[fitsbits] WCS papers new draft released - Papers I, II, III

[Eric Greisen]

New versions of the three World Coordinate System papers are now
available from

   http://www.aoc.nrao.edu/~egreisen/

and

   http://www.atnf.csiro.au/~mcalabre/

The new versions are colour-coded to highlight changes made since the
last public release.

   Paper I: Changes made since 10/1/01 highlighted, green: oldest
            changes, blue: since 10/19/01, red: December

   Paper II: last released on 2000/11/07, all changes since that date
             are highlighted in red.

   Paper III: changes made since 10/1/01 are highlighted: green
              oldest, blue since 10/19/01 red: December.

We trust that the colour-coding will readily allow interested parties
to discern all non-trivial changes.

The most important changes stemmed from the decision to extract all
material relating to "distortions" into a separate Paper IV
provisionally titled "Representations of Distortions in FITS World
Coordinate Systems" which is currently under development.  The
material removed includes the "pixel regularization correction" from
Paper I, the polynomial distortion function attached to the TAN
projection and the DSS header construction example from Paper II, and
the polynomial spectral distortion correction from Paper III.

Work on Paper IV also convinced us to revert to the PCj_i + CDELTj
construct.  This was replaced by CDj_i late in 1998 to obtain quick
agreement for the WCS proposal among various dissenting parties.  That
quick agreement did not eventuate.  Therefore we have felt free to
revert to what we believe is a much better formalism, as explained in
Paper I.  However, the current proposal requires that the CDj_i be
fully supported in translation.  Please read the discussion in Paper I
before responding to this change.

A summary of the main changes are:

   Paper I:
	* CDj_i -> PCj_i + CDELTj.
        * Introduction of WCSAXES.
        * Specification of default values for CRPIXi, etc.
        * Formalization of BINTABLE and PIXLIST keywords.
        * Units in comment fields.
        * Removal of pixel regularization correction.
        * Specification of both PCj_i and CDj_i matrices.

   Paper II:
	* Detailed explanation and formal generalization of the meaning
	  of (phi_0,theta_0).
	* Introduction of user-specified (phi_0,theta_0).
	* Generalization of AZP to tilted planes of projection.
	* Introduction of SZP, slant zenithal projection.
        * Removal of polynomial distortion function from TAN.
	* Removal of the DSS header construction example.
	* Mathematical discussion of the matrix which results from
	  translating AIPS convention headers.
	* New header interpretation example 3.
	* New header construction example 1.
	* Derivation of the slant orthographic projection in aperture
	  synthesis radioastronomy.

   Paper III (now co-authored by Frank Valdes):
        * Removal of polynomial spectral distortion correction.
	* New section on gratings and prisms.
	* New header interpretation example.
	* Discussion of real-world complications arising from Doppler
	  correction.
	* Reversion to PC+CDELT.

A new version of WCSLIB which supports the generalized AZP and SZP
projections is available from

   http://www.atnf.csiro.au/computing/software/wcslib.html

This also contains an improved version of PGSBOX. 

Mark Calabretta, ATNF   (Mark.Calabretta at atnf.csiro.au)
Eric Greisen, NRAO      (egreisen at nrao.edu)
2001/12/12