FITS keywords

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 and data type as the FITS keyword. Illegal characters in FITS keywords 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 a direct translation is not always possible. If the MIDAS descriptor is compatible with a prime FITS keyword (i.e. its name is 8 or less characters and its value consists of only one number or string), it will be written as a prime FITS keyword. Care must be taken to avoid that MIDAS descriptors with names overlapping standard FITS keywords names are used. This may lead to unpredictable results when such frames are saved in FITS format and restored. Descriptors which contain a full stop ('.') or start with underscore ('_') are assumed to be hierarchical and will be store as HIERARCH cards (see Section 7.1.3). Also in this case, the descriptor must be single values.

All other 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 may not be decoded by other readers.