Set day_obs_offset in Instrument to be consistent with translator

The translator already had a ROLLOVER TIME used to calculate
observing_day if it was not found in the header.

python/lsst/obs/lsst/_instrument.py

@@ -46,6 +46,17 @@
 PACKAGE_DIR = getPackageDir("obs_lsst")
 
 
+class classproperty:  # noqa: N801
+    """Simple decorator that can emulate a class property.
+    """
+
+    def __init__(self, func):
+        self.fget = func
+
+    def __get__(self, instance, owner):
+        return self.fget(owner)
+
+
 class LsstCam(Instrument):
     """Gen3 Butler specialization for the LSST Main Camera.
 
@@ -89,6 +100,15 @@ class LsstCam(Instrument):
     obsDataPackage = "obs_lsst_data"
     visitSystem = VisitSystem.BY_SEQ_START_END
 
+    @classproperty
+    def day_obs_offset(cls) -> int:  # noqa: N805
+        """Offset to use when calculating day_obs.
+
+        Do not cache for now. Need to ensure that subclass values do not
+        get overridden by the value from this class.
+        """
+        return round(cls.translatorClass._ROLLOVER_TIME.to_value("s"))
+
     @property
     def configPaths(self):
         return [os.path.join(PACKAGE_DIR, "config"),

python/lsst/obs/lsst/translators/imsim.py

@@ -15,6 +15,7 @@
 import logging
 import astropy.units as u
 from astropy.coordinates import Angle, AltAz
+from astropy.time import TimeDelta
 
 try:
     import erfa
@@ -58,6 +59,9 @@ class LsstCamImSimTranslator(LsstSimTranslator):
 
     cameraPolicyFile = "policy/imsim.yaml"
 
+    _ROLLOVER_TIME = TimeDelta(0, scale="tai", format="sec")
+    """This instrument did not offset the observing day."""
+
     @classmethod
     def can_translate(cls, header, filename=None):
         """Indicate whether this translation class can translate the

python/lsst/obs/lsst/translators/lsst.py

@@ -178,7 +178,8 @@ class LsstBaseTranslator(FitsTranslator):
 
     _ROLLOVER_TIME = TimeDelta(12*60*60, scale="tai", format="sec")
     """Time delta for the definition of a Rubin Observatory start of day.
-    Used when the header is missing. See LSE-400 for details."""
+    Used when the header is missing. See LSE-400 or SITCOMTN-032 for details.
+    """
 
     @classmethod
     def __init_subclass__(cls, **kwargs):

python/lsst/obs/lsst/translators/lsst_ucdcam.py

@@ -14,6 +14,7 @@
 
 import logging
 import astropy.units as u
+from astropy.time import TimeDelta
 
 from .lsst import LsstBaseTranslator
 
@@ -58,6 +59,9 @@ class LsstUCDCamTranslator(LsstBaseTranslator):
 
     cameraPolicyFile = "policy/ucd.yaml"
 
+    _ROLLOVER_TIME = TimeDelta(0, scale="tai", format="sec")
+    """This instrument did not offset the observing day."""
+
     @classmethod
     def can_translate(cls, header, filename=None):
         """Indicate whether this translation class can translate the

python/lsst/obs/lsst/translators/phosim.py

@@ -18,6 +18,7 @@
 import astropy.units as u
 import astropy.units.cds as cds
 from astropy.coordinates import Angle
+from astropy.time import TimeDelta
 
 from astro_metadata_translator import cache_translation, merge_headers
 from astro_metadata_translator.translators.helpers import (
@@ -62,6 +63,9 @@ class LsstCamPhoSimTranslator(LsstSimTranslator):
 
     cameraPolicyFile = "policy/phosim.yaml"
 
+    _ROLLOVER_TIME = TimeDelta(0, scale="tai", format="sec")
+    """This instrument did not offset the observing day."""
+
     @classmethod
     def can_translate(cls, header, filename=None):
         """Indicate whether this translation class can translate the

tests/test_gen3.py

@@ -28,6 +28,7 @@
 
 import numpy as np
 
+from astropy.time import TimeDelta
 from astro_metadata_translator import ObservationInfo
 from lsst.obs.lsst import (LsstCam, LsstComCam, LsstCamImSim, LsstCamPhoSim,
                            LsstTS8, LsstTS3, LsstUCDCam, Latiss, LsstComCamSim,
@@ -134,6 +135,11 @@ def checkInstrumentWithRegistry(self, cls, testRaw):
             self.assertEqual(cameraGeomDetector.getName(), cameraGeomDetector2.getName())
             self.assertEqual(cameraGeomDetector.getSerial(), cameraGeomDetector2.getSerial())
 
+        # Check that the day_obs offset in the Instrument  is consistent with
+        # the translator rollover time.
+        self.assertEqual(TimeDelta(instrument.day_obs_offset, format="sec", scale="tai"),
+                         cls.translatorClass._ROLLOVER_TIME)
+
     def testLsstCam(self):
         testFpath = "lsstCam/raw/2019-03-22/3019032200002/3019032200002-R10-S22-det035.fits"
         self.checkInstrumentWithRegistry(LsstCam, testFpath)