keyword units

[Peter Teuben]

William Pence wrote:
> 
>     The following convention for encoding the physical units of FITS
> keyword values was recently developed during discussions about the data
> products for some future missions.  We would appreciate any comments
> about this proposal or on any other conventions that may already exist
> for encoding keyword units.
> 
>      Two routines for reading or writing keyword units using this
> convention have already been implemented in the latest version of the
> CFITSIO library (V1.25) on a trial basis.  Assuming that the response
> is favorable, a corresponding pair of subroutines will be added in the
> next release of the Fortran FITSIO library.
> 
> -----------------------------------------------------------------------
> 
>              Proposed Convention for FITS Keyword Units
> 
>                       William Pence (HEASARC)
>                       Jonathan McDowell (ASC)
>                       Clive Page  (Leicester)
> 
>                           14 July 1997
> 
> One deficiency of the current FITS Standard is that it does not define
> a specific convention for recording the physical units of a keyword
> value.  The TUNITn keyword can be used to specify the physical units of
> the values in a table column, but there is no comparable convention for
> keyword values.  The comment field of the keyword is often used for
> this purpose, but the units are usually not specified in a well defined
> format that FITS readers can easily recognize and extract.
> 
> To solve this deficiency, we propose a convention in which the keyword
> units are enclosed in square brackets as the first token in the keyword
> comment field; more specifically, the opening square bracket should
> immediately follow the slash '/' comment field delimiter and a single
> space character.  The following examples illustrate keywords that use
> this convention:
> 
> 1234567890123456789012345678901234567890123456789012345678901234567890123456789
> EXPOSURE=               1800.0 / [s] elapsed exposure time
> V_HELIO =                16.23 / [km s**(-1)] heliocentric velocity
> LAMBDA  =                5400. / [angstrom] central wavelength
> FLUX    = 4.9033487787637465E-30 / [J/cm**2/s] average flux
> 
> In general, the units named in the IAU(1988) Style Guide are
> recommended, with the main exception that the preferred unit for angle
> is 'deg' for degrees.
> 
> This convention is optional and does not preclude other conventions.
> 

Novel idea! 

Comments/wonderings:

 1) Would it not be useful to add a keyword to "version" this 
	convention. In case it ever gets upgraded/modified? 
	Should it also not be a comment? But it could also 
	be a real keyword, in which case CFITSIO and others
	can more easily make (ab)use of it.

 2) What kind of convention is used for the convention of how
    to "express" the formulae of the units? It sounds like you
    are using a fortran convention (** for power), but with
    a blank for multiplication. Can the notation be rigourously
    written down or is there room for confusion?

	[sorry, I still don't have my copy of the IAU style guide,
	and the answer may be in that. Isn't there a web page on
	this by now?]

 3) what is the convention when something is e.g. log(flux),
    is it 
	[log(J/cm**2/s)]
    where we all understand this is 10-based log, and not something
    else?


- peter