Use sky_to_fov() also for RADEC FoVAlignment in make_map_background_irf

gammapy/data/metadata.py

@@ -107,6 +107,8 @@ def from_header(cls, header, format="gadf"):
             "PRESSURE",
             "RELHUM",
             "NSBLEVEL",
+            "CREATOR",
+            "HDUVERS",
         ]
         optional = dict()
         for key in optional_keywords:

gammapy/data/observations.py

@@ -15,6 +15,7 @@
 from astropy.units import Quantity
 from astropy.utils import lazyproperty
 import matplotlib.pyplot as plt
+from gammapy.irf import FoVAlignment
 from gammapy.utils.deprecation import GammapyDeprecationWarning, deprecated
 from gammapy.utils.fits import LazyFitsData, earth_location_to_dict
 from gammapy.utils.metadata import CreatorMetaData, TargetMetaData, TimeInfoMetaData
@@ -67,7 +68,7 @@
     aeff = LazyFitsData(cache=False)
     edisp = LazyFitsData(cache=False)
     psf = LazyFitsData(cache=False)
-    bkg = LazyFitsData(cache=False)
+    _bkg = LazyFitsData(cache=False)
     _rad_max = LazyFitsData(cache=True)
     _events = LazyFitsData(cache=False)
     _gti = LazyFitsData(cache=True)
@@ -101,7 +102,7 @@
         self.aeff = aeff
         self.edisp = edisp
         self.psf = psf
-        self.bkg = bkg
+        self._bkg = bkg
         self._rad_max = rad_max
         self._gti = gti
         self._events = events
@@ -116,6 +117,28 @@
         except AttributeError:
             return f"<pre>{html.escape(str(self))}</pre>"
 
+    @property
+    def bkg(self):
+        """Background of the observation."""
+        bkg = self._bkg
+        # used for backward compatibility of old HESS data
+        try:
+            if (
+                bkg
+                and self._meta
+                and self._meta.optional
+                and self._meta.optional["CREATOR"] == "SASH FITS::EventListWriter"
+                and self._meta.optional["HDUVERS"] == "0.2"
+            ):
+                bkg._fov_alignment = FoVAlignment.REVERSE_LON_RADEC
+        except KeyError:
+            pass
+        return bkg
+
+    @bkg.setter
+    def bkg(self, value):
+        self._bkg = value
+
     @property
     def meta(self):
         """Return metadata container."""
@@ -148,7 +171,7 @@
     def available_hdus(self):
         """Which HDUs are available."""
         available_hdus = []
-        keys = ["_events", "_gti", "aeff", "edisp", "psf", "bkg", "_rad_max"]
+        keys = ["_events", "_gti", "aeff", "edisp", "psf", "_bkg", "_rad_max"]
         hdus = ["events", "gti", "aeff", "edisp", "psf", "bkg", "rad_max"]
         for key, hdu in zip(keys, hdus):
             available = self.__dict__.get(key, False)

gammapy/irf/core.py

@@ -33,6 +33,8 @@ class FoVAlignment(str, Enum):
 
     ALTAZ = "ALTAZ"
     RADEC = "RADEC"
+    # used for backward compatibility of old HESS data
+    REVERSE_LON_RADEC = "REVERSE_LON_RADEC"
 
 
 class IRF(metaclass=abc.ABCMeta):

gammapy/makers/tests/test_spectrum.py

@@ -7,6 +7,7 @@
 from regions import CircleSkyRegion
 from gammapy.data import DataStore
 from gammapy.datasets import SpectrumDataset
+from gammapy.irf import FoVAlignment
 from gammapy.makers import (
     ReflectedRegionsBackgroundMaker,
     SafeMaskMaker,
@@ -125,6 +126,11 @@ def test_spectrum_dataset_maker_hess_dl3(spectrum_dataset_crab, observations_hes
 
     datasets = []
     for obs in observations_hess_dl3:
+
+        assert obs.meta.optional["CREATOR"] == "SASH FITS::EventListWriter"
+        assert obs.meta.optional["HDUVERS"] == "0.2"
+        assert obs.bkg.fov_alignment == FoVAlignment.REVERSE_LON_RADEC
+
         dataset = maker.run(spectrum_dataset_crab, obs)
         datasets.append(dataset)
 

gammapy/makers/utils.py

@@ -3,7 +3,7 @@
 import warnings
 import numpy as np
 import astropy.units as u
-from astropy.coordinates import Angle, SkyOffsetFrame
+from astropy.coordinates import Angle
 from astropy.table import Table
 from gammapy.data import FixedPointingInfo
 from gammapy.irf import BackgroundIRF, EDispMap, FoVAlignment, PSFMap
@@ -81,12 +81,15 @@ def _get_fov_coords(pointing, irf, geom, use_region_center=True, obstime=None):
             fov_lon, fov_lat = sky_to_fov(
                 altaz_coord.az, altaz_coord.alt, pointing_altaz.az, pointing_altaz.alt
             )
-        elif irf.fov_alignment == FoVAlignment.RADEC:
-            # Create OffsetFrame
-            frame = SkyOffsetFrame(origin=pointing_icrs)
-            pseudo_fov_coord = sky_coord.transform_to(frame)
-            fov_lon = pseudo_fov_coord.lon
-            fov_lat = pseudo_fov_coord.lat
+        elif irf.fov_alignment in [FoVAlignment.RADEC, FoVAlignment.REVERSE_LON_RADEC]:
+            fov_lon, fov_lat = sky_to_fov(
+                sky_coord.icrs.ra,
+                sky_coord.icrs.dec,
+                pointing_icrs.icrs.ra,
+                pointing_icrs.icrs.dec,
+            )
+            if irf.fov_alignment == FoVAlignment.REVERSE_LON_RADEC:
+                fov_lon = -fov_lon
         else:
             raise ValueError(
                 f"Unsupported background coordinate system: {irf.fov_alignment!r}"