Add crude ERA from LST method and a test for it.

python/lsst/obs/base/makeRawVisitInfo.py

@@ -240,6 +240,15 @@ def popMjdDate(self, md, key, timesys=None):
             self.log.warn("Could not parse {} = {} as an MJD date: {}".format(key, mjdDate, e))
         return BadDate
 
+    @staticmethod
+    def eraFromLstAndLongitude(lst, longitude):
+        """
+        Convert local sidereal time and longitude (both in degrees) to an approximate Earth Rotation Angle.
+
+        NOTE: if we properly compute ERA via UT1 a la DM-8053, we could remove this method.
+        """
+        return lst - longitude
+
     @staticmethod
     def altitudeFromZenithDistance(zd):
         """Convert zenith distance to altitude (lsst.afw.geom.Angle)"""

tests/testMakeRawVisitInfo.py

@@ -267,6 +267,12 @@ def testPopMjdDate(self):
             missingDate = self.makeRawVisitInfo.popMjdDate(md, "BADKEY")
             self.assertEqual(missingDate, DateTime())
 
+    def testEraFromLstAndLongitude(self):
+        LST = 90*degrees
+        Longitude = 50*degrees
+        era = self.makeRawVisitInfo.eraFromLstAndLongitude(LST, Longitude)
+        self.assertEqual(era, LST-Longitude)
+
     def testAltitudeFromZenithDistance(self):
         for zdDeg in (0, 35.6, 89.999, 90.0):
             desAltDeg = 90-zdDeg