[AstroPy] computing latitude/longitude from RA/Dec

[Michael Hoenig]

Hi,

I currently have an image of the night sky and matching RA/Dec 
coordinates of the center.  I also have the azimuth and elevation the 
camera was pointed at, and the time.  I am trying to use this info to 
calculate the latitude and longitude on Earth where the picture was 
taken from, using AstroPy methods...

latitude = Dec +/- zenith distance, this is easy.

longitude = LST - GMST, where LST = LHA + RA.  This one I'm having some 
more trouble with...

I can create a Time() object, and then use .sidereal_time('mean') to get 
the LST, but of course that requires the location to begin with.

I can create a SkyCoord object from my RA and Dec, but is seems a simple 
transform_to() isn't sufficient either.

What am I missing here?  Is this possible in AstroPy, or do I need to 
resort to something like PyEphem or Skyfield?

Thanks for any help!
Michael