[iaufwg] CRPIXj clarification

[William Pence]

Dear IAU FITS Working Group Members,

It appears that the discussion on FITSBITS regarding the proposed change 
to the new draft FITS Standard to clarify the definition of the CRPIXj 
keyword has concluded, so the next step is for the IAUFWG to formally 
approve the change.  This is the final issue that needs to be resolved 
to complete the work on the new FITS Standard.

The latest draft of the proposed changes is shown below.  The wording is 
essentially what was proposed by Mark Calabretta last month, and 
incorporating a few other small suggestions from Bill Thompson, Dick 
Shaw, Steve Allen, and Rob Seaman.  I believe this proposed wording is a 
good compromise that I hope  everyone can accept without much further 
delay.  For the record, all the other members of the FITS technical 
panel have recently endorsed this proposed wording.

Before proceeding with the final IAUFWG vote on this matter, I should 
first call for any final discussion on this matter.  If you have any 
comments or suggestions regarding this proposal, please make them here 
within the next few days.

regards,
Bill

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Proposed change to the wording of the new draft of the FITS Standard:

1) Insert the following paragraph at the start of Sect. 8.1:

   Rather than store world coordinates separately for each datum,
   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]
     Location of the reference point in the image 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 $(1.0, 1.0, \ldots)$.