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

[Christoph Deil]

> On 15 Nov 2014, at 18:00, Erik Tollerud <erik.tollerud at gmail.com> wrote:
> 
> Hi Tom and Joseph,
> 
> Originally, Galactic coordinates were defined to ~arcmin precision relative to FK4 (and it's not clear one way or another if this is supposed to include E-terms or not). Since then there have been recalibrations to FK5, although there's not an "official" version as far as we could find (just an appendix in a paper).
> 
> So I think what's happening here is that the "shortcut" FK5-> Galactic transform is being used, which gives somewhat different results because it made different arbitrary choices. We could just turn that off if we want to always be sure to use the FK4 transformation.
> 
> It's not really clear which is the "right" answer, though. I think our original thinking was that no one really uses Galactic for subarcsec precision, and there FK4 transformations are a lot slower, so this "shortcut" makes sense...
> 
> 

Can’t you get speed and internal consistency in this case by hard-coding the results you get via FK4?

If no, then probably precision and consistency for Galactic coordinates is more important than speed for most users?

Christoph


> 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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