Subject: WCS proposal draft updated

From: egreisen at primate.CV.NRAO.EDU (Eric Greisen)
Date: Tue, 16 Aug 94 12:44:13 EDT

I have placed new versions of the draft paper "Representations of
Celestial Coordinates in FITS" by Eric W. Greisen and Mark Calabretta
in the anonymous ftp area of fits.cv.nrao.edu called
fits/documents.wcs where all documents on this topic may be found.
The older June 1993 and August 1993 versions are also there in the
.tex and .ps.all forms with appropriate names.  The latest form is
there as wcs.tex and as wcs.ps.all.Z, wcs.ps.most.Z, wcs.ps.some.Z,
and wcs.ps.none.Z depending on how many of the figures you want to
print.

The main difference between the June and August 1993 versions was
the re-introduction of the CDELTn keywords.  This means the the
CDiiijjj matrix is a pixel-to-pixel conversion matrix with no units.
Philosophically, it just feels better to avoid mixing extremely
different units (e.g. frequencies with CDELT's of order 10**6 or more
and angles with CDELT's of 10**(-3) or less).  There is also some
accuracy questions that would arise in dealing with such a matrix.
The other reason to pull apart the originally proposed CDi_j matrix
was to allow for more than one description of an axis without having
to have a second whole CDi_j matrix (or n**2 such matrices in bad
cases).

The changes to the current version include

(1) The SIN projection was given two parameters to allow for slant
orthographic projections such as those produced by interferometers
whose baselines all lie in a plane (east-west interferometers, VLA
snapshots, etc.).

(2) The NCP projection's conversion to SIN was corrected and given an
alternate conversion using the slant parameters.

(3) The CUNITn keyword was added to allow users to document whatever
units they use.

(4) A set of keywords to give 2nd, 3rd, ... 9th forms of description
for an axis were added.

(5) A short usage note for random-groups interferometer data was added
as Appendix C.3.

(6) The discussion of units was expanded and strengthened.

(7) The discussion of which keywords apply to the array axis and which
to the coordinate axis was reworded and promoted from a footnote to
the main text.

(8) An error in Table 1 (faces for Quadrilateralized sphere
representations) was corrected (face numbers 3 and 4 were switched).

(9) Minor spelling errors and typos were corrected.

I hope that you will fetch, print and read this document.  I intend to
present a poster on it at the ADASS meeting in Baltimore (Sept 25-28)
and it will be discussed at a "Birds of a Feather" session as well.

Eric W. Greisen
National Radio Astronomy Observatory