Actually, I wonder if the arguments to that function would make more sense than the zenith angle and parallactic angle here?  

I'm trying to think of what the most common use case would be, and I think typically, you would have a single `zenith_coord`, and objects at different locations (specified by `obj_coord`).  So I think the most likely usage pattern would prefer those parameters, rather than `zenith_angle` and `parallactic_angle` having to be calculated for each object.  

Not that the effect varies that much over the size of the typical exposure I realize, but we might as well encourage people to get it right by varying the `obj_coord` appropriately.