To avoid possible misinterpretations and naming conflicts for keywords describing acquisition parameters, ESO has adopted a hierarchical keyword convention for this purpose using the keyword HIERARCH. Such keywords have the following syntax:
HIERARCH domain level-1 ... level-n keyword = value / comment
where the domain always is ESO. Several hierarchical
levels may exsist with a keyword and associated value at the lowest
level. Naming and parameter follow the general FITS rules when
applicable ( e.g. max. 8 characters). The levels defined for the
ESO domain are given in Table 
where also an
abbreviation character is specified. FITS readers which do not know
this convension should save the HIERARCH-keywords as comments
following standard FITS rules. The standard specification of the FITS
Acquisition and Archive format used by ESO can be found in
[7]. This document also gives a full definition of all
hierarchical keywords in the ESO-domain.
Table: Levels defined for ESO hierarchical FITS keywords and the abbreviations used for the translations to MIDAS descriptor names
The hierarchical structure provides a convenient and clear way to separate information concerning different subsystems. The full concatenated name can however be much longer than the 15 character limit for MIDAS descriptors making it necessary to use a name translation scheme.
Descriptor names of hierarchical keywords start with an underscore
character ' _'. Domain and level names are abbreviated to single
characters where E is used for the ESO-domain. Other
abbreviations are listed in Table . Some levels may
also have an associated index which is maintained in the translation.
The domain and level part is separated with an underscore
character from the keyword which follows the rules for non-standard
keywords. The hierarchical keyword:
HIERARCH ESO TEL FOCU SCALE = 1.489 / Focus length (deg/m) = 5.36"/mm
will yield the MIDAS descriptor name _ETF_SCALE while
HIERARCH ESO INS OPTI-3 ID = 'ESO#427 ' / Optical element identifier
will become the descriptor _EIO3_ID. When writing such descriptors out again, the reverse translation is applied.