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Section 3

FITS File Organization

3.1

Overall File Structure

A FITS file shall be composed of the following FITS structures, in the order listed:

· Primary header and data unit (HDU)

· Conforming Extensions (optional)

· Other special records (optional, restricted)

A FITS file composed of only the primary HDU is sometimes referred to as a Basic

FITS file, or a Single Image FITS (SIF) file, and a FITS file containing one or more

extensions following the primary HDU is sometimes referred to as a Multi-Extension

FITS (MEF) file.

Each FITS structure shall consist of an integral number of FITS blocks which are

each 2880 bytes (23040 bits) in length. The primary HDU shall start with the first FITS

block of the FITS file. The first FITS block of each subsequent FITS structure shall

be the FITS block immediately following the last FITS block of the preceding FITS

structure.

This standard does not impose a limit on the total size of a FITS file, nor on the

size of an individual HDU within a FITS file. Software packages that read or write

data according to this standard could be limited, however, in the size of files that are

supported. In particular, some software systems have historically only supported files

up to 231 bytes in size (approximately 2.1 × 109 bytes).

3.2

Individual FITS Structures

The primary HDU and every extension HDU shall consist of 1 or more 2880-byte header

blocks immediately followed by an optional sequence of associated 2880-byte data blocks.

FITS Standard