Subject: FITS tables of astronomical catalogues and Units

From: francois at SIMBAD.u-strasbg.fr (Francois Ochsenbein)
Date: Mon, 19 Sep 94 10:23:55 EDT

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Message-Id: <9409161659.AA26838 at SIMBAD.u-strasbg.fr>
From: francois at SIMBAD.u-strasbg.fr (Francois Ochsenbein)
To: fitsbits-request at fits.CV.NRAO.EDU
Subject: FITS tables of astronomical catalogues and Units
Date: Fri, 16 Sep 94 18:59:30 +0200

At CDS (Centre de Donnees astronomiques de Strasbourg) we have a large
quantity (> 900) of astronomical catalogues, most of these being made
of one or more tables.
Descriptions of all these tables are being homogeneized according to
standards (document referenced below) also adopted by the ADC . One of 
the important features of this standard is the facility of converting
any of these documented tables into FITS (ascii table extension), and 
presently over 500 catalogues have such descriptions. 

One of the key issues in the interpretaion of tabular data is the 
understanding of the units in which the data are expressed.
The rules adopted in this standard for specifying the units are more 
rigid than the OGIP definitions, and since hundreds of tables using 
these conventions already exist and are in use, we feel it could be 
worthwhile to let the FITS community aware of these conventions for discussion.

The complete definitions concerning units are detailed in the document; 
they can be resumed by the 4 rules:
- - the symbol of any unit is a single word (no embedded space)
- - a numeric factor can only precede the symbols, e.g.  0.01arcsec/yr
						   not arcsec/(100yr)
- - operators are only the dot (.) for the multiplication, 
  the slash (/) for the division, and integer powers (+ or -).
  with classical precedence rules.
  Examples:   km/s  or km.s-1           W/m2/Hz   or  W.m-2.Hz-1
- - the list includes about 50 basic unit symbols: SI, IAU extensions, atomic
  physics extensions (sorry, CGS units are excluded!)

Despite of the large number of tables examined, we never found any necessity
to introduce non-integer powers or functions of units like e.g.  log(K) 
to express "logarithm of Kelvin" --- and in fact log(K) is rather a scale, 
not a physical unit!

Besides the question of units, the document also addresses other conventions,
like a simple way of designating related columns (typically a value and 
its related error)

Any comment, suggestion, etc would be appreciated !
Francois Ochsenbein / CDS Strasbourg, France
================================================================================
Document on the "Astronomical Catalogues at CDS: Adopted Standards"
is available on URL ftp:://cdsarc.u-strasbg.fr/pub/cats/doc.ps
                    (TeX and dvi versions also available)
Hundreds of asatronomical catalogues at URL
		    http:://cdsweb.u-strasbg.fr/Cats.html
================================================================================

        _/_/_/_/_/_/ . . .             Centre de Donnees astronomiques de
      _/                               Strasbourg
     _/                                http://cdsweb.u-strasbg.fr/CDS.html
    _/        _/_/  _/_/_/             
   _/      _/      _/    _/            Francois Ochsenbein
    _/_/_/_/      _/    _/_/_/_/       Internet: francois at simbad.u-strasbg.fr
         _/      _/    _/      _/         Phone: +33 88 35 84 11
          _/_/  _/_/_/        _/            Fax: +33 88 25 01 60
                             _/            Post: Observatoire Astronomique
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