One limitation 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 analogous 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 problem, CFITSIO uses a local 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 immediately follows the slash '/' comment field delimiter and a single space character. The following examples illustrate keywords that use this convention:
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.
The fits_read_key_unit and fits_write_key_unit routines in CFITSIO read and write, respectively, the keyword unit strings in an existing keyword.