.FITS

Flexible Image Transport System

Introduction

FITS is the standard data format of astronomy, first published in 1981 and maintained by the IAU FITS Working Group; version 4.0 of the standard dates from 2016. It stores images, data cubes and tables together with human-readable text headers that describe the instrument, the observation and the mapping from pixels to positions on the sky.

Each pixel is a measured value, such as a photon count, stored as an integer or a floating point number. Observatories and space telescopes publish their data as FITS, and astrophotographers use it to keep their raw frames. The extensions .fits, .fit and .fts are all common.

Software

Structure

A FITS file is a sequence of 2880-byte blocks grouped into header and data units (HDUs). Each header is a list of 80-character ASCII lines called cards, in the form KEYWORD = value / comment, ended by an END card and padded with spaces to a whole block. The primary header must begin with SIMPLE = T, then BITPIX (the data type: 8, 16, 32 or 64 for integers, -32 or -64 for floats), NAXIS and the size of each axis.

The data follows the header as big-endian numbers, padded to a whole block. Further HDUs, called extensions, can hold more images (IMAGE) or tables (TABLE for text and BINTABLE for binary).

  1. 0Primary header 80-character cards, from SIMPLE to END2880 × n bytes
  2. …Primary data the image or data cube, optional2880 × n bytes
  3. …Extensions repeated: a header, then its image or table2880 × n bytes
A FITS file: header and data units in 2880-byte blocks