FITS data format. The IAUFWG has approved the submission of this
document and the registration of these two MIME types.
4. FITS Definition
FITS is defined by a document approved by the International
Astronomical Union (IAU) and published in the journal Astronomy &
Astrophysics [NOST]. Conventions for additional keywords used in
FITS files are proposed by interested parties and negotiated and
reviewed by ad hoc committees of the FITS community. If such usage
of additional keywords is approved by national committees and the
IAUFWG then they become new reserved keywords in the FITS standard
and are published in an ongoing series of papers (e.g., [WCS1,
WCS2]).
Copies of the standard documents can be found at the following sites:
http://fits.gsfc.nasa.gov/
http://archive.stsci.edu/fits/
http://www.cv.nrao.edu/fits/
http://heasarc.gsfc.nasa.gov/docs/heasarc/fits.html
Although a brief structure and feature description is provided in
this section as background information, the reader is directed to the
FITS standards documents to obtain complete feature and technical
details.
4.1. FITS Structure
A FITS file consists of a sequence of one or more header and data
units (HDUs) optionally followed by special records. The structure
of a FITS file is based on blocks with a length of 2880 8-bit bytes
(23040 bits). This size was chosen because it is evenly divisible by
the byte and word lengths of all known computer systems. All FITS
files have lengths that are integral multiples of this block size.
Each FITS header consists of a sequence of one or more 2880-byte
blocks that hold 36 80-character records (36*80=2880). The records
consist of ASCII keyword/value pairs plus optional comments. The
character set is the 7-bit printing ASCII codes, including the ASCII
space. In particular, the control codes CR, LF, FF, TAB and NUL are
not used in FITS headers. The keywords are up to 8 characters in
length. Some keywords are mandatory, and their meaning is rigidly
prescribed. Among these are keywords that describe the structure and
size of the subsequent data array. The standard reserves other
keywords for the purpose of conveying specifically defined items of
metadata. Keywords that are neither mandatory nor reserved may be
inserted with semantics that are defined by local conventions. (Some
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