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