[fitswcs] Units along spectral axes

[Pedro Osuna]

Dear William and all,

the problem mentioned by William is what led my colleague Jesus Salgado
and myself to formulate a possible solution using simple dimensional
analysis to handle units (in the context of the IVOA).

We basically propose that any unit should carry its dimensions and
scaling factor (based on SI, for instance) so that, for example, the
unit Jansky would appear like:


name=Jansky
symbol=Jy
scale=1E-26
dimeq=MT**-2


parsing is not needed anymore as the unit is not defined by the name but
by the dimensions MT**-2 1E-26. 


This solves the problem mentioned by William, which is quite common.

We would have liked to propose the fits consortium to add two parameters
for the description of the units:

CDIMEQ
CSCALEQ

in this case, William's example would appear like:

CTYPEka  = 'WAVE-V2W'
CUNITka  = 'Angstrom'
CDIMEQ    = 'L'
CSCALEQ  = '1E-8' 
RESTWAVa = 3.04E-8


besides solving many problems with the parsing (normally problematic due
to the different standards of same things) it allows for clients to
handle more complex problems. 

This proposal deals with the definition of the units themselves
(carrying their own dimensions) and not with how to convert between
different physical quantities (which happens to be just one of its
applications).

Based on a commend from Dr. Greisen on this very same list to our idea,
we wrote a small paper on this issue with an example that shows how to
make use of this description of units to handle more complex problems
(in this case, spectra in VO context).

You can access the draft at:

http://esavo.esa.int/vodocs/DimensionalAnalysis_VOSpectra.pdf

Cheers,
Pedro Osuna

ESA 
Science Archives System Engineer
Science Archive Team
European Space Astronomy Centre
(ESAC/ESA)
e-mail: Pedro.Osuna at esa.int
Tel + 34 91 8131314
---------------------------------                                                                                
European Space Astronomy Centre
European Space Agency
P.O. Box 50727
E-28080 Villafranca del Castillo
MADRID - SPAIN
On Thu, 2005-06-09 at 17:50, William Thompson wrote:
> 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.
-- 
Pedro Osuna Alcalaya

ESA 
Science Archives System Engineer
Science Archive Team
European Space Astronomy Centre
(ESAC/ESA)
e-mail: Pedro.Osuna at esa.int
Tel + 34 91 8131314
---------------------------------                                                                                
European Space Astronomy Centre
European Space Agency
P.O. Box 50727
E-28080 Villafranca del Castillo
MADRID - SPAIN