SED Data Model: Questions and Comments

[Pedro Osuna]

Dear all,

about the units handling, I think the units should be handled by the
clients, not the Data Model. The only information the client needs is
the dimensions (equation) of the units together with the scale factor,
therefore the units don't have to be stored in any specific format. 
We have proposed to include those in the SSA protocol and they have been
included in the SED data model. We have shown in several places how well
this works for flux densities. The Quantity Data Model should include
the dimensions as well. 
For other things like photometry, other information like the zero point
and zero flux is needed, and I think we still lack a proper data model
including photometry (in my opinion, all the information needed to
convert whichever unit to flux should be self contained in the metadata;
for instance, photometry data should be accompanied by zero point-zero
flux or filter profile or whatever that would allow a user (client) to
convert to flux densities; obviously, "standards" like Johnson_J are not
worth in this case as Jonhnson_J seems to be different for different
people).
We are finishing a paper with a rigorous formalism on how to deal with
the units problem, but we've already presented it in many places. Will
circulate this paper soon.

Cheers,
Pedro.
 

  
On Wed, 2005-02-16 at 10:17, David Berry wrote:
> On Tuesday 15 February 2005 12:42 pm, Gilles DUVERT wrote:
> > If 'Quantity' is, or will be, able to deal with such things as flux
> > conversions from/to air wavelength vs. humidity and frequency,etc..  and
> > Janskys from/to Cousins/Johnson magnitudes, at least to a large extent
> > and great accuracy, all the best. In that case all units or combinaisons
> > thereof supported by Quantity should be authorized in, e.g., the SED DM.
> > But at the same time one could use the Quantity DM to convert the data
> > beforehand to specific quantities that would be the only  ones referred
> > to in the SED DM, such as basic  MKSA units. And make the DM simpler.
> 
> The issue is ensuring that the model includes all the necessary meta-data
> needed to convert *from* MKSA units to other more esoteric systems. For
> instance, if all flux density-like data is required to be stored in
> W/m^2/Hz (or do I mean W/m^2/Angstrom?) within the DM but the end user
> wants to see antenna temperature, then the DM needs to include all the
> necessary efficiency factors, etc, needed to do this conversion. Likewise,
> converting to/from magnitudes.
> 
> Then what if the meta data needed to convert from your estoteric system to
> the standard DM system are only poorly known? It is surely possible that
> useful science could be done in the esoteric system, without ever
> converting to the standard system. But if the DM *insists* that all data
> is converted to a standard system, then you are immediately introducing
> errors which need not be there.
> 
> So what system should the DM use for flux density-like data?
> 
> 
> David
-- 
Pedro Osuna Alcalaya

 
Software Engineer
Science Archive Team
European Space Astronomy Centre
(ESAC/ESA)
e-mail: Pedro.Osuna at esa.int
Tel + 34 91 8131314
---------------------------------                                                                                
European Space Astronomy Centre
European Space Agency
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