Photometry in VO Spectrum Model

[Anita Richards]

On Mon, 23 Oct 2006, Doug Tody wrote:
>
> I can see that one might want to plot a spectrum in magnitude units in
> a standard system.  A good question to consider however, is whether this
> feature belongs in the user interface, where the spectrum is plotted in
> whatever units the user prefers, or in the protocol.  This is similar to
> displaying RA/DEC in sexagesimal units, while we use decimal degrees in
> the protocol.  This sort of conversion is easily handled in the user
> interface.
>

Unfortunately it is not easy, that is the problem.  At least, I am assured 
by optical astronomers that for highly accurate conversion you need a raft 
of metadata and even to know the nature of the object.  That is also the 
case for converting X-ray counts to physical units.

Part of the discussion which we have been having in VOTech has considered 
a two-level conversion.  That is, if a standard magnitude system is 
specified, or a simple conversion factor for X-ray counts, you can convert 
to physical units to within a few percent accuracy.  NB you also need to 
know the effective wavelength for the optical filter to get the spectral 
axis in physical units, or similar for the X-ray band.

For higher accuracy, we have discussed the possibility of metadata 
including a unique reference which would allow a VO query to the parent 
instrument archive to associate zeropoints etc. with that particular 
observation as required.  This may happen in the near future for a few 
archives but will not be exhaustive (if anyone can find a web reference 
for e.g. plate N mag as in USNO-B, I will buy them a very large beer....) 
so we need to preserve the possibility of passing data in these bizarre 
units and hope taht tools will allow an easy manual option for specifying 
conversion factors.

cheers
a



- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dr. Anita M. S. Richards, AstroGrid 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).