Keywords in the FITS headers are stored in MIDAS files as descriptors. The naming conventions are not identical. It is therefore necessary to perform a mapping between them. In general, the MIDAS descriptors will get the same name as the FITS keyword except that illegal characters for descriptors ( e.g. ' -') will be substituted by underscore ('_'). Keywords which describe the data structures are handled in a special way. Special name mapping is given in the following sections.
It is simple to map FITS keywords into MIDAS descriptors because the latter can have longer names and store arrays of values. The other way the translation is non-unique. Thus, general MIDAS descriptors are encoded in a special section of the header as HISTORY cards. The format used preserves all the descriptor information but is only known to MIDAS and therefore cannot be decoded by other readers.