1 Overview

An archival format must be utterly portable and self-describing, on the assumption that, apart from the transcription device, neither the software nor the hardware that wrote the data will be available when the data are read. "Preserving Scientific Data on our Physical Universe'', p. 60. Steering Committee for the Study on the Long-Term Retention of Selected Scientific and Technical Records of the Federal Government, [US] National Research Council, National Academy Press 1995.

1.1 Purpose

This standard formally defines the FITS format for data structuring and exchange that is to be used where applicable as defined in Sect. 1.3. It is intended as a formal codification of the FITS format that has been endorsed by the IAU  for transfer of astronomical data, fully consistent with all actions and endorsements of the IAU and the IAU FITS Working  Group (IAUFWG). Minor ambiguities and inconsistencies in FITS as described in the original papers are eliminated.

1.2 Scope

This standard specifies the organization and content of FITS data sets, including the header and data, for all  standard FITS formats: Basic FITS, the  random groups  structure, the ASCII table  extension, the image  extension, and the binary table  extension. It also specifies minimum structural requirements for new extensions  and general principles governing the creation of new extensions. It specifies the relation between physical block sizes  and logical records for FITS files on bitstream devices and sequential media. For headers, it specifies the proper syntax for card images and defines required  and reserved  keywords. For data, it specifies character and value representations and the ordering of contents within the byte stream. It defines the general rules to which new extensions are required to conform.

   
1.3 Applicability

This standard describes an extensible data interchange format particularly well suited for transport and archiving of arrays and tables of astronomical data. The IAU  has recommended that all astronomical computer facilities support FITS for the interchange of binary data. It has been NASA policy for its astrophysics projects to make their data available in FITS format. This standard may also be used to define the format for data transport in other disciplines, as may be determined by the appropriate authorities.

1.4 Organization of this document

Section 3 is a glossary of definitions, acronyms, and symbols. In Sect. 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 . Section 5 provides details about header card image syntax and specifies those keywords required  and reserved  in a primary header. Section 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 (Sect. 7) and the current standard  extensions: ASCII table, image , and binary table  (Sect. 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 .1, provides a formal expression of the keyword/value syntax for header card images described in Sect. 5.2. Appendix .2 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 .6 describes aspects of the implementation of FITS on physical media not covered by the blocking agreement. Appendix .11 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 .12 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 .13), a list of the ASCII character set  and a subset designated ASCII text  (Appendix .14), a description of the IEEE  floating point format (Appendix .15), and a list of the extension type  names that have been reserved as of the date this document was issued (Appendix .18). Appendix .19 is a list of NOST documents, including earlier versions of this standard.