[Quantity] How to store a list of positions

[David Berry]

Ed,
   Having said in my previous message that I am not happy with the idea of
using a Quantity to describe an axis as a discrete set of axis values, I
should possible explain that I *do* recognise that there will be occasions
when you want to store a set of discrete axis values in a Quantity. I may
have given the impression that this is not possible in my model. I do
not think that describing an axis of (say) an image is such an occasion,
but on the other hand it should be possible to use a Quantity to store
(say) a set of star positions given as (RA,Dec) pairs.

The distinction between this situation and the one we have usually been
discussing (that of a flux Quantity), lies in the value which the
Quantity is being used to represent. I see a Quantity as being:

Quantity
   ValueList
   WCS
   Units
   ... (other stuff, e.g. may be errors, quality flags, etc)

in which both WCS and Units are optional FrameSets describing the
transformations from "what you have" (raw pixel value and pixel indices)
into "what you want" (calibrated data value and world coords in various
alternative systems).

Now in a flux Quantity, the Units frameset describes how to transform the
numerical values stored in the ValueList into scientifically meaningful
flux values and also describes the resulting flux system (physical units,
etc). The frameset may contain more than one possible "flux" system (e.g.
it may describe how to convert the raw pixel values into antenna
temperature, or it may contain several different calibrations). Also, if
each value in the ValueList can be said to have a "position" in one or
more "world coordinate systems" then the WCS FrameSet will describe how to
transform list index into these world coordinates.

On the other hand, if we want to use a Quantity to describe a set of star
positions, the ValueList will hold numerical (RA,Dec) pairs (say). Now, as
described above it is the job of the Units component to describe the values
stored in the ValueList, therefore the Units component will hold a
FrameSet which describes the (RA,Dec) system in use (epoch,equinox,etc)
and how to transform these positions into any other relevant systems.
In this case it is probable that there is no significance at all in the
list index used to access the ValueList, and so in this case the WCS
component would either be empty or omitted all together.

Summary: The Units component describes the *numerical values" in the
ValueList, and the WCS describes the *list index* within the ValueList.

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
OX11 0QX