Merge pull request #5262 from meeseeksmachine/auto-backport-of-pr-525…

…7-on-v1.2.x

Backport PR #5257 on branch v1.2.x (Pass `events.time_ref` to `observation.pointing.to_fits_header`in `observation.write`)

gammapy/data/observations.py

@@ -576,7 +576,9 @@ def write(
         events = self.events
         if events is not None:
             events_hdu = events.to_table_hdu(format=format)
-            events_hdu.header.update(self.pointing.to_fits_header())
+            events_hdu.header.update(
+                self.pointing.to_fits_header(time_ref=events.time_ref)
+            )
             hdul.append(events_hdu)
 
         gti = self.gti

gammapy/data/tests/test_observations.py

@@ -378,6 +378,8 @@ def test_observation_write(tmp_path):
     obs = Observation.read(
         "$GAMMAPY_DATA/hess-dl3-dr1/data/hess_dl3_dr1_obs_id_023523.fits.gz"
     )
+    mjdreff = obs.events.table.meta["MJDREFF"]
+    mjdrefi = obs.events.table.meta["MJDREFI"]
     path = tmp_path / "obs.fits.gz"
 
     obs.meta.creation.origin = "test"
@@ -393,6 +395,9 @@ def test_observation_write(tmp_path):
     assert obs_read.obs_id == 23523
     assert_allclose(obs_read.observatory_earth_location.lat.deg, -23.271778)
 
+    assert_allclose(obs_read.events.table.meta["MJDREFF"], mjdreff)
+    assert_allclose(obs_read.events.table.meta["MJDREFI"], mjdrefi)
+
     # unsupported format
     with pytest.raises(ValueError):
         obs.write(tmp_path / "foo.fits.gz", format="cool-new-format")