[iaufwg] Call for vote on the new FITS Standard

[Mark Calabretta]

On Tue 2008/05/06 18:03:44 -0400, William Thompson wrote
in a message to: IAU-FWG <iaufwg at nrao.edu>

Dear Bill,

>Nowhere does it state that CRPIXj runs from 1 to N, instead of from
>0 to N-1.  I've found that the distinction between these two cases
>trips up many novice FITS users, and is the most common mistake made.

I agree with the sentiment but not the proposed change to the definition
of CRPIXja.

In the language of Eq. 8.1, CRPIXja simply records r_j (so it wouldn't
be a bad idea for the definition of CRPIXja to refer back to that).  In
turn, r_j is just a specific value of p_j.

The problem is that p_j, i.e. "pixel coordinates", are never properly
defined.  They are actually a generalization of storage coordinates
i.e. the integer array indices used to address the data array - refer
to http://www.atnf.csiro.au/people/mcalabre/WCS/Intro/WCS04.html.

Basically there needs to be a statement somewhere near Eq. 8.1 referring
to the "index count" defined in Sect. 3.3.2 with a statement that
p_j = 1.0 coincides with I_j = 1, i.e. the index of the first datum on
axis j, and that p_j increments in step with I_j.  Non-integral p_j may
be understood to indicate interpolation between two data values and is
disallowed for certain "conventional" axis types.

As for visualising FITS image data (a completely separate issue), there
should also be a statement somewhere that when a datum is represented
as a "pixel", I_j, and hence integral p_j, coincides with the centre of
the pixel.

Also, the reference to the HEALPix paper on p120 should be MNRAS, 381,
865.

Regards, Mark