[AstroPy] fits coordinate system keyword

[Paul Kuin]

In the WCS II standard paper ( M. R. Calabretta and E. W. Greisen:
Representations of celestial coordinates in FITS
http://www.aanda.org/articles/aa/pdf/2002/45/aah3860.pdf) on page 1079 the
is a table defining some variables used. In particular 'a' is alternate
version code 'blank or [A-Z]. In table 2 on page 1082, the definition is
RADESYSa, meaning that valid versions are either RADESYS, RADESYSA,
RADESYSB, ... , RADESYSZ.

So it always was defined as a valid. Similar, EQUINOXa/. I've seen that
mostly used as just EQUINOX. If the header has just one WCS block, or a
preferred one, that is useful to have.

Let's see what Bill Pence says. Maybe there were some changes after all.



On Mon, Oct 5, 2015 at 8:34 PM, Thomas Robitaille <
thomas.robitaille at gmail.com> wrote:

> Hi Paul,
>
> Just out of curiosity, was RADECSYS ever a keyword in the FITS standard,
> as opposed to RADESYS? (which is the currently valid keyword). If so, in
> what version of the standard?
>
> Cheers,
> Tom
>
> Paul Kuin wrote:
> > Actually, one of the basic tenets of FITS is to be completely backward
> > compatible, so any keyword that was valid in the past is valid now. So
> > that warning is also inaccurate. If it was standard, it still is, and
> > ever will be.
> >
> >
> >
> > On Mon, Oct 5, 2015 at 5:25 PM, Erik Bray <embray at stsci.edu
> > <mailto:embray at stsci.edu>> wrote:
> >
> >     On 10/05/2015 12:17 PM, Paul Kuin wrote:
> >
> >
> >
> >         I am getting very annoyed of the warning that keeps popping up
> >
> >         WARNING: FITSFixedWarning: RADECSYS= 'FK5' / default
> >         RADECSYS is non-standard, use RADESYSa. [astropy.wcs.wcs]
> >
> >         I'm not sure when the FITS standard changed, but MOST legacy
> >         fits files (I use)
> >         will have the RADECSYS keyword. It works, and I do not see why
> >         someone felt the
> >         need to keep this warning being popped up all the time.
> >
> >
> >     Probably as a warning to remind authors of new files that they
> >     shouldn't be using that keyword.  I don't know that it necessarily
> >     needs to be displayed every time reading in a WCS except maybe to
> >     head off cases of ambiguity.  But I don't know this particular issue
> >     well enough to be sure if the warning is appropriate in all cases or
> >     not.
> >
> >         I think that by default it should be off.
> >
> >         Since you all probably think it should be there, I would like to
> >         know how to
> >         turn it off ?  any suggestion on how to do that?
> >
> >
> >     http://docs.astropy.org/en/stable/warnings.html
> >
> >
> >     Erik
> >
> >     _______________________________________________
> >     AstroPy mailing list
> >     AstroPy at scipy.org <mailto:AstroPy at scipy.org>
> >     https://mail.scipy.org/mailman/listinfo/astropy
> >
> >
> >
> >
> > --
> >
> > * * * * * * * * 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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