[AstroPy] Telescope pointing with astropy, atmospheric refraction, and precession

[Tony Willis]

I've been following this thread with interest. About 2 years ago I went 
through the effort of computing and comparing apparent position 
calculations from astropy, skyfield, PyEphem and CASA Measures. I'm 
attaching the script I used to test the output using the radio source 
3C147. Lines up to about 160 just have some conversion functions I put 
in so the script was more or less self-contained. The actual tests begin 
at about line 161. The outputs generally agree to within about 0.5 arcs 
or so. Personally I tend to use CASA Measures code for this sort of 
thing. That particular part of CASA was written by Wim Brouw, who used 
to be on the IAU SOFA board. But both astropy and skyfield give very 
close answers.

Cheers

Tony Willis

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