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

[Tim Jenness]

I was under the impression that SOFA deliberately ignores Galactic
coordinates. I can't find any mention of Galactic in the SOFA/ERFA source
and PAL (a SLALIB port to C using ERFA where it can) had to use the SLALIB
Galactic code.

-- 
Tim Jenness

On Sat, Nov 15, 2014 at 3:13 PM, Paul Kuin <npkuin at gmail.com> wrote:

> I asked the chair of the IAU astrometry commission. Basically the current
> coordinate system is ICRS, and the galactic frame is defined with reference
> to that with the same accuracy. I think that is in a few milli arcsec
> accuracy. The SOFA implementation is to be consulted for actual algorithms,
> and I think this has been (or will be) implemented in astropy as the ERFA
> package.
>
> In summary, I would think the differences found by Joseph are way too
> large. It may be that the Kapteyn algorithms are also not compliant.  Test
> should be done against the results of the ERFA/SOFA software.
>
> Thats all the help I can give.
>
> Paul
>
> On Sat, Nov 15, 2014 at 5:32 PM, Thomas Robitaille <
> thomas.robitaille at gmail.com> wrote:
>
>> I've played around with this a bit more, and I found that if I set:
>>
>> _ngp_B1950 = FK4NoETerms(ra=192.25*u.degree, dec=27.4*u.degree)
>>
>> (not FK4) then I explicitly set:
>>
>> Galactic._ngp_J2000 = _lon0_B1950.transform_to(FK5)
>>
>> then if I set:
>>
>> Galactic._lon0_J2000 = Angle(122.931918539, u.degree)
>>
>> Then the astropy FK5 -> Galactic and FK5 -> FK4 -> Galactic
>> transformations agree within ~0.1mas, and our results are in agreement
>> with other codes. I'll open an issue on the astropy repo and we can
>> discuss the technical details there.
>>
>> Tom
>>
>> Erik Tollerud 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...
>> >
>> > 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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>> >
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>
>
>
> --
>
> * * * * * * * * http://www.mssl.ucl.ac.uk/~npmk/ * * * *
> Dr. N.P.M. Kuin      (n.kuin at ucl.ac.uk)
> phone +44-(0)1483 (prefix) -204927 (work)
> mobile +44(0)7806985366  skype ID: npkuin
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