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2.3 Generalized Extensions

  The special records provision of the original FITS design made it possible to extend the format. There were two principal reasons for wanting to extend FITS beyond the simple header/array structure:

For example, the table extensions have allowed tables, lists, etc., associated with a data matrix to be written in the same FITS file as the data matrix, implicitly establishing the relationship among the different pieces of information.

The method chosen was to define ``extensions'', HDUs, which, like the primary HDU, are composed of a header consisting of card images in ASCII text with keyword=value syntax, followed by data. There could be many kinds of extensions, each with a different defined data format. Structuring extensions in this way made it easy to modify software that read the FITS header for the primary array to read extension headers as well. Information about the extension data would appear in the extension header in a way specified by the rules for that extension. All logical records would be 23040 bits (=2880 8-bit bytes), as the original paper describing FITS prescribed for information following the primary HDU. The HDU itself is called an extension. The design is called an extension type.

The requirement that no revision to FITS could cause an existing FITS file to go out of conformance dictated a number of the basic rules governing the construction of new extensions.

In the original FITS data sets, the Basic FITS structure of header and array appeared at the start of the file. Therefore, extensions would appear only after the primary Basic FITS header and array. Because the initial array ended only at the end of a 23040-bit record, an extension would always start a new record.

It was envisioned that most FITS extensions would become standard in the same way as Basic FITS, through acceptance by the astronomical community and endorsement by the IAU. An extension would go through a development period, initially being used only by a subset of the FITS community that would refine it. Other extensions might be in use only within a limited group and might never become standard at all. Now, a FITS file may include many extensions of different types. Given that the order of adoption of extensions as standard cannot be predicted with certainty, it would not have been wise to prescribe an order of extension types within a file. Suppose standard extensions were required to appear first, and the fourth extension on a data set were to become standard. Then, the data set would go out of conformance. It was thus agreed that extensions might appear in any order in a FITS file.

With extensions appearing in any order, those extensions a user might want to or be able to read could be separated by extensions with which the user would be unfamiliar; the user might wish to read, say, only the third and seventh and skip all the rest. The user should not have to know anything about the structure of the intervening extensions to be able to read the ones of interest. To make this process possible, two general rules were specified:

The software reading the FITS file would have a list of types of extensions that it could handle. By reading the type name from a standard location in the header, the software would be able to determine whether or not it could handle this extension. If it couldn't, it could at least calculate how many records it would have to skip to reach the beginning of the next extension.

A complete set of rules, described as the Generalized Extensions agreement (Grosbøl et al. 1988; hereafter FITS Paper III), was endorsed by the IAU in 1988. These rules appear in section 3.3.

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