[AstroPy] astropy.coordinates vs kapteyn.celestial Coordinate Transformations

[Joseph Booker]

Hello,

I've been porting pyregion to use astropy instead of kapteyn, and tests
with coordinate system conversions are slightly off.

I think I've narrowed down the problem to my expectation that this should
be nearly zero:

In [21]: from astropy.coordinates import SkyCoord

In [22]: from kapteyn import celestial

In [23]: a = SkyCoord('292.03306305555554d 1.7592747222222223d',
frame='galactic').transform_to('fk5'); print(a)
<SkyCoord (FK5: equinox=J2000.000): ra=171.158093022 deg,
dec=-59.2630875829 deg>

In [24]: celestial.sky2sky(celestial.galactic, celestial.fk5,
[292.03306305555554], [1.7592747222222223])
Out[24]: matrix([[ 171.15816386,  -59.26319319]])

In [25]: SkyCoord('171.15816386d -59.26319319d',
frame='fk5').separation(a).to('arcsecond')
Out[25]: <Angle 0.4019071919711007 arcsec>


My question is: am I misunderstanding something about these coordinate
transformations to make them not equivalent? A third of an arcsecond is
significantly big deviation, particularly for HST or interferometry. AFAIK
fk5 is J2000 in both libraries and galactic coordinates have no concept of
epoch or equinox time.

Thanks,
Joseph Booker
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