A plehtora of Quantities

[Guy Rixon]

> > I suggest "count" and "ratio" for the others. I don't think that putting
> > quasi-units like "CCD" in a Q is going to work; better to write "count" and
> > infer the quasi-unit from context.
>
> I could live with that - the Frame could contain other meta-data saying
> "we are counting CCDs here" - possible a UCD?

Possibly a UCD...or it may be implicit from the position of the Q in the DM.

> > BTW: "pixel" is not a count but a length in arbitrary units.
>
> I would need to disagree here I think. True  - a pixel has a size, but
> then so do forests and you wouldn't want to measure distance in "forests".
> The point is that the size of a pixel (and a forest) is variable, and so
> does not correspond directly to a measure of length. What you do is you
> use your WCS information to map a pixel "count" into a physical "world
> coordinate". [Of course you can gave fractional pixel counts as well!]
> Just for the oldies amongst us, here's a bit of fortran:
>
>       DOUBLE PRECISION FRED( 10 )
>
>       DO I = 1, 10
>          WRITE(*,*) FRED( I )
>       END DO
>
> What are the units of I? "Pixels"?

"Freds" possibly? :)

Seriously, one doesn't count arbitrary pixels.  One always means "pixels in
the current image" which _usually_ implies a physical or angle-on-sky extent.
That extent may be accessible to the s/w (via WCS or via some FITS keyword
giving the pixel size) or it may be known to the user. Either way, there is an
implicit conversion to linear/angular units.  Basically, pixel is a special
case of a length unit whose magnitude varies with context.

Guy Rixon 				        gtr at ast.cam.ac.uk
Institute of Astronomy   	                Tel: +44-1223-337542
Madingley Road, Cambridge, UK, CB3 0HA		Fax: +44-1223-337523