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

[Christoph Deil]

> On 14 Nov 2014, at 21:39, Joseph Booker <joe at neoturbine.net> wrote:
> 
> 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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Hi Joseph,

it must be some small difference in the definition of the Galactic or FK5 system, but I don’t know what it is.

Differences of `astropy.coordinates` against `kapteyn.celestial` for Galactic to FK5 conversion at the 0.4 arcsec level have been there since the initial implementation of `astropy.coordinates`:
http://www.astropy.org/coordinates-benchmark/summary.html <http://www.astropy.org/coordinates-benchmark/summary.html>

If you or any sky coordinate experts have time to work on coordinate checks, please file issues or pull requests here:
https://github.com/astropy/coordinates-benchmark/ <https://github.com/astropy/coordinates-benchmark/>

Thanks for porting `pyregion` to use `astropy` and doing such detailed checks!

Christoph


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