Units

The units named in the IAU (1988) Style Guide are recommended for array and keyword values, with the exception of measurements of angles. This set of units includes the SI units and additional units that are standard in astronomy, for example, magnitudes. Using BUNIT and TUNIT$n$ keywords will remove any confusion; similarly, the comment field following CTYPE$n$ keywords can be used to specify the units. Fractional units should not be used. If, for example, for space reasons, temperatures must be listed in the file in units of tens of degrees, the units specified by BUNIT should be degrees, and scaling (section 3.1.2.1) should be used to transform from the file values to physical values. Always explicitly describe non-standard units. A copy of the IAU Style Guide recommendations is available on the IAU World Wide Web site, currently at http://www.lsw.uni-heidelberg.de/iau/units.html.

For celestial coordinate systems specified with the CTYPE$n$, CRVAL$n$, CDELT$n$, and CROTA$n$ keywords, or with the keywords used in the world coordinates conventions described in section 4, the use of decimal degrees for the angular measurements is required, and degrees are recommended as the units for other angular measurements (with, for example, the value of BUNIT = 'deg ').