[fitswcs] Fwd: New WCS conventions posted to the Registry of FITS Conventions

[Frank Valdes]

Good point David.  I was aware that the some of the PV had evolved in the final standard to mean specific things but did not really think of it.  As you say this should be made explicit that this convention does not follow those rules so that a TPV WCS means to interpret the PV coefficients as described.  I will modify my version of the technical description and ask that it be updated on the copy hosted by the FITS Support Office.

Frank

On Oct 21, 2011, at 1:26 PM, David Berry wrote:

> The same two points I made after the announcement of TPV on fitsbits:
> 
> 1) I think the description of the convention should also explain 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.
> 
> This is probably implicit in the current description of TPV, but
> making it explicit may avoid confusion.
> 
> 2) In terms of support for TPV, version 5.7-3 of the starlink AST
> library supports the TPV convention (AST is the WCS library used by
> DS9).
> 
> David
> 
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