(RFC 4047 published April 2005, subtype last updated April 2005)

MIME media type name: application

MIME subtype name: fits

Required parameters: none

Optional parameters: none

Encoding considerations: binary

FITS files can be quite large. When transferred via HTTP it may be
efficient for the transaction to make use of content-coding or
transfer-coding values such as "gzip", "compress", or "deflate".

Security considerations:

FITS provides a means of transporting arrays and tables of data and
keyword/value pairs of metadata. The standard FITS keywords are
either mandatory or reserved. Mandatory keywords provide information
necessary for correct interpretation of the data; reserved keywords
merely provide standard bits of metadata. As such, the current
standard FITS keywords do not pose security risks.

A FITS file author may insert additional keywords with semantics that
are not described by the standard. Parties exchanging FITS files may
employ locally defined conventions that use various keywords and
their values to induce actions on the part of the recipient. There
are existing local conventions where such keywords are used to
request the reading of other files and/or URIs. There are other
local conventions where such keywords are used to modify the state of
a telescope and/or instrument. The security implications of local
conventions such as these SHOULD be analyzed by the parties employing
them.

Interoperability considerations:

FITS files have been successfully transported between wildly
different computers since 1979. The difficulty most likely to be
encountered by a FITS application is inability to acquire the
computational resources required by a very large FITS file.

Published specification:

The specification for this content type is published as a series of
papers in refereed astronomical journals:

Hanisch, R., et al., "Definition of the Flexible Image Transport
System (FITS)", Astronomy & Astrophysics, 376, p. 359, 2001.

Greisen, E. and M. Calabretta, "Representations of world coordinates
in FITS", Astronomy & Astrophysics, 395, p. 1061, 2002.

Calabretta, M. and E. Greisen, "Representations of celestial
coordinates in FITS", Astronomy & Astrophysics, 395, p. 1077, 2002.

Copies of these specifications can also be found via:

http://fits.gsfc.nasa.gov/
http://archive.stsci.edu/fits/
http://www.cv.nrao.edu/fits/
http://heasarc.gsfc.nasa.gov/docs/heasarc/fits.html

Applications that use this media type:

There are many astronomical image viewing and data reduction
applications including, but not limited to, the following list:

IRAF http://iraf.noao.edu/
AIPS http://www.aoc.nrao.edu/aips/
AIPS++ http://aips2.nrao.edu/
MIDAS http://www.eso.org/projects/esomidas/
ds9 http://hea-www.harvard.edu/RD/ds9/
fv http://heasarc.gsfc.nasa.gov/ftools/fv/
Aladin http://aladin.u-strasbg.fr/
Starlink http://star-www.rl.ac.uk/
Miriad http://bima.astro.umd.edu/miriad/
STSDAS http://www.stsci.edu/resources/software_hardware/stsdas
PROS http://hea-www.harvard.edu/PROS/pros.html
CIAO http://cxc.harvard.edu/ciao/
XANADU http://heasarc.gsfc.nasa.gov/docs/xanadu/xanadu.html
HESSI http://hesperia.gsfc.nasa.gov/ssw/hessi/doc/
FITSview http://www.nrao.edu/software/fitsview/
XMM-SAS http://xmm.vilspa.esa.es/external/xmm_sw_cal/sas_frame.shtml

At the present time many of these applications are not designed to
support use as viewers of "application/fits" files in association
with web browsers.

Additional information:

A FITS file described with the media type "application/fits" SHOULD
conform to the published standards for FITS files as determined by
convention and agreement within the international FITS community. No
other constraints are placed on the content of a file described as
"application/fits".

A FITS file described with the media type "application/fits" may have
an arbitrary number of conforming extension header and data units
(XHDUs) that follow its mandatory primary header and data unit
(PHDU). The XHDUs may be one of the standard types ("IMAGE",
"TABLE", and "BINTABLE") or any other type that satisfies the
"Requirements for Conforming Extensions" (section 4.4.1 of [NOST]).
The PHDU or any "IMAGE" XHDU may contain zero to 999 dimensions with
zero or more pixels along each dimension.

The PHDU may use the random groups convention, in which the dimension
of the first axis is zero and the keywords GROUPS, PCOUNT and GCOUNT
appear in the header. NAXIS1=0 and GROUPS=T is the signature of
random groups; see section 7 of the Definition of FITS paper [NOST].

Recommendations for application writers:

An application intended to handle "application/fits" SHOULD be able
to provide a user with a manifest of all of the HDUs that are present
in the file and with all of the keyword/value pairs from each of the
HDUs.

An application intended to handle "application/fits" SHOULD be
prepared to encounter XHDUs that contain either ASCII or binary
tables, and to provide a user with access to their elements.

An application which can modify FITS files or retrieve FITS files
from an external service SHOULD be capable of writing such files to a
local storage medium.

Complete interpretation of the meaning and intended use of the data
in each of the HDUs typically requires the use of heuristics that
attempt to ascertain which local conventions were used by the author
of the FITS file.

As examples, files with media type "application/fits" might contain
any of the following contents:

- An empty PHDU (containing zero data elements) followed by a table
HDU that contains a catalog of celestial objects.

- An empty PHDU followed by a table HDU that encodes a series of
time-tagged photon events from an exposure using an X-ray detector.

- An empty PHDU followed by a series of IMAGE HDUs containing data
from an exposure taken by a mosaic of CCD detectors.

- An empty PHDU followed by a series of table HDUs that contain a
snapshot of the state of a relational database.

- A PHDU containing a single image along with keyword/value pairs of
metadata.

- A PHDU with NAXIS1=0 and GROUPS=T followed by random groups data
records of complex fringe visibilities

Magic number(s): "SIMPLE = T"

Jeff Uphoff of the National Radio Astronomy Observatory (NRAO) has
contributed database entries for the magic number file which is used
by the Unix "file" command. Magic number files with these entries
are distributed with a variety of Unix-like operating systems. In
addition to recognizing a FITS file using the string given above, the
Uphoff entries also recognize the data type of the pixels in the
PHDU.

File extension(s): fits

This file extension SHOULD NOT be interpreted as a prescription.

The FITS standard originated in the era when files were stored and
exchanged via magnetic tape; it does not prescribe any nomenclature
for files on disk. Various sites within the FITS community have
long-established practices where files are presumed to be FITS by
context. File extensions used at such sites commonly indicate
content of the file instead of the data format.

In the absence of other information it is reasonably safe to presume
that a file name ending in ".fits" is intended to be a FITS file.
Nevertheless, there are other commonly used extensions; e.g., ".fit",
".fts", and many others not suitable for listing in a media type
registration.

Intended usage: Common

Persons to contact for further information:

"Steve Allen" <sla&ucolick.org>
"Don Wells" <dwells&nrao.edu>

Author/Change controller:

"Steve Allen" <sla&ucolick.org>

The IAU FITS Working Group may authorize changes to this document.