[fitswcs] Exhortation

[Patrick Wallace]

On Thu, 16 Jul 1998, Don Wells wrote:

> Isn't it true that you are the responsible person for the IAU
> commission effort to create and distribute the official algorithms for
> these calculations?

Yes - through the SOFA initiative.

> Will these algorithms include officially-blessed
> transformations from the older reference frames to ICRS?

Yes - once we get our act together.  The best available FK5->ICRS information
currently available is in the preamble to the Hipparcos catalogue, and there
was a paper at the IAU General Assembly last year as well (but beware -
two columns in the vital table were swapped).  The rotation is of order
30 mas at present, and isn't going to explain the 500 mas mismatch of
the radio/optical contours.

> Should the
> section in G&C include some discussion of how to use these algorithms
> to implement the transformations in the FITS context?

Yes.  But bear in mind that the relevance of these old frames has
diminished greatly since the introduction of ICRS.  Most existing
data are of intrinsically worse accuracy than the FK5->ICRS average
rotation, and any serious attempt to relate genuinely FK5-based data
would have to take into account the zonal errors in FK5 to be of any
practical, as opposed to canonical, use. 


Patrick Wallace
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Starlink Project Manager                        Internet:  ptw at star.rl.ac.uk
Rutherford Appleton Laboratory                       Tel:    +44-1235-445372
Chilton, Didcot, Oxon OX11 0QX, UK                   Fax:    +44-1235-446667
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