The zenith angle and position angle are a bit tricky to figure out.  It might be worth providing a helper function (maybe in the dcr module) that would take CelestialCoords for the object (ra,dec) and the zenith (or alternatively the hour angle and the latitude of the telescope) that could return the zenith_angle and position_angle needed for this function.  I think those are the values you would normally need to calculate these angles at least.