a Quantity to rule them all? Not quite.

[David Berry]

Alberto,

> It looks to me that all these discussions are "forgetting" the role of
> any higher level data model. By reading your emails, it even seems to me
> that Quantity covers all aspects and no higher level models are needed.

That is certainly not our intention. A Quantity is suppose to be a
general purpose box into which you can put an array of numerical values
representing measurements (theoretical or experimental) of some single
phenomenon. The idea of Observation is that it then groups together
Quantities (and other structures) holding measurements of the various
phenomena which make up a single observation.

> In the above examples,  I would say that the Observation DM should be
> consulted to get to know which quantity necessites which mapping info.
> And the mapping info could depend on various instrument specific details.
> Examples:
> The mapping from pixels to sky coordinates might require specialised
> geometric correction solutions.
> The mapping from counts (ADU) to flux might involve a zeropoint along with
> the knowledge of "exposure maps".

I think the WCS information is rightly in the Quantity rather than the
Observation because two Quantities within the same Observation may have
different WCS. In an Observation representing a mosaic camera for instance
each chip will have its own WCS (that is, the relationship between pixel
coords and sky coords will be different for each chip),and so it is
appropriate to store the WCS for each individual chip in the Quantity
which holds the pixel values for that chip.

The point is that "WCS" (in its most general sense) can potentially be
associated with *any* array of measured values, no matter what values are
stored in the array. And things which are potentially common to all arrays
should be in Quantity.

> If I understand correctly David's proposal, I like the getMapping method.
> The question is: how does it work? where is it getting the mapping info
> from?

A description of the Mapping is stored inside the CoreQuantity. But the
idea of the Mapping class is that it is a general purpose class for
transforming arbitrary numbers, and it can be used in any context, not
just within Quantity. So if there was some need to store a Mapping
directly in an Observation, it could do.


David


----------------------------------------------------------------------
Dr David S. Berry    (dsb at ast.man.ac.uk)

STARLINK project		 |	Centre for Astrophysics
(http://www.starlink.ac.uk/)	 |	University of Central Lancashire
Rutherford Appleton Laboratory	 |	PRESTON
DIDCOT				 |	United Kingdom
United Kingdom			 |	PR1 2HE
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