chunks (256MB by default) to keep zarr happy.
-    chunks = [chunk_by_size(xds[column_name]) for xds in data_xds_list]
+    chunks = [chunk_by_size(xds[column_name]) for xds in xdso]
 
     # Repeat of above call but now with correct chunking information.
     data_xds_list = xds_from_storage_ms(
@@ -67,9 +87,12 @@ def backup():
         chunks=chunks
     )
 
+    # Use existing functionality. TODO: Improve and expose DDID selection.
+    xdso = filter_xds_list(data_xds_list, args.field_id)
+
     bkp_xds_list = xds_to_zarr(
-        data_xds_list,
-        f"{args.zarr_dir.url}::{timestamp}-{ms_name}-{column_name}.bkp.qc",
+        xdso,
+        f"{args.zarr_dir.url}::{label}-{ms_name}-{column_name}.bkp.qc",
     )
 
     dask.compute(bkp_xds_list, num_workers=args.nthread)
diff --git a/quartical/calibration/calibrate.py b/quartical/calibration/calibrate.py
index 83ed022b..dba5f2f2 100644
--- a/quartical/calibration/calibrate.py
+++ b/quartical/calibration/calibrate.py
@@ -253,6 +253,18 @@ def make_visibility_output(
 
     itr = enumerate(zip(data_xds_list, mapping_xds_list))
 
+    if output_opts.subtract_directions:
+        n_dir = data_xds_list[0].dims['dir']  # Should be the same on all xdss.
+        requested = set(output_opts.subtract_directions)
+        valid = set(range(n_dir))
+        invalid = requested - valid
+        if invalid:
+            raise ValueError(
+                f"User has specified output.subtract_directions as "
+                f"{requested} but the following directions are not present "
+                f"in the model: {invalid}."
+            )
+
     for xds_ind, (data_xds, mapping_xds) in itr:
         data_col = data_xds.DATA.data
         model_col = data_xds.MODEL_DATA.data
diff --git a/quartical/data_handling/selection.py b/quartical/data_handling/selection.py
index 4f6f9301..bd0f66c3 100644
--- a/quartical/data_handling/selection.py
+++ b/quartical/data_handling/selection.py
@@ -1,7 +1,13 @@
-def filter_xds_list(xds_list, fields, ddids):
+def filter_xds_list(xds_list, fields=[], ddids=[]):
 
-    filter_fields = {"FIELD_ID": fields,
-                     "DATA_DESC_ID": ddids}
+    # If we specify an int, make it a list. Might be worth improving.
+    fields = [fields] if isinstance(fields, int) else fields
+    ddids = [ddids] if isinstance(ddids, int) else ddids
+
+    filter_fields = {
+        "FIELD_ID": fields,
+        "DATA_DESC_ID": ddids
+    }
 
     for k, v in filter_fields.items():
         fil = filter(lambda xds: getattr(xds, k) in v, xds_list)