Subject: wcs.ps

From: Francois Ochsenbein <francois at simbad.u-strasbg.fr>
Date: Wed, 31 Mar 1999 17:37:29 +0200

I read with interest the "Representation of world coordinates
in FITS" proposal -- this convention widely used will benefit
the publication! But I was suprised to find there the
Appendix B with the OGIP conventions on units in FITS that,
as far as I know, were never approved (was WCS approved ?)

There are the following points of disagreements:

1. Compared to the IAU Style Manual:
   In table3: 
   - the radian (symbol rad) is missing
   - the Ohm sould be written with a capitalized O,
         to be consistent with its origin (physicist's name)
         and uppercase greek symbol.
   In table 4: 
   - the symbol of the year is the single-letter a (not yr)
   - the sun-based units (solar mass at least) is missing
   - the letter 'b' alone is the symbol of the barn (cross-section)
   - for the Angstroem: should be capitalized for the same reasons
     as Ohm above -- and should or not an 'e' appended after the
     'o' to reflect the original accentuation of the o ?

2. The usage of the ** for exponentiation is not really a good
   idea -- it only makes sense for fortran programmers.
   Why not a single-letter symbol like the carret (^) most
   likely familiar to many more persons ? 

3. The usage of the blank is not a good idea either, even though
   it looks more like the printed text, it complicates its
   acquisition from simple-minded software (e.g. Perl) without
   better readibility  -- "count /s" doesn't look better than
   "count/s" 

4. Some usual abbreviations like "ct" for count, or "ph" for
   photon, could be accepted .
   And some "miscellaneous" units could be useful as well like
   "beam" or "bit"

5. Finally, the usage of the trigonometric functions applied to
   units raises the question of the units of their argument and
   therefore the inconsistency with the inverse function, the degree
   being adopted instead of the radian. My personnal suggestion
   would be to drop completely the trigonometric functions applied
   to non-dimensionless units -- I never found such expression in
   the literature, and this would be bad physics anyway !

Are these conventions about units in FITS definitive ?
The documentation of astronomical catalogues available from
the data centres (ADC, CDS, ...) describe over 60,000 columns;
roughly 50% of these columns are unitless, and therefore about
30,000 columns have attached units carefully standardized
to allow automatic conversions. (description at 
http://vizier.u-strasbg.fr/doc/catstd-3.2.htx ) 
I would hesitate to change these anyway... but would appreciate
any comment / argumentation !

======================================================
Francois Ochsenbein
         Centre de Donnees astronomiques de Strasbourg
Internet: francois at astro.u-strasbg.fr
   Phone: +33 3 88 150 755
     Fax: +33 3 88 150 740
    http://cdsweb.u-strasbg.fr/CDS.html
    Post: Observatoire Astronomique
          11, rue de l'Universite
          67000 STRASBOURG, France 
======================================================