[iaufwg] CRPIX clarification

[William Pence]

Below, is the latest version of the CRPIXj clarification that 
incorporates Bill Thompson's revision to the sect. 8.1 paragraph and has 
"conventional" in italics.  This does not include Steve Allen's 
suggestion of using "initial pixel" instead of "first pixel" in the 
definition of CRPIXj;  I'll do so if others think the current wording 
could be a source of confusion by programmers who might interpret  the 
"first pixel" to be the pixel following the "zeroth pixel" and hence is 
really the second pixel in the array.

On a procedural note, here is how I propose we complete this process of 
producing the final standard document:

1.  IAU FWG members have 1 more week to post suggestions here on the 
iaufwg email list on how to clarify the use of CRPIXj.

2.  On Tuesday, May 27th, I will post an announcement on the FITSBIT 
email list to say that the IAU-FWG has approved the new FITS standard 
document along with a proposed clarification on the usage of CRPIXj.

3.  We then allow 3 weeks for any further comments or suggestions from 
the FITSBITS community (which includes the regional FITS committees) on 
the proposed clarification of CRPIXj.

4.  Assuming that the discussion of the CRPIXj clarification on FITSBITS 
is relatively non-controversial, we will incorporate the resulting final 
version of the proposed clarification of CRPIXj into the official FITS 
standard.  The IAU-FWG will only conduct another formal vote on this if 
a) there is some controversy that must be resolved, or b) if any IAU FWG 
member specifically requests a format vote.

Let me know if you have any comments about the process outlined above.

Here then, is the latest version of the proposed change:

1) Insert the following paragraph 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}.
   Integral pixel coordinate values coincide with the corresponding array
   indices, while fractional pixel coordinate values lie between array
   indices and thus imply interpolation.  Pixel coordinate values are
   defined at all points within the image lattice and outside it (except
   along {\em conventional} axes, see Sect. 8.5). They form the basis of
   the world coordinate formalism in FITS depicted schematically in Fig.
   8.1.

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

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


-- 
____________________________________________________________________
Dr. William Pence                       William.Pence at nasa.gov
NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
Greenbelt MD 20771                      +1-301-286-1684 (fax)