Fix Lo L2 processing pointing set midpoint_j2000_et calculation (#3011)

Co-authored-by: Menlo Innovations - CAVA Project <harrison@menloinnovations.com>

Author: pleasant-menlo

imap_processing/ena_maps/ena_maps.py

@@ -804,8 +804,8 @@ def midpoint_j2000_et(self) -> float:
             The midpoint value [J2000 ET] of the pointing set.
         """
         epoch_delta = met_to_ttj2000ns(
-            self.data["pointing_end_met"].data - self.data["pointing_start_met"].data
-        )
+            self.data["pointing_end_met"].data
+        ) - met_to_ttj2000ns(self.data["pointing_start_met"].data)
         return float(ttj2000ns_to_et(self.epoch + epoch_delta / 2))
 
 

imap_processing/tests/ena_maps/test_ena_maps.py

@@ -307,8 +307,9 @@ def test_init(self, lo_pset_ds):
         # check that the midpoint_j2000_et property is equal to the expected value
         assert lo_pset.midpoint_j2000_et == ttj2000ns_to_et(
             lo_pset.epoch
-            + met_to_ttj2000ns(
-                lo_pset_ds.pointing_end_met - lo_pset_ds.pointing_start_met
+            + (
+                met_to_ttj2000ns(lo_pset_ds.pointing_end_met)
+                - met_to_ttj2000ns(lo_pset_ds.pointing_start_met)
             )
             / 2
         )