3.3.2 Reserved Keywords for Extension Headers

  These keywords are not required but, if used, must have the meaning and syntax defined here. They may appear anywhere between the required keywords and the END keyword. These keywords are reserved for all extension headers and need not be specifically reserved for an individual new extension.

• EXTNAME (character) can be used to give a name to the extension to distinguish it from other extensions of the same type. For example, if there are several tables on a tape, they could be distinguished by their values of EXTNAME. The name may have a hierarchical structure giving its relation to other files (e.g., ``map1.cleancomp''). Remember, however, that the reader should be able to retrieve the data properly using only the first eight characters of the string. Conventions for hierarchical structures of extension names have not been established.

  The distinction between the name and the type is that the type, the value of XTENSION, describes the structure of the extension, while the value of EXTNAME provides a name for the extension but says nothing about its structure. For example, the header for an ASCII table containing information about Seyfert galaxies could have XTENSION = 'TABLE ' and EXTNAME = 'SEYFERTS'.

• EXTVER (integer) is a version number which can be used with EXTNAME to identify an extension. The default value is 1.
• EXTLEVEL (integer) specifies the level of the extension in a hierarchical structure. The default value for EXTLEVEL should be 1.

Because the term ``type'' refers to the structure of an extension as given by the value of XTENSION, user definition of a EXTTYPE keyword is not recommended, though not strictly forbidden by the FITS rules.

As a sample application of hierarchical structure, consider an observing session in which several objects are observed. In addition to the actual observations, other information, say, about the observing system configuration or physical state, might be present. Some information might apply to the entire observing session, while other information would apply only to the observations of an individual object. In this case, an extension at EXTLEVEL = 1 could contain the information applying to the entire observing session as part of the header, while some information about the observations of individual objects could appear in the associated data in a master table. Each row in the master table would contain information about the observations of one object. One entry in each row would be the name of a table containing observations of that object. The observations would appear in separate table extensions with EXTLEVEL = 2 and EXTNAME equal to the name given in the master table. This concept has yet to be implemented in data for archive or distribution, and any software to support it is still in the developmental stage.

The grouping proposal, discussed in section 5.3, describes an alternative method of creating hierarchies.