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

[Thomas Robitaille]

It is true that most other tools are more consistent:

http://www.astropy.org/coordinates-benchmark/summary.html

(see the fk5 to galactic section). It seems like we should be achieving
1-10mas agreement with other codes.

The weird thing is that fk5 -> fk4 agrees with other codes, as does fk4
-> galactic. I think this is because astropy.coordinates also defines
the fk5 -> galactic transformation directly so it may be related to
this. It may be that the direct fk5 -> galactic transformation is not
quite right.

Erik T.: can you comment on this?

Cheers,
Tom

Christoph Deil wrote:
>> On 14 Nov 2014, at 21:39, Joseph Booker > 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)
>>
>>
>> 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]: 
>>
>>
>> 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
>
> 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/
>
> Thanks for porting `pyregion` to use `astropy` and doing such detailed
> checks!
>
> Christoph
>
>
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