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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.

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.


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Next: Creating New Extensions Up: Extensions Previous: Required Keywords for an