2.3. Keyword value units
The original FITS paper (Wells et al. ) assumed that the units along each axis could be implied simply by the contents of the CTYPEi keyword. This has not turned out to be true in general. Therefore, we propose adding a new indexed, keyword
with which the units of CRVALi and CDELT i may be specified. Restrictions on the nature and range of units, if any, will be determined by the agreements applying to the specific axes. If they are not so limited, units should conform with the recommendations of the IAU Style Manual (McNally ). Particular conventions for CUNITi values are discussed in Sect. 4. Case will be significant in the values assigned to CUNITi since, for example, it is necessary to represent both milli and Mega prefixes for units such as mJy and MJy. The values assigned to CUNITi cannot exceed 68 characters, but may well be longer than the 8 characters which has been a traditional - but optional - limit for character-valued but non-mandatory keywords.