[fitsbits] FIT Keyword data dictionaries

[Patrick Wallace]

On Mon, 6 Nov 2000, Mark Calabretta wrote:

> Could we stick to right-handed spherical coordinates everywhere in FITS
> please?  Celestial coordinate systems in WCS paper 2 (q.v.) are explicitly
> right-handed.

I wish this was feasible, but where there's an established convention you
can end up in trouble.  East +ve longitudes are right-handed, and match
the convention that most people use these days, so we're OK there.  And
the equatorial, galactic, ecliptic systems are always right-handed.  But
we're stuck with a left-handed convention for hour angle (west +ve),
though perhaps that's of limited relevance for FITS.

The real bugbear is azimuth, where a left-handed convention (north thru
east) is firmly established.

The pointing-analysis and telescope control software that I do uses south
thru east azimuths internally to ensure that everything is right-handed,
while using the normal left-handed convention for all user-interface
purposes.  This is a fruitful source of confusion and I have lingering
doubts about the wisdom of the policy I've adopted.

Patrick Wallace
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Starlink/HMNAO                                  Internet:  ptw at star.rl.ac.uk
Rutherford Appleton Laboratory                       Tel:    +44-1235-445372
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