VOPlot - Tool for plotting VOTables released.

[Markus Dolensky]

Doug Tody wrote:
 > While at ESO a couple of weeks ago the topic of spectra came up in
 > connection with SIA - the ESO folks were interested in this as well.
 > Images and spectra are two sides of the same coin.  SIA could be extended
 > to prototype some sort of simplified uniform access to spectra, or we
 > could do something new.  Both images and spectra should be dealt with as
 > components of VO data access.



Hi Niall,

Here's the ad hoc solution chosen for the AVO science demo. The SEDs 
were expressed as a VOTable which got dynamically created by 
Aladin/VizieR and handed over to a little SED plot utility:

--------------

For each object there is a <TABLE ID="" name=""> element. The 'ID' 
attribute is 'abused' for event handling/display purposes between some 
java components. The optional 'name' attribute contains a catalog name 
and object identifier taken from VizieR/CDS.
E.g.: <TABLE ID="528661881" name="II/234/ubvrijk 19311">

Each TABLE contains at least 2 FIELD elements (flux and frequency) with 
optional error column(s). Arbitrary further columns are ignored.

For the sake of the demo we agreed to use the following UCDs:
Flux (Jy)             : PHOT_FLUX_DENSITY
error in flux         : ERROR
central frequency (Hz): OBS_FREQUENCY
error in frequency    : ERROR

The following FIELD attributes are used:
{ name, ucd, unit, datatype }
Attribute 'precision' was specified, but not actually used.

E.g.: <FIELD name="frequency" ucd="OBS_FREQUENCY" unit="Hz" 
datatype="float" precision="E5"></FIELD>

-------------

Maybe it's useful to look at the user requirements to understand the 
rationale for above solution:
http://www.euro-vo.org/internal/Avo/AvoSoftwareRepos/sed_urd.txt

Obviously there are a number of assumptions built in but we are 
interested in finding a more general convention.
Some improvements would be straight forward like supporting different 
units or allow to choose from arbitrary UCDs.

-------------

Other steps are more tricky: An important step not described in the 
VOTable is how to convert magnitudes given in some VizieR catalog to 
flux density as required by the SED plotter. For the AVO demo it was 
done only for some selected catalogs by consulting relevant publications 
and looking up zero points in some FITS headers.

Markus