Fixed kapteyn transforms to explicitly specify the epoch of observation for the B1950 conversion, and the equinox for the ecliptic conversion
Explicitly precess astropy.coordinates fk4 result to B1950
From David Berry:
1) The differences between kapteyn and (idl,pyast,tpm) on the J2000 to B1950 tests is reduced seriously if you specify the epoch of observation explicitly in kapteyn/convert.py. That is, change
This reduces the median difference from 0.217 arc-sec, to 0.0044 arc-sec (this must mean that kapteyn is using a different default epoch of observation to the other systems).
2) Improved agreement in J2000->ecliptic can be achieved by specifying the equinox explicitly in kapteyn/convert.py. Change:
This causes kapteyn and IDL have zero difference. The kaptyn->pyast median difference goes down from 0.04 to 0.036. The remaining difference between kapteyn and pyast seems to be down to:
a) DIfferent obliquity models. kaptyn uses the IAU 2000 model, and pyast uses the IAU 2006 model. b) Different precession models. kaptyn uses IAU 1976 and pyast uses IAU 2006.
Reverting the models used by pyast gives zero difference between kapteyn and pyast for the J2000->Ecliptic test