Standards for Astronomical Catalogues, Version 2.0 (pdf)
(February 2000)

3.3  Labels

Remember that a label is always a single word: it cannot contain any embedded blank or space. Other characters are in principle accepted, e.g. parentheses or other punctuation signs, with the following restrictions:

• the characters  $ @ ! ` \ ^ ~ & are not accepted in a label
• the first character of a label can't be one of the symbols   + - * / = . # _ ' "
• the last character of a label can't be one of the symbols   , ;
• the various kinds of parentheses and brackets   ( ) [ ] { }, when used, must be balanced
• a label can't be made of digits only
• a single-character label must be alphabetic.
• the label — is used for filling bytes, like the colon fond sometimes between the hours and minutes, or a x existing between the major and minor axises of a galaxy.

Some commonly used labels are presented in label. This table is not an exhaustive list of all possible column labels; it merely represents the standards adopted for the most common columns headings — like celestial coordinates, times and dates, and some of the most frequently found parameters. The VizieR service contains all definitions of all colums; these definitions are stored in the METAcol (field meta-data) table which can be queried like any of the tables accessible in VizieR. For instance, finding the columns having oscillator strength in their explanation can be found by entering the constraint *oscillator strength* (the * matches any number of characters) in the explain field of the METAcol

Some comments on the labels defined in label:

• unicity principle: two columns can't have the same label in a table (the — label however can be present several times). When a table contains for instance the equatorial position in two different systems, B1950 and J2000, choose the labels RAh ...DEs for the basic (original) position, and choose another set of names, e.g. RA1950h ...DE1950s for the other set of positions.

• Julian dates should be complete – no truncated Julian date, no Modified Julian Date (MJD) which are the source of many confusions.

• The following suffixes can be appended to a label to designate the components of time or angle:

  .Y	Year part (in order to avoid any ambiguity, the year should include all 4 digits)
  .M	Month name or number in range [1⋅⋅⋅12]
  .D	Day number in month (2 digits, range [1⋅⋅⋅31]) or year (3 digits, range [1⋅⋅⋅366])
  .h	Hour (range [0⋅⋅⋅24[)
  .m	Minutes (range [0⋅⋅⋅60[))
  .s	Seconds (range [0⋅⋅⋅60[))
  .d	Degrees (range [0⋅⋅⋅360[)

  For instance, the various components of an observation date and time can be labelled Obs.Y, Obs.Y, Obs.M, Obs.D, Obs.h, Obs.m.

• remember that labels are normally written at the top of each column; very long names should be avoided, especially for short (a couple of bytes) columns
• ... but names should however not be too cryptic !
• Columns containing file names should contain file or File in their labels – e.g. FITSfile to indicate an associated image in FITS format

A parameter has frequently associated values, and we have adopted the rule of association with the one-letter-underscore prefix: if a column is obviously associated to another one — typically mean errors or uncertainty flags — we use one of the underscore prefixes listed in prefix.

Usual mathematical functions may be specified in the label, with parentheses or a dot; for instance, the logarithm of the effective temperature could be labelled log(Te) or log.Te.

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