spectral WCS in FITS

[Anita Richards]

> From: Andreas Wicenec <awicenec at eso.org>
>
> I think we should start with the following concept:
>
1.
> - Convert FITS binary tables to VOTable with <STREAM> elements or binary
> attachments, i.e. don't touch the binary part, but just describe it.
>
2.
> - Convert FITS ASCII tables to fully tagged VOTables with <TABLEDATA>
> elements.

Thanks Andreas, that is what I hoped was the case.  As you say below,
there is no point in converting anything more complex entirely to VOTable,
as complex FITS binary data are unusable (or vastly less accessible at
best) in that form any way.

However, to go back to Bob's original question, is there anything in (or
missing from) the WCS Spectral Coordinates document which makes (2.) (or
its reverse) difficult? If not, then I agree with Dave Berry's comment
that we should not force VOTable and FITS to be identical.


best wishes

Anita

>
> The reverse conversions should then also be possible (see my example).
> The cross conversion between the two is more tricky and requires that the
> converter needs to know how to deal with the binary part. This is also
> possible, but not with plain XSLT although it can be done by using external
> functions called by the xslt processor. I did bot try it yet, but I guess it
> is not really complicated for simple binary tables. The problem starts when
> you want to deal with complicated radio or other interferometric data which
> are represented using hierarchical table structures with many multiplicity
> levels (baselines, basebands, polarisations, frequency channles). VOTable is
> not yet able to fully deal with this kind of data and the way it is
> implemented in FITS is also not really self-describing, i.e. there is a lot
> of implicit knowledge necessary to interpret the complete data structure of
> something like an AIPS++ measurement set. On the other hand the amount of
> data and the complexity of such structures makes pure ASCII representations
> pretty bulky and nobody really gains from being able to read it or load it
> into an editor, because it is almost impossible to do anything reasonable
> interactively anyway.
>
> Cheers,
> Andreas
>
> On Thursday 27 November 2003 20:13, Anita Richards wrote:
> > Yes but "The binary data is still kept as external links in a STREAM
> > element"
> >
> > If that is what we are talking about then there is no problem... what I
> > *thought* was the problem included the possibility of converting spectral
> > data in the form of FITS tables entirely to VOTable (probably OK?) and
> > back again.  But if no-one wants to do that, that is fine...
> >
> > a
> >
> > - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> > Dr. Anita M. S. Richards, AVO Astronomer
> > MERLIN/VLBI National Facility, University of Manchester,
> > Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
> > tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).
> >
> > On Thu, 27 Nov 2003, Marco C. Leoni wrote:
> > > Anita,
> > >     about the conversions FITS<--->VOTable there was the proposal from
> > > Andreas, please have a look at:
> > > http://www.ivoa.net/forum/votable/0217.htm
> > >
> > >
> > > Cheers,
> > >     Marco
> > >
> > > Anita Richards wrote:
> > > >On Thu, 27 Nov 2003, Markus Dolensky wrote:
> > > >>>However
> > > >>>unless I ahve misunderstood the debate, there is no question that the
> > > >>> data _can_ be handled, it is simply a question of how,
> > > >>
> > > >>The question is whether we can perform the conversion between VOTable
> > > >> and FITS format using off the shelve tools (XSLT). If not we need to
> > > >> write Astronomy specific parsers which 'understand' the data whereas
> > > >> we would prefer to apply just syntax rules instead.
> > > >>
> > > >>Markus
> > > >
> > > >For FITS to VOTable, there is also the tool being developed by VO India
> > > >http://vo.iucaa.ernet.in/~voi/votableStreamWriter.htm
> > > >which is indended to eventually cope with FITS.
> > > >
> > > >But what about the reverse? That is more difficult, surely?
> > > >
> > > >cheers
> > > >a
> > > >
> > > >- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> > > >Dr. Anita M. S. Richards, AVO Astronomer
> > > >MERLIN/VLBI National Facility, University of Manchester,
> > > >Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
> > > >tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).
>
>