To: [fitsbits] 
Subject: Start of the TPV and ZPX FITS Convention Comment Periods
From: David Berry d.berry at jach.hawaii.edu
Date: Fri Oct 7 03:23:13 EDT 2011

Two points:

1) I think the description of the convention should also say that If
FITS reading software encounters a TPV header, then the standard
meanings attached to PV1_0 to PV1_4 (where i is the index of the
longitude axis) are suspended in favour of the meanings attached to
these keywords by the TPV convention. In the published standard these
PV values record information about the native coords of the fiducial
point and celestial pole (see section 2.6 of FITS-WCS paper II), but
in the TPV convention, they record polynomial distortion coefficients.
So in the TPV convention, the native coords of the celestial pole need
to be obtained from the LONPOLE and LATPOLE keywords, and the native
coords of the fiducial point are assumed to take their default values
for a TAN projection.

2) Version 5.7-3 of the starlink AST library also supports the TPV convention

David Berry

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To: [fitsbits] 
Subject: Start of the TPV and ZPX FITS Convention Comment Periods
From: Lucio Chiappetti lucio at lambrate.inaf.it
Date: Mon Oct 17 13:04:42 EDT 2011


On Thu, 6 Oct 2011, William Pence wrote:

> This is to announce the start of the Public Comment Period on the TPV 
> and ZPX FITS world coordinate system conventions developed at NOAO.

Assuming as usual that the conventions are already in use, there is no 
sense in comments which require a change in the definition or 
implementation of the convention, but just on the completeness of the 
documentation.

> The TPV and ZPX conventions build on the standard TAN and ZPN WCS
> projections, respectively, with the addition of a general polynomial
> distortion correction.

The two conventions seem to use two different formalisms to tackle a 
similar problem.

(1) the TPV uses a set of PV keywords for the coefficients, which is a
     practical and clear solution. Although it is perhaps a pity not to
     have another kwd indicating the order of the polynomial (i.e. the
     number of coeffs to expect), the documentation is clear enough in
     defining the rules for missing PV kwd defaults.

(2) However the HTML documentation for the TPV is not clear in the
     fact it does not state that CTYPE1/2 should be suffixed with
     --TPV, which one can infer from the sample header listing.

     This key issue to tag the convention shall be EXPLICITLY STATED
     in the HTML documentation

(3) in fact the HTML documentation for the ZPX does state that !
     So in this respect it is better.

(4) the fact ZPX uses not a clear set of coefficients but codes
     them into the long string given by the concatenation of the
     WAT keywords (the rules for the decomposition are clearly
     described in the documentation) is at variance with the simpler
     mechanism of the PV keywords for TPV. A similar mechanism would
     have been more elegant, but it is too late now to complain.

-- 
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