Adopt LSST boresightRotAngle convention for imsim

The LSST convention for the boresight rotation angle is such that,

At boresightRotAngle = 0, the focal plane pixels are alligned:
+x: E->W (-ve RA), +y: S->N (+ve Dec)

To match this convention, the imsim data requires the following:

boresightRotAngle = 90 deg - ROTANGLE

where ROTANGLE is the FITS header card in the raw imsim data.

Prior to the "translator" method of standardizing metadata from obs
packages to the consistent set of LSST conventions, this rotation
was being accommodated in lsstCameraMapper.py.  This removes the
latter accommodation and adds it to the imsim translator.

python/lsst/obs/lsst/lsstCamMapper.py

@@ -159,11 +159,10 @@ def assemble_raw(dataId, componentInfo, cls):
     exposure.getInfo().setVisitInfo(visitInfo)
 
     boresight = visitInfo.getBoresightRaDec()
-    rotangle = visitInfo.getBoresightRotAngle()
+    boresightRotAngle = visitInfo.getBoresightRotAngle()
 
     if boresight.isFinite():
-        exposure.setWcs(getWcsFromDetector(exposure.getDetector(), boresight,
-                                           90*geom.degrees - rotangle))
+        exposure.setWcs(getWcsFromDetector(exposure.getDetector(), boresight, boresightRotAngle))
     else:
         # Should only warn for science observations but VisitInfo does not know
         logger.warn("Unable to set WCS for %s from header as RA/Dec/Angle are unavailable" %

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

@@ -14,6 +14,7 @@
 
 import logging
 import astropy.units as u
+from astropy.coordinates import Angle
 
 from astro_metadata_translator import cache_translation
 from astro_metadata_translator.translators.helpers import tracking_from_degree_headers
@@ -49,7 +50,6 @@ class ImsimTranslator(LsstSimTranslator):
         "dark_time": ("DARKTIME", dict(unit=u.s)),
         "exposure_time": ("EXPTIME", dict(unit=u.s)),
         "detector_serial": "LSST_NUM",
-        "boresight_rotation_angle": ("ROTANGLE", dict(unit=u.deg)),
     }
 
     cameraPolicyFile = "policy/imsim.yaml"
@@ -92,3 +92,9 @@ def to_boresight_airmass(self):
         if altaz is not None:
             return altaz.secz.to_value()
         return None
+
+    @cache_translation
+    def to_boresight_rotation_angle(self):
+        angle = Angle(90.*u.deg) - Angle(self.quantity_from_card("ROTANGLE", u.deg))
+        angle = angle.wrap_at("360d")
+        return angle