§3 is a glossary of definitions, acronyms, and symbols. In §4, this document describes the overall organization of a FITS file, the contents of the first (primary) header and data, the rules for creating new FITS extensions , and the relation between physical block sizes and logical records for FITS files on bitstream devices and sequential media. The next two sections provide additional details on the header and data, with a particular focus on the primary header . §5 provides details about header card image syntax and specifies those keywords required and reserved in a primary header. §6 describes how different data types are represented in FITS. The following sections describe the headers and data of two standard FITS structures , the now deprecated random groups records (§7) and the current standard extensions: ASCII table, image , and binary table (§8). Throughout the document, deprecation of structures or syntax is noted where relevant. Files containing deprecated features are valid FITS, but these features should not be used in new files; the old files using them remain standard because of the principle that no change in FITS shall cause a valid FITS file to become invalid.
The Appendixes contain material that is not part of the standard. The first, Appendix A, provides a formal expression of the keyword/value syntax for header card images described in §5.2. Appendix B provides examples of widely accepted FITS conventions that are not part of the formal FITS standard. It describes three conventions in use with the binary table extension -- one for handling multidimensional arrays, one for including variable length arrays, and one for arrays of substrings. Appendix C describes aspects of the implementation of FITS on physical media not covered by the blocking agreement. Appendix D is the appendix to the agreement endorsed by the IAUFWG for a new format for keywords expressing dates. The new format uses a four-digit value for the year, and thus eliminates any ambiguity in dates from the year 2000 and after. This appendix is not part of the formal agreement. It contains a detailed discussion of time systems. It has been slightly reformatted for stylistic compatibility with the remainder of this document. Appendix E lists the differences between this standard and the specifications of prior publications; it also identifies those ambiguities in the documents endorsed by the IAU on which this standard provides specific rules. The next four appendixes provide reference information: a tabular summary of the FITS keywords (Appendix F), a list of the ASCII character set and a subset designated ASCII text (Appendix G), a description of the IEEE floating point format (Appendix H), and a list of the extension type names that have been reserved as of the date this document was issued (Appendix I). Appendix J is a list of NOST documents, including earlier versions of this standard.