Structure of FITS files

A FITS file is a collection of Header Data Units (HDUs). The first HDU of a FITS file is called the Primary HDU, other HDUs are called extenstions. Each HDU consists in two parts: the header part is a list of FITS keywords which have a name and, usually, a value and a comment; the data part may be a multi-dimensional array or a table. The primary HDU and so-called FITS Image extensions are HDUs storing a multi-dimensional array in their data part, while FITS Table extensions are HDUs storing a table in their data part. The HDUs of a FITS file are indexed by their number (starting at 1 for the primary HDU) or by their name (specified by the value of the HDUNAME or EXTNAME keyword of their header part). The first HDU which must be an image may be empty. Arrays stored in FITS Image extensions have elements of the same numeric type and may have 1 to 999 dimensions. Tables stored in FITS Image extensions may have a variable number of columns identified by their names, their entries (at a given row and column) are called cells and may have multiple dimensions. Generally, all cells of a given column have the same data type and dimensions. FITS headers are stored in textual form encoded with a restricted subset of the ASCII characters. Although FITS supports ASCII Tables, the data part of FITS HDUs is generally in binary format (in big endian byte order and following IEEE stabdard for floating point values).

References and related resources:

• The IAU FITS Working Group maintains the FITS standard and endorses its evolution.

• The NASA FITS Standard Document hosts the offical reference document that defines the FITS format.