[QUANTITY] Plea for pragmatism

[Doug Tody]

Hi Alberto -

> > Did I leave anything out?  Polarization perhaps, but that might be better
> > dealt with elsewhere.
> 
> To answer you question I think that the missing info has
> to do with the Errors associated on all those quantities, 
> oops sorry, let me call them parameters not to confuse things here;
> mainly I refer to the astro-spectro-photo-metric accuracy.
> An image might be of interest to me because satisfies some coverage
> criterion, but I might want to discard it because the astrometric
> precision is not the one required for my study.
> 
> Hence: Coverage(how the 5D volume is sampled)+Errors.

But these are not mere parameters but "bandpass" data models.  Each is
essentially a value (the refValue) plus "error bars" (the hi/lo values).

 
> What else ? The seeing. 
> How the 5D volume is sampled is not enough ...
> Resolution is intrinsic to the instrument/camera,
> while the seeing has to do with the overall atmosphere+dome+optical design.
> I don't like this since it's breaking the simmetry.
> 
> Hence: 5D volume + errors + seeing ?
> 
> Unless we replace "Resolution" with the covolution of Resolution and seeing.

All of these models are assumed to be observational models calibrated to
the sky, not instrumental models.  Hence the spectral bandpass is not just
for a filter, but also for the telescope, instrument, atmosphere, etc.
The spatial bandpass would reflect both instrumental resolution and,
at the low end, seeing effects (note also that for instruments such as
interferometers the spatial frequency response and filling factor will
in general not be flat as it is for O/IR instruments).


> The refValue/hiValue/loValue are a very good starting point,
> the zero order approximation, we can certainly go further
> and defines things like exposure maps etc, to describe more precisely
> the coverage, but that can come later. The zero order approximation,
> or "summary data model" as you call it,
> will always be valid, as you were saying.

Exactly.
 

> I think it should not be difficult to come up quickly with 
> a simple data model to describe the coverage; and it will greatly
> help the development in other WG, like DAL, VOQL and UCD.

Right!  I don't think it should be all that hard to come up with these
simple models.  In doing so however, we will have to work out a lot of
important things, like how to formally define a data model, how to
represent one in XML (schema), how to represent one in VOTable (UTYPE
or equivalent), how to relate or associate entities, how to use
such information conveniently in queries, and so forth.

	- Doug