[QUANTITY] Plea for pragmatism

[amicol at eso.org]

Quoting Doug Tody <dtody at nrao.edu>:

> 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).

Maybe we are not on the same wavelength here, let's try to clarify this
please. Let's take the case of "flux bandpass":

>     flux bandpass
>         range of flux values in data
>         hiValue, loValue, units, fillFactor (no refValue)
>         loValue is also known as the limiting flux or magnitude
>         hiValue is saturation limit or maximum flux

Here loValue is the limiting flux, and hiValue is the saturation limit.
those two values have nothing to do with the photometric accuracy,
eg, the error on the zeropoint. Where do you describe that ?

Similarly the sky coverage (be it a circle or a polygon) should
have an associated astrometric accuracy (any vertex position is knwown
within a certain precision).

Also, for the spatial bandpass I thought you meant 
loValue to be the resolution, and hiValue the field of view; 
did I missinterpreted that ?
 
> > 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).

Agreed, but the pixel scale is still an important piece of information
and it is useful to know whether an image is oversampled or not.
And, yes, the PSF might vary within the field of view.
 
> 
> > 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
> 


Alberto