[fitswcs] Units along spectral axes

[William Thompson]

Folks:

I've been working on incorporating Paper III in my IDL software, and came to
realize that it's vitally important that the program be able to parse the units
specification in order to implement the non-linear projections.  For example, if 
the axis keywords include

	CTYPEka  = 'WAVE-V2W'
	CUNITka  = 'Angstrom'
	RESTWAVa = 3.04E-8	/[m] Rest wavelength

the program has to know how to convert Angstroms into meters in order to compare
it with the rest wavelength.  I've written a units parser routine based on Paper 
I, but it could be easily confused by deviations (e.g. 'Ang' or 'angstroms' 
instead of 'Angstrom').

The current version of the paper includes the statement

> The IAU-standard prefixes for scaling the units are described in Paper I and
> should be used with all coordinate types, except that the dimensionless ones
> are not scaled.

I suggest that this be strengthened to point out the need for strict adherence 
to the system outlined in Paper I.

I also suggest that the caption for Table 6 be changed to

	Table 6.  Grism parameters, their default values, and required units.
	                                                      ^^^^^^^^

to emphasize that the units of the parameters are fixed, and do not vary with 
the spectral units.

-- 
William Thompson
NASA Goddard Space Flight Center
Code 612.1
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson.1 at gsfc.nasa.gov