[AstroPy] FITS files and column descriptions

Paul Kuin npkuin at gmail.com
Thu Nov 7 16:59:46 EST 2013

Some years ago, I was leading the NASA Astronomical Data Center, now gone.
I worked with CDS to have a way to present a readable table along with the
ability to automatically generate a FITS file.  That lead to the "ReadMe"
files that accompany the catalogs from CDS, which is storing the metadata.

I think it is important to make sure no metadata are lost in the conversion
process. Sure, the FITS standard says so-and-so, but in practice, one has
to make amends so the standard can evolve.

I have only used the astropy saving to tables a little bit because I got
annoyed that there was loss of (meta) data.

The reason I write is, that there needs to be some serious thought about
this issue. as flagged by Yannick. My (biased) feeling  is that you cannot
go around having a second metadata file when going to ascii. Doing
otherwise will lead to a plethora of ways to save the data file. Better to
keep the data file clean, and put the jumk in a meta data file.



On Thu, Nov 7, 2013 at 9:20 PM, Yannick Roehlly <yannick.roehlly at lam.fr>wrote:

> Hi all,
> I have some remarks / questions concerning the column descriptions in
> FITS table files (and VO).
> 1. Long column description
> The FITS format requires that a column description must be 68
> characters or less. Some tools like topcat / stills allows longer
> descriptions because they use a kind of VO-table metada section in the
> FITS header. If you open such file with astropy.io.fits and save it
> again, you loose the long description. In fact, it's not an astropy
> problem as these description as not in the "real FITS" column
> descriptions in the header (TCOMxx), it is just because astropy does
> not save the "VO-table" part of the topcat file.
> Maybe it's worth raising a warning when opening such files, like "Hey,
> you may loose metadata!" because one may be surprise not to have all
> its column description.
> 2. astropy.table and column descriptions in FITS files
> When opening a FITS file with astropy.tables(with last git checkout)
> one does not have access to column descriptions. When saving an
> astropy.io.table to FITS, the column description are lost. But when
> opening a FITS file with description and saving it again to FITS, the
> column descriptions are still here. I suppose that it's because
> astropy.io.fits is under the hood.
> Is this because column description handling is not implemented or do I
> miss something?
> 3. Column descriptions in VO-Table
> When saving a table to VO-Table, astropy prettily indent the xml, e.g.
>      IAU Name
> But when reading these tables with topcat, this one add the spaces to
> the description ("     IAU Name"). Maybe astropy should write the
> description on one line.
> Regards,
> Yannick
> --
> Mystics always hope that science will some day overtake them.
>                 -- Booth Tarkington
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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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