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<H2><A NAME="SECTION001113000000000000000">
FITS keywords</A>
</H2>
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.
<P>
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>i.e.</I> 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 <TT>HIERARCH</TT> cards (see
Section <A HREF="node116.html#ch07:hierarchical-keywords">7.1.3</A>). Also in this case, the
descriptor must be single values.
<P>
All other MIDAS descriptors are encoded in a special section of the
header as <I>HISTORY</I> cards. The format used preserves all the
descriptor information but is only known to MIDAS and may not be
decoded by other readers.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-09</I>
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