Photometry and SED models at Nara

[Miguel Cerviño]

Hi Anita and Jonathan

I just read the discussion about "colors", spectral index related with the Photometry and SED DM.
I think it is a very good case although not just affect the Photometry and SED DataModel.
(I include here the relevant lines)

>> Back to the SED, we should also consider the extensions to spectral index 
>> and spectral curvature (possibly higher orders) which are the coefficients 
> 
> So, this concerns an image product, spectral index maps ("color maps" in the optical),
> or a time series product, color versus time, 
> with a complicated "y" axis. It's complicated because the metadata
> needs to point to two Band definitions. I think one could have a 
> "Photometry Color/Spectral Index" standard that tells you how to 
> record in metadata that a time series, image, etc. has a particular color as
> its data values. That would use the Photometry standard (but not the SED)
> but could be a short separate document, since colors do raise some interesting issues.


I have several examples that are related with spectra like the one you quoted:
colors, line equivalent widths, continuum indices (as it is the ratio of two bands/wavelengths)
hardness ratios (for X rays) etc... but also, at the end, any kind of "product" where
a ratio is involved like mass-luminosity relations, abundance ratios, a lot of
theoretical results etc...

I think that a simple basic approach without a priori assumptions about which quantities
provide the "index" is more useful. In the line of the "physical quantity" presented
in "Simple Spectral Lines Data Model Version 1.0" 
(http://www.ivoa.net/Documents/SSLDM/20101004/index.html)
it would be just an extended physical quantity expressed as the composition of other two 
(maybe more) physical quantities related by a mathematical operation (usually take a ratio and a log)
Of course there is an additional issue in the characterization of the resulting error since
there would be a covariance between the quantities and/or some hypothesis about the independence
of the quantities must be done but...

It is just an draft idea, I think that it is so natural and simple that
may work for a wide number of cases with a relatively easy implementation/description
in a data model.

Unfortunately It is not possible to me to attend the Nara Interop, so I 
acknowledge the use of the mailing-list for discussion (I will we aware of
the presentations etc in the wiki any case :)

cheers and I hope you have a nice interop.

Miguel