A Brief Introduction to the Flexible Image Transport System (FITS)
The discussion on this page represents the views of the FITS
Support Office. It does not necessarily represent the official
position of the IAU FITS Working Group.
Contents
- What FITS Is
- A format for data transport
- How FITS Evolves
- A process that ensures community involvement and acceptance
- What FITS Is Not
- Not just images
FITS (Flexible Image Transport System) is a data format designed to
provide a means for convenient exchange of astronomical data between
installations whose standard internal formats and hardware differ. A
FITS data file is composed of a sequence of Header Data Units (HDUs).
The header consists of keyword=value statements, which describe the
organization of the data in the HDU and the format of the contents. It
may provide additional information, for example, about instrument status
or the history of the data. The data follow, structured as the header
specifies. The data section of the HDU may contain a digital image, but,
except for the first HDU, it doesn't have to. Other possible
formats include tables and multidimensional matrices that are not
images. An HDU need not contain data. If the first HDU contains data,
they must be organized as an array of one or more dimensions; in
subsequent HDUs, called extensions, other organizations are possible,
consistent with certain rules. The "Image" in
FITS comes from the original use of the format to
transport digital images, but it's not just for images any more.
FITS supports 5 data types in primary or IMAGE extension
data arrays: 8-bit unsigned binary integers, 16-bit twos-complement
signed binary integers, 32-bit twos-complement signed binary integers,
32-bit IEEE-754 standard floating point numbers, and 64-bit IEEE-754
floating point numbers. For signed integers, the byte that includes
the sign bit is first and the byte that has the 1-bit as its least
significant bit is last.
FITS does not support the 16-bit unsigned integer data type
generated by many analog/digital converters. Conforming FITS files
can be produced from such data by subtracting 32768 (decimal) from the
converter output before writing to the file, while setting the
BZERO keyword in the FITS header equal to 32768 and the
BSCALE keyword equal to 1. A FITS reader will then add
32768 to the value in the file, restoring the original value, before
interpreting it. Whether a 16-bit unsigned data type should be added,
and if so, how, is controversial. There were extensive discussions in
the sci.astro.fits
newsgroup in
February
- April, 1994 and again in
February -
March, 1997. (The links go to the directory in the
sci.astro.fits archive where discussion for the months in
question can be found.)
The
International
Astronomical Union FITS Working Group (IAUFWG)
was given authority over FITS matters by the 1988
IAU
General Assembly. The IAUFWG is associated with the
Working Group on Astronomical Data. The current chair is D. Wells
(NRAO) and the vice-chair is
E. Raimond (NFRA).
When the developer of a data set finds that it does not fit well into
the primary HDU or a standard extension format, a new design may be
developed. No change can be made that would cause existing FITS files
to be out of conformance -- the "once FITS, always FITS"
rule. Because software to read FITS files uses the type name of an
extension to determine whether or not the software can read the
extension, extension type names must be unique. The IAUFWG maintains
a list of extension type names that have
been registered; the list is at the FITS Support Office. A unique
name for any new extension type, even a developmental extension or one
that will be used only locally, must be registered with the
IAU FITS Working Group, optionally through the
FITS Support Office as its agent.
After astronomical community discussion, a
formal proposal is distributed. This proposal is discussed by the
community and may be further modified. Tests are run using the new
format to confirm that it can be practically used for data transport.
If the astronomical community reaches a consensus that the proposal
should be adopted as standard FITS, and if successful data transfer
using the proposed extension can be demonstrated, it is submitted for
ratification to the regional committees -- the European FITS Committee,
the Japanese FITS Committee, and the American Astronomical Society
Working Group on Astronomical Software
(WGAS) FITS Committee. Following approval by the regional
committees, it is submitted to the
Approval by the Working Group establishes it as a
standard extension.
In addition to the formal rules, a number of conventions are widely
observed. Some conventions are used throughout the community others
only within a particular discipline such as high energy astrophysics
or single dish radio astronomy. Usually, the originator(s) of a
convention will circulate an initial proposal for comments among a
small group in the same discipline or installation. After this
proposal has been refined based upon these comments, it is put out for
public comment, usually by announcement to the
sci.astro.fits
newsgroup of a URL from which it can be retrieved. Comments at this
time may lead to additional changes. If the affected community
accepts the convention, installations will start using it in their
FITS files. Since failure to use a convention is not a violation of
the FITS rules, FITS readers unaware of it must not terminate with an
error or give incorrect results when encountering it.
FITS is not principally a graphics format designed for the transfer of
pictures; it does not incorporate "FITS viewers," packages
for decoding the data into an image. Users must develop or obtain
separate software to read and display the data from the FITS file.
There are a number of different
packages for particular
applications and hardware, but there is no single standard package
for all applications.
Other FITS Support Office pages
The FITS Support Office is hosted by the
HEASARC
(High Energy Astrophysics Science Archive Research Center)
(Electronic Mail) fits @ fits.gsfc.nasa.gov
(Telephone) +1-301-286-4599
Last revised: 12 September 2002
Responsible NASA representative: Dr. William D. Pence