Subject: World Coordinates

From: thompson at stars.gsfc.nasa.gov (William Thompson, code 682.1, x2040)
Date: Mon, 9 Nov 1992 22:29:00 GMT

After having read the world coordinates proposal, I am still unclear as to how
the developers would describe this very simple problem:

	Given: A slit spectrograph with a single pointing position.  The
	dimensions of the data are two: one spatial coordinate and one
	spectral coordinate.  However, to describe the pointing, two
	spatial coordinates are needed, as well as the orientation of
	the slit, and the spectral dispersion and central wavelength.
	In other words, the data has only two dimensions, but it takes
	three dimensions to properly describe it.

Secondly, it appears that this convention is only relevant to skymapped
data--which I admit applies to most astronomical data.  However, it's hard to
see the relevance to solar images, for example.

Bill Thompson

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From: hanisch at stsci.edu (Bob Hanisch, ST ScI)
Date: Tue, 10 Nov 1992 14:47:36 GMT

>see the relevance to solar images, for example.

The title of the draft probably should have been more restrictive, i.e.,
"Representation of Sky Projection Coordinate Systems Within FITS" or something
like that.  At the WCS meeting in Charlottesville in 1988 we decided to tackle
this problem first, and that was again agreed upon at the Boston meeting last
week.  The example cited above is actually not so simple, since things are
rarely this straightforward.  The dispersion axis is usually not perpendicular
to the spatial axis, and indeed is usually curved.  The dispersion relation 
itself is frequently nonlinear.  The simple case described above CAN be
handled within FITS, and is supported within a number of systems (e.g., radio 
data cubes are really the same thing -- the dispersion axis is usually the 
third axis, but this need not be the case).

At the Boston meeting there were volunteers to work on adding solar and
planetary coordinate systems to the draft.  If you are interested in
participating in this effort, please contact Winnifred Williams at NOAO
(wwilliams at noao.edu), who agreed to coordinate this work.
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