use sky_to_fov also for RADEC FoVAlignment

gammapy/makers/utils.py

@@ -257,6 +257,33 @@ def make_map_background_irf(
     )
     coords["energy"] = geom.axes["energy"].edges.reshape((-1, 1, 1))
 
+            # for backwards compatibility, obstime should be required
+            if obstime is None:
+                warnings.warn(
+                    "Future versions of gammapy will require the obstime keyword for this function",
+                    DeprecationWarning,
+                )
+                obstime = pointing.obstime
+
+            pointing_altaz = pointing.get_altaz(obstime)
+            altaz_coord = sky_coord.transform_to(pointing_altaz.frame)
+
+            # Compute FOV coordinates of map relative to pointing
+            fov_lon, fov_lat = sky_to_fov(
+                altaz_coord.az, altaz_coord.alt, pointing_altaz.az, pointing_altaz.alt
+            )
+        elif bkg.fov_alignment == FoVAlignment.RADEC:
+            fov_lon, fov_lat = sky_to_fov(
+                sky_coord.ra, sky_coord.dec, pointing_icrs.ra, pointing_icrs.dec
+            )
+        else:
+            raise ValueError(
+                f"Unsupported background coordinate system: {bkg.fov_alignment!r}"
+            )
+
+        coords["fov_lon"] = fov_lon
+        coords["fov_lat"] = fov_lat
+
     bkg_de = bkg.integrate_log_log(**coords, axis_name="energy")
     data = (bkg_de * d_omega * ontime).to_value("")