spectral data

[Anita Richards]

In response to Frank Valdes' comprehensive and very readable posting,
> Incorporating spectra in the Next Phase of the VO ..
I think there are two more things which flow out of this:
- errors
- conversions and conventions

These are covered in e.g. Greisen et al 2002 Representations of spectral
coordinates in FITS - is this published yet?

and we need to think about two sorts of conversion
1) Accuracy needed for answering queries - here we need to know the
bandpass shape etc. etc. if the user wants high accuracy.
2) Accuracy for Registry Resource metadata and crude conversions -
we can probably ignor filter/instrumetn-specific and other details.  In
other cases also, e.g. line ratios from a single dataset, useful results
can be obtained without all the information one would have in an ideal
world.

I take issue with one comment:

> However, the 4D proposal is to not make the extension too general, and
> hence, complex. Instead, simply add two parameters to cover the vast
> majority of observational parameter space of interest to astronomers.

Unfortunately - or fortunately - that is a very dangerous thing to say.
Polarization is one example of extra axes in image data, let alone other
sorts of data like time-series along moving coordinates or visibility
data.

Just one example in the image domain: Monitioring of SiO maser emission.
Experiments currently published produce:
Multiple epochs of RA-Dec-Frequency datacubes  (4 axes)
in Stokes I Q U V			       (another 4 axes?)
in 2 or more lines of SiO	               (another >2 axes?)

I am not suggesting we start by figuring out how to represent this but we
need to be able to easily add dimensions, and I think we need to tackle
polarization after we have a working consensus on spectral coordinates.

In addition, some data use an extra dimension or dimensions for
uncertainties, masks etc.


Regarding Ivo Busko's document on Generic Spectral Data Structures, I
guess this is just for handling certain types of data? as it does not seem
to cover the 3 or 4 D cases, e.g. datacubes, described by Frank Valdes.
This is not a criticism, as I am sure Specview is very good in its domain,
but it will not meet all the near future needs of the VO.

thanks
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).