[AstroPy] astropy.coordinates vs kapteyn.celestial Coordinate Transformations
Paul Kuin
npkuin at gmail.com
Sun Nov 16 11:42:30 EST 2014
Hi Thomas,
Good question.
I browsed yesterday some of the papers of commission 8 members that have
been linked from the SOFA web pages.
There are two ways to look at this issue. The first one is that a
"complete" set of observations can define a reference system. One such
coming soon is that based on the GAIA mission, so based on optical sources.
The system is typically tied to very remote sources, so Quasars. The
current system is based on radio interferometric observations of quasars,
and the next system (ICRS3) is probably being a refinement on that. We
won't at first be dealing with that, but the errors in the positions are
going to be of the order of 40-70 micro-arcseconds!
I'm not sure what the position of the specialists in astrometry and
coordinate systems is with regard to maintain the link to the older
systems, like FK4, FK5, the old galactic coordinate system prior to the
1959 revision, etc. It seems to be a neglected area, and may be an issue
with historical data. I just cc:-ed Norbert Zacharias, since I am getting
out of my depth here. It may just be that we have touched upon an area that
needs attention, or that we need to find the right person or publications.
Regards,
Paul
On Sun, Nov 16, 2014 at 3:37 PM, Thomas Robitaille <
thomas.robitaille at gmail.com> wrote:
> Hi Paul,
>
> Thanks for the input - as an aside, do you have any insight into whether
> the IAU is considering proposals such as:
>
> http://www.aanda.org/articles/aa/pdf/2011/02/aa14961-10.pdf
>
> to re-define the Galactic coordinate system relative to ICRS? Having
> Galactic coordinates defined relative to FK4 at the moment is not ideal
> and introduces a lot of confusion, so re-defining galactic coordinates
> relative to ICRS would be a nice solution.
>
> Cheers,
> Tom
>
>
> Paul Kuin 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 <mailto: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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> >
> >
> >
> > --
> >
> > * * * * * * * * http://www.mssl.ucl.ac.uk/~npmk/ * * * *
> > Dr. N.P.M. Kuin (n.kuin at ucl.ac.uk <mailto: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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--
* * * * * * * * 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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