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SECTION 3. FITS FILE ORGANIZATION

F. An up to date list is also maintained on the FITS Support Office web site.

3.4.1.2

Size Specification

The total number of bits in the data of each extension shall be specified in the header

for that extension, in the manner prescribed in §4.4.1.2.

3.4.2

Standard Extensions

A standard extension is a conforming extension whose organization and content are

completely specified in §7 of this standard. Only one extension format shall be approved

for each type of data organization.

3.4.3

Order of Extensions

An extension may follow the primary HDU or another conforming extension. Standard

extensions and other conforming extensions may appear in any order in a FITS file.

3.5

Special Records (Restricted Use)

Special records are 2880-byte FITS blocks following the last HDU of the FITS file

that have an unspecified structure that does not meet the requirements of a conforming

extension. The first 8 bytes of the special records must not contain the string `XTENSION'.

It is recommended that they do not contain the string `SIMPLE  '. The contents of special

records are not otherwise specified by this standard.

Special records were originally designed as a way for the FITS format to evolve

by allowing new FITS structures to be implemented. Following the development of

conforming extensions, which provide a general mechanism for storing different types of

data structures in FITS format in a well defined manner, the need for other new types

of FITS data structures has been greatly reduced. Consequently, further use of special

records is restricted and requires the approval of the IAU FITS Working Group.

3.6

Physical Blocking

3.6.1

Bitstream Devices

For bitstream devices, including but not restricted to logical file systems, FITS files

shall be interpreted as a sequence of 1 or more 2880-byte FITS blocks, regardless of the

physical blocking structure of the underlying recording media. When writing a FITS

file on media with a physical block size unequal to the 2880-byte FITS block length,

any bytes remaining in the last physical block following the end of the FITS file should

FITS Standard