Hierarchical keywords

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 exist 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). Such keywords are mapped to MIDAS descriptors with names starting with the domain, then all levels and ending with the keyword where the components are separated with the full stop character ('.'). The hierarchical keyword:

HIERARCH ESO TEL FOCU SCALE = 1.489 / Focus length (deg/m) = 5.36"/mm

will yield the MIDAS descriptor name ESO.TEL.FOCU.SCALE while

HIERARCH ESO INS OPTI-3 ID = 'ESO#427 ' / Optical element identifier

will become the descriptor ESO.INS.OPTI-3.ID. When writing such descriptors out again, the reverse translation is applied.

In MIDAS releases prior to 96NOV, a special encoding scheme was used to translate hierarchical keywords name to MIDAS descriptor name shorter than 16 characters. This is not needed anymore since long descriptor name are supported from the 96NOV release. For backwards compatibility, a C-flag is avaliable in the INTAPE/FITS command providing the pre-96NOV name mapping.

The hierarchical structure provides a convenient and clear way to separate information concerning different subsystems. The definition of FITS keywords used by ESO for data acquisition can be found in the Data Interface Control Document [7]. This document also gives a full definition of all hierarchical keywords in the ESO-domain. FITS readers which do not know this convention should save the HIERARCH-keywords as comments following standard FITS rules.