🚀 go-pugleaf

In article 9NOV199218295135@stars.gsfc.nasa.gov, thompson@stars.gsfc.nasa.gov (William Thompson, code 682.1, x2040) writes:
>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

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@noao.edu), who agreed to coordinate this work.
---
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