[AstroPy] fits coordinate system keyword

Thomas Robitaille thomas.robitaille at gmail.com
Mon Oct 5 17:17:25 EDT 2015


I think there is a misunderstanding:

RADECSYS is not valid

RADESYS is valid

(note the extra C). We are not debating that RADESYS is valid - it is -
but the warning concerns the common mis-spelling of RADE*C*SYS. Does
this make sense?

Cheers,
Tom

Paul Kuin wrote:
> 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 <mailto: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>
>     > <mailto: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
>     >
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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>
>     <mailto:n.kuin at ucl.ac.uk <mailto:n.kuin at ucl.ac.uk>>)
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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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