[iaufwg] CRPIXj clarification

[Mark Calabretta]

On Wed 2008/06/25 12:23:40 -0400, William Pence wrote
in a message to: Mark Calabretta <mcalabre at atnf.csiro.au>
and copied to: IAU-FWG <iaufwg at nrao.edu>

Dear Bill,

>In hopes of speeding up this process, let's step back a minute to review 

We don't seem to be saving much time!

>the start of section 8.1, as well as modifying the definition of CRPIXj 
>itself to state that the first pixel has a value of 1.

For the record, my response is appended (Eric concurred).

>clause referring to random groups was dropped after Eric Greisen raised 
>an issue with it.  The clause was simply eliminated, rather than spend 
>more time debating how to reword it, for several reasons: it is 

The discussion finished at the point where the parenthetical comment was
changed from "(e.g. as is done for random groups data)" to "(e.g. as is
done for the random parameters using PTYPEn in random groups data)".
Noone demurred.  That was the result of the discussion and that's what
it should be changed to.

>an issue with it.  The clause was simply eliminated, rather than spend 
>more time debating how to reword it, for several reasons: it is 
>irrelevant to the William Thompson's concern about the starting point of 
>CRPIXj, 

That itself is irrelevant.

> it is not essential to the mean of the sentence it is embedded 
>in,

It provides the best example that I can think of (within FITS), but if
you can come up with an alternative then so much the better (as I have
already said).

> and because the reference to group parameters within random groups 
>is too obscure to be helpful to most readers.

They can find out about them if they wish, and if they don't then it
doesn't matter (as I have already said).

Regards, Mark

>>>

Date: Tue, 13 May 2008 16:34:12 +1000
From: Mark Calabretta <mcalabre at atnf.csiro.au>
To: Eric W Greisen <egreisen at nrao.edu>
Cc: William Pence <William.Pence at nasa.gov>
Subject: Re: [iaufwg] Call for vote on the new FITS Standard

On Mon 2008/05/12 13:00:16 -0400, William Pence wrote
in a message to: Eric W Greisen <egreisen at nrao.edu>,
      Mark Calabretta <mcalabre at atnf.csiro.au>

>Since you are the experts, could you 2 propose a specific change to the 
>wording of the draft FITS Standard to address this issue?

Dear Eric,

I propose that the following paragraph be inserted at the start of
Sect. 8.1:

  Rather than store world coordinates separately for each datum (e.g. as
  is done for random groups data), the regular lattice structure of a
  FITS image offers the possibility of defining rules for computing
  world coordinates at each point.  As stated in Sect. 3.3.2 and
  depicted in Fig. 3.1, image array data are addressed via {\em integral
  array indices} that range in value from 1 to NAXISj on axis j.
  Recognizing that image data values may have an extent, for example an
  angular separation, spectral channel width or time span, and thus that
  it may make sense to interpolate between them, these integral array
  indices may be generalized to floating-point {\em pixel coordinates}.
  While integral pixel coordinate values coincide with the corresponding
  array indices, pixel coordinates are defined at all points within the
  image lattice and outside it (except along conventional axes, see
  Sect. 8.5).  They form the basis of the world coordinate formalism in
  FITS depicted schematically in Fig. 8.1.

Several other changes would be desirable:

1) Remove the italics from "pixel coordinates" on p75 (they are used in
   the first mention above).

2) Change the definition of CRPIXj on p77 to

   CRPIXj - [floating-point, indexed, default: 0.0]
       Pixel coordinate element of the reference point for axis j,
       corresponding to $r_j$ in Eq. (8.1).  Note that the reference
       point may lie outside the image.

   If Bill Thompson is still unhappy with this he might be satisfied
   if the following is added to the last sentence:

       ...and that the first pixel in the image has pixel coordinates
       $(1.0, 1.0, \ldots)$.

   although this is a statement about pixel coordinates in general, not
   just CRPIXj.

How does that sound to you?

Regards, Mark