[fitsbits] IAU FWG approval of the new FITS Standard

[William Pence]

Dear Colleagues,

I am pleased to announce that the IAU FITS Working Group has unanimously 
approved the draft 3.0 version of the FITS Standard as the new official 
version of this document (pending 1 further clarification, described 
below).  This new version of the document contains numerous updates and 
clarifications that reflect the evolution of the FITS format since the 
previous version of the Standard was approved in 1999.  One notable 
addition is a new section on world coordinate systems that summarizes 
the conventions defined in 3 separate WCS papers that were published in 
2002 and 2006.  The current draft of this new document is available on 
the FITS Support Office web site at 
http://fits.gsfc.nasa.gov/fits_draft.html.

The final version of the new FITS Standard document will be made 
available on the FITS Support Office web site as soon as one remaining 
issue that was raised during the final review of the document is 
resolved:  several of the IAU FITS Working Group members felt that the 
definition of the CRPIXj WCS keyword should be clarified to state that 
the numbering of the pixels along each axis in a FITS image runs from 1 
to N and not 0 to N-1.  This has apparently been a source of confusion 
to some FITS users, and could cause the coordinate system that is 
defined in some FITS images to have an erroneous 1 pixel offset.

A set of 3 small revisions to the wording of the WCS section of the 
Standard has been proposed to resolve this issue, as shown below. 
Please review this proposed change, and reply here with any further 
comments or suggestions you may have.


Regards,
Bill Pence, on behalf of the IAU FITS Working Group

=============================================================

The following changes to the wording of the new FITS Standard document 
have been proposed to clarify the use of the CRPIXj keyword.  The 
current version of this document is available at 
http://fits.gsfc.nasa.gov/fits_draft.html.

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]
     Location of the reference point in the image for axis j, in units
     of pixels, 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)