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

Sat Nov 15 20:33:26 EST 2014

That may be correct. It then depends on the definition of the galactic
coordinates. I went looking for it. In Fits II Calabretta and Greisen
mention that the Glon and Glat are reserved for the systems defined in
Adriaan Blaauws paper:

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1960MNRAS.121..123B&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

This is in 1950.0 Equinox, so FK4. Of course the pole and zero longitude
can be converted into J2000 (FK5) to get the FK5 positions. It is possible
that that were not done correctly.

Since astropy galactic coordinates probably are tied to the FITS
definition, this is the correct reference, I think.

Paul

On Sat, Nov 15, 2014 at 10:37 PM, Tim Jenness <tim.jenness at gmail.com> wrote:

> 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
>> Mullard Space Science Laboratory  – University College London  –
>> Holmbury St Mary – Dorking – Surrey RH5 6NT–  U.K.
>>
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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
Mullard Space Science Laboratory  – University College London  –
Holmbury St Mary – Dorking – Surrey RH5 6NT–  U.K.
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