[fitswcs] TAN+poly & astrometric discussions in WCS paper-2

[Don Wells]

Dear FITS WCS ad hoc task force,

The mailing list 'fitswcs at nrao.edu' is still alive, even though we
have not used it for about a year. During that time Greisen and
Calabretta have made the changes that we and the ADASS'98 BOF session
specified, and at the ADASS'99 BOF session the three draft documents
were approved for submission to the FITS regional committees, with the
understanding that some details of papers 2 and 3 were unfinished at
that time.

In particular, several individuals have been corresponding with
Calabretta since ADASS'99 regarding two issues in paper-2: 

 (1) demonstration that the proposed TAN+polynomial function will
 adequately represent the geometry of certain imaging systems which
 have the 'dual point-of-symmetry' problem, and

 (2) addition of explanatory text regarding the relation of
 TAN+polynomial to existing astrometric practice.

Mark has made substantial progress in both of these areas. I believe
that paper-2 is nearly ready to be submitted to the regional
committees, but first I would like for the experts on the 'fitswcs'
list to review and comment on the current draft. The latest
(unadvertised) draft version is at

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

The relevant text, which addresses questions which astrometrists are
likely to ask plus the definition of the terms of TAN+poly, is in
Section 4.1.2 ("TAN: distorted gnomonic") of the above URL.  Regarding
issue (1) , Mark has a Web page on his numerical experiments:

    Notes on the TAN projection 
    http://www.atnf.csiro.au/~mcalabre/TAN/index.html

Is there any reason to think that WFPC2 and the SAO camera will be
harder to represent than DEIMOS? Is there any reason to think that the
BENDXY model, which was developed for the AAO Schmidt, will not also
be satisfactory for POSS2 and ESO and other Schmidt systems? What
other distorted cameras need to be checked against TAN+poly before we
approve the notation?  I would like to see comments from the various
experts on 'fitswcs' on these and related questions before I submit
paper-2 to the committees for formal approval.

It will be appropriate for 'fitswcs' people to solicit comments from
astrometrists they know in order to assure that Mark addresses their
concerns in the text of paper-2.

I invite any who are reluctant to comment openly on the 'fitswcs'
mailing list to send comments to me privately in order to guide me in
making decisions about the readiness of the WCS documents for
submission. 

Regards,
Don Wells [Chair, IAU-FWG Working Group]
-- 
  Donald C. Wells      Scientist - GBT Project        dwells at nrao.edu
                    http://www.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA