Network Working Group S. Allen

Network Working Group S. Allen
Request for Comments: 4047 UCO/Lick Observatory
Category: Informational D. Wells
National Radio Astronomy Observatory
April 2005

MIME Sub-type Registrations for
Flexible Image Transport System (FITS)

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

This document describes the registration of the Multipurpose Internet
Mail Extensions (MIME) sub-types to be used by the international
astronomical community for the interchange of Flexible Image
Transport System (FITS) files. The encoding is defined by the
published FITS standard documents. The FITS format has been in use
since 1979, and almost all data from astronomical observations are
interchanged by using FITS.

Table of Contents

1. Introduction.................................................. 2
2. Conventions Used in this Document............................. 2
3. Overview...................................................... 2
4. FITS Definition............................................... 3
4.1. FITS Structure.......................................... 3
4.2. History of FITS Features................................ 5
4.3. Stability of the FITS definition........................ 6
4.4. Portability of FITS files............................... 7
4.5. Application Programming Interfaces to FITS.............. 7
4.6. FITS File Conformance Testing........................... 8
4.7. Archives That Distribute FITS Files..................... 8
5. IANA Considerations........................................... 9
5.1. Registration of application/fits........................ 10
5.2. Registration of image/fits.............................. 14
6. References.................................................... 19
6.1. Normative References.................................... 19

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6.2. Informative References.................................. 20
7. Security Considerations....................................... 21
8. Contributors.................................................. 21
9. Acknowledgements.............................................. 22
Authors' Addresses................................................ 22
Full Copyright Statement.......................................... 23

1. Introduction

The FITS file format [FITS] was designed in order to facilitate the
interchange of astronomical image data between observatories. FITS
provides a means of transporting arrays and tables of data and
keyword/value pairs of metadata. FITS is defined by standards
documents that are approved by the International Astronomical Union
(IAU, http://www.iau.org/) and published in refereed journals.

Before the inception of HTTP, astronomers used the Internet to
exchange FITS files. Multiple unofficial media types for FITS files
[ASU] came into use shortly after the inception of the WWW and have
remained in use. Currently (2005) the international astronomical
community is pursuing many cooperative efforts (e.g., [IVOA], [NVO],
[AstroGrid], [AVO]) to produce web services that provide astronomical
data. The exchange of FITS files is a fundamental element of the
prototypes for these web services [SIAP]. The astronomical community
has to agree to use one set of media types for FITS files in order to
promote interoperability of its various services.

In its simplest form, FITS is used as a means of transporting
astronomical image data in a raster form along with coordinate
information and other standard and locally defined metadata. In such
applications FITS is much like the well-known TIFF format [TIFF] with
the addition of the GeoTIFF tags [GeoTIFF]. However, FITS is capable
of describing a much broader range of data than 2-dimensional
rasters. A consensus has developed in the FITS community that two
media types are needed: one for images and one for all other cases.

2. Conventions Used in this Document

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [Require].

3. Overview

This document describes the registration of the MIME media sub-types
"application/fits" and "image/fits".

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In 1988 the International Astronomical Union formed the FITS Working
Group (IAUFWG) to oversee matters pertaining to the evolution of the
FITS data format. The IAUFWG has approved the submission of this
document and the registration of these two MIME types.

4. FITS Definition

FITS is defined by a document approved by the International
Astronomical Union (IAU) and published in the journal Astronomy &
Astrophysics [NOST]. Conventions for additional keywords used in
FITS files are proposed by interested parties and negotiated and
reviewed by ad hoc committees of the FITS community. If such usage
of additional keywords is approved by national committees and the
IAUFWG then they become new reserved keywords in the FITS standard
and are published in an ongoing series of papers (e.g., [WCS1,
WCS2]).

Copies of the standard documents can be found at the following sites:

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

Although a brief structure and feature description is provided in
this section as background information, the reader is directed to the
FITS standards documents to obtain complete feature and technical
details.

4.1. FITS Structure

A FITS file consists of a sequence of one or more header and data
units (HDUs) optionally followed by special records. The structure
of a FITS file is based on blocks with a length of 2880 8-bit bytes
(23040 bits). This size was chosen because it is evenly divisible by
the byte and word lengths of all known computer systems. All FITS
files have lengths that are integral multiples of this block size.

Each FITS header consists of a sequence of one or more 2880-byte
blocks that hold 36 80-character records (36*80=2880). The records
consist of ASCII keyword/value pairs plus optional comments. The
character set is the 7-bit printing ASCII codes, including the ASCII
space. In particular, the control codes CR, LF, FF, TAB and NUL are
not used in FITS headers. The keywords are up to 8 characters in
length. Some keywords are mandatory, and their meaning is rigidly

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prescribed. Among these are keywords that describe the structure and
size of the subsequent data array. The standard reserves other
keywords for the purpose of conveying specifically defined items of
metadata. Keywords that are neither mandatory nor reserved may be
inserted with semantics that are defined by local conventions. (Some
local conventions have later been adopted into standardized
practice.) The end of the header is signified by a block containing
the keyword "END". A simple example of a FITS header for a digital
image, using only keywords that were specified in the initial FITS
Agreement of March 1979 [FITS], is as follows:

1 2 3 4 5 6
1234567890123456789012345678901234567890123456789012345678901234567..

SIMPLE = T / file does conform to FITS standard
BITPIX = 16 / 16-bit twos-complement pixel values
NAXIS = 2 / 2-dimensional image
NAXIS1 = 512 / first axis length
NAXIS2 = 512 / second axis length
COMMENT -----------------------------------------------------
COMMENT FITS (Flexible Image Transport System) format is defined
COMMENT in 'Astronomy and Astrophysics', volume 376, page 359;
COMMENT bibcode: 2001A&A...376..359H
COMMENT -----------------------------------------------------
ORIGIN = 'Lick Observatory ' /
DATE = '2003-11-22T05:23:45' / when this file was written
END

(These 13 records are followed by 23 80-character records of ASCII
spaces, to pad the header block to the 2880-byte FITS block size.)

The first keyword/value pair, SIMPLE=T, is the signature of FITS; all
FITS files begin with these characters. BITPIX is the second keyword
of all FITS files; legal values for it are 8, 16, 32, -32 and -64.
NAXIS is the third keyword of all FITS files; legal values for it are
0 to 999. NAXIS=0 is legal, and implies that there is no image data
matrix associated with the header (and the NAXISi keywords must not
be present); this value is common for files that will be given the
media type "application/fits" as discussed in section 5.1 of this
document. NAXIS=2 is the value most often used for files that will
be given the media type "image/fits", as discussed in section 5.2 of
this document (NAXIS=3 FITS images are also common). The COMMENT
records will be ignored by FITS reading software, but are used here
to specify the precise journal citation for the FITS standard, an
item of information that is important for archiving on timescales of
decades. The ORIGIN keyword is commonly used in FITS files to encode
the name of the institution where the file was produced. The DATE
keyword is used to convey the timestamp for the file (whereas the

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keyword DATE-OBS is used to convey observation start times). FITS
headers commonly contain a vast variety of additional keywords used
to encode metadata. In particular, a digital image header will often
include keywords to specify the precise celestial coordinates of the
pixels of the 2-D matrix, with conventions that became part of the
FITS standard in 2002 (see [WCS2]). Finally, the above example
demonstrates how, in addition to the COMMENT records, 60% of the
bytes in keyword=value records in FITS headers are reserved for
comments. Although the simple header example above is contained in
only one FITS block, multi-block FITS headers are commonly
interchanged.

Following each header is a data unit that consists of a sequence of
zero or more 2880-byte blocks. In accordance with the description in
the keywords of their header, these blocks contain an N-dimensional
data array, optionally followed by other small groups of array data.
In most cases, the data array represents either an N-dimensional
array of image pixel values or a 2-dimensional array of tabular data.

Following the HDUs a FITS file may contain zero or more 2880-byte
blocks of special records. The standard does not specify anything
about their content. (This convention for special records is not
known to have been used for any purpose other than prototyping new
elements of the standard, such as random groups and BINTABLE.)

The initial HDU in a FITS file is known as the primary HDU (PHDU);
any subsequent HDU is known as an extension HDU (XHDU). The keyword
content of the PHDU is distinct (a PHDU requires a slight alteration
before it can become an XHDU, and most types of XHDU cannot become a
PHDU). A PHDU may have a data array consisting of zero elements, and
this will often be the case for FITS files intended to communicate
tables and multiple images. An XHDU may be one of several standard
types, or it may be another conforming type. Standard types of XHDU
include "IMAGE" (containing an N-dimensional data array similar in
most respects to the PHDU), "TABLE" (containing a 2-dimensional table
of ASCII character data), and "BINTABLE" (containing a 2-dimensional
table of binary data whose elements may themselves be multi-
dimensional arrays).

4.2. History of FITS Features

In 1981 the original definition of FITS described a single HDU
containing one multi-dimensional image array [FITS], as illustrated
by the sample header shown in section 4.1. Subsequent agreements
have used the original framework of HDUs and keyword/value pairs to
extend FITS while preserving the validity of all existing files.
FITS now has standard means of describing multiple arrays of image
data and/or multiple tables of numeric and character information.

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Brief highlights of the history of FITS

- 1979: Initial FITS Agreement and first interchange of files

- 1980: Random groups convention developed

- 1981: Published original (single HDU) definition [FITS]

- 1981: Published random groups definition [GROUPS]

- 1982: Formally endorsed by the IAU

- 1988: Defined rules for multiple HDUs [XTENSION]

- 1988: FITS Working Group established by IAU [IAUFWG]

- 1988: Extended to include ASCII tables [TABLE]

- 1990: Extended to include IEEE floating-point data

- 1994: Extended to multiple image arrays [IMAGE]

- 1995: Extended to binary tables [BINTABLE]

- 1997: Adopted a Y2K-compliant date format

- 2001: Reiterated existing standard in one paper [NOST]

- 2002: Approved conventions for world coordinates [WCS1, WCS2]

4.3. Stability of the FITS Definition

After the adoption of FITS by the IAU in 1982, some in the emerging
community of FITS users realized that there would be tension between
the need for archival stability and the need for evolution. In order
to satisfy both of these requirements they set up a controlled
parliamentary process. At the time of the Generalized Extensions
Agreement [XTENSION], the meta-agreement that controls the evolution
of FITS, the FITS community adopted the guiding principle "Once FITS,
always FITS". Under this rule, no change may be made to FITS that
invalidates existing files. Changes to the FITS standard occur only
after action by the IAU FITS Working Group (FWG). The FWG acts only
after approval by regional working groups that coordinate FITS
activity in various parts of the world.

FITS has been adopted as the archival format for image data and
interferometric data obtained by many ground-based observatories and
for all data from many spacecraft. Many astrophysical archives store

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their data in FITS format, and most astronomical catalog data has
been transcribed into FITS files. The many terabytes of data in
these archives contribute to the stability of the FITS standard.

4.4. Portability of FITS files

Eric Greisen, one of the authors of the original document [FITS],
relates that in 1979

[t]he first FITS files were written by a PL/I program on an IBM
360 under OS/MFT (32-bit, twos-complement numbers and 8-bit EBCDIC
characters) and were read by a Fortran program executing on a CDC
6400 under SCOPE (60-bit, ones-complement numbers and 6-bit
"Display Code" characters). [Remark]

Subsequent evolution of computing hardware and FITS over 25 years has
not degraded this ability to transfer the data content.

The structure of FITS files is extremely general, and this
necessarily complements the nature of astronomical data. FITS is
used to store observations of the entire electromagnetic spectrum
from radio to gamma rays, from ground-based observatories and from
spacecraft. FITS is also used to communicate physical properties
other than radiation intensity; these may be inferred from
observations or calculated by theoretical models. The pedigree of
data in a FITS file typically varies among disciplines; FITS may be
used to store raw and uncalibrated data, completely reduced and
calibrated data, or both. Nevertheless, the FITS standard provides
that the syntactic content of the data and metadata are unambiguously
available to posterity.

Observatories have developed numerous local conventions for the
storage and transfer of data peculiar to their instrumentation and
purview. Application software for handling FITS files from different
regions of the electromagnetic spectrum has been largely disjoint.
For a FITS file that consists of multiple HDUs there are no widely
established conventions governing the meaning of, interrelations
between, and suggested use of the data sets. Recognition of any
local conventions used for FITS data has often been based on
heuristics of the additional (non-standard) keyword/value pairs.
Fully understanding the semantic content of a FITS file usually
requires an external data dictionary.

4.5. Application Programming Interfaces to FITS

Although the definition of FITS is expressed in terms of the bit
content of the files, there are widely supported application
programming interfaces (APIs) which simplify the task of manipulating

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FITS files. Interfaces exist for many languages and operating
systems. A partial list of APIs follows:

CFITSIO http://heasarc.gsfc.nasa.gov/fitsio/
fitsTcl http://heasarc.gsfc.nasa.gov/ftools/fv/fitsTcl_home.html
WCSLIB http://www.atnf.csiro.au/people/mcalabre/WCS/
PyFITS http://www.stsci.edu/resources/software_hardware/pyfits
WCSTools http://tdc-www.harvard.edu/software/wcstools/
FUNTOOLS http://hea-www.harvard.edu/RD/funtools/
IDLASTRO http://idlastro.gsfc.nasa.gov/
fitsy http://cfa-www.harvard.edu/~john/fitsy/
IUEDAC http://archive.stsci.edu/iue/iuedacfits.html
Mathematica http://documents.wolfram.com/v5/Built-inFunctions/
GraphicsAndSound/ImportAndExport/
AdditionalInformation/Import.html
MatLab http://www.mathworks.com/access/helpdesk/help/techdoc/
ref/fitsread.shtml?cmdname=fitsread

Current lists of more APIs can be found at
http://fits.gsfc.nasa.gov/fits_libraries.html

List of applications that use FITS are found in the IANA
registrations of the media types.

4.6. FITS File Conformance Testing

FITS files can be tested for conformance to the Definition of FITS
rules [NOST, WCS1, WCS2] with an application named "fitsverify",
which is available at

http://heasarc.gsfc.nasa.gov/docs/software/ftools/fitsverify/

in the form of executable binary files for Solaris, Linux, and
Windows platforms, as well as in source code. Although "fitsverify"
has not been endorsed by the IAUFWG, users should be aware that the
designer of the program was the Secretary of the Technical Panel that
produced the published FITS standard [NOST].

4.7. Archives That Distribute FITS Files

As noted in section 4.3 of this RFC, massive (multi-terabyte) data
archives that contain and/or distribute FITS files contribute to the
stability of the FITS standard. There are numerous publicly
available archives of FITS files derived from both space and ground-
based observations that span the entire range of the electromagnetic
spectrum from radio to gamma-ray wavelengths. The following are
examples of such archives, in no particular order:

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Telescope(s) URLs for archive access
------------ ------------------------------------------------------
KPNO,CTIO,.. http://archive.noao.edu/nsa/
VLT,HST,.. http://archive.eso.org/
Subaru http://smoka.nao.ac.jp/
SDSS http://www.sdss.org/dr3/
CFHT http://cadcwww.dao.nrc.ca/cfht/cfht.html
VLA,VLBA,GBT http://e2e.aoc.nrao.edu/archive/archive_describe.html
HST,MAST http://archive.stsci.edu/
HEASARC http://heasarc.gsfc.nasa.gov/w3browse/
Chandra http://cxc.harvard.edu/cda/
LaPalma http://archive.ast.cam.ac.uk/ingarch/
BIMA http://bimaarch.ncsa.uiuc.edu/
Keck-DEIMOS http://archive.deep.ucolick.org/
ComptonGRO http://cossc.gsfc.nasa.gov/archive/index.html
Spitzer,.. http://www.ipac.caltech.edu/
AAT http://www.aao.gov.au/archive/
HIPASS http://www.atnf.csiro.au/research/multibeam/
multibeam.html
JCMT http://salish.dao.nrc.ca:8080/jcmt/intro.html
COBE,WMAP http://lambda.gsfc.nasa.gov/
EVN http://www.jive.nl/archive/scripts/listarch.php
Gemini http://gemini.ast.cam.ac.uk/sciops/data/dataIndex.html
XMM-Newton http://xmm.vilspa.esa.es/external/xmm_data_acc/xsa/

5. IANA Considerations

The general nature of the full FITS standard requires the use of the
media type "application/fits". Nevertheless, the principal intent
for a great many FITS files is to convey a single data array in the
PHDU, and such arrays are very often 2-dimensional images. Several
common image viewing applications already display single-HDU FITS
files, and the prototypes for virtual observatory projects specify
that data provided by web services be conveyed by the data array in
the PHDU. These uses justify the registration of a second media
type, namely "image/fits", for files which use the subset of the
standard described by the original FITS standard paper [FITS].

We note that the media type "image/gif" [MIME2] admits raster images
that are three dimensional, because animated GIF images contain two
spatial dimensions and one temporal dimension. We note that the
media types "image/vnd.dwg" and "image/vnd.dxf" admit data that
include three-dimensional vectors and curves as well as objects
created by using constructive solid geometry. Following these
precedents for the "image" media type, we specify that "image/fits"
MAY be used to describe FITS PHDUs that have other than two
dimensions. We expect that most files described as "image/fits" will
have two-dimensional (NAXIS=2) PHDUs.

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5.1. Registration of application/fits

To: [email protected]
Subject: Registration of Standard MIME Media type application/fits

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.

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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.

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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.

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- 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" <[email protected]>
"Don Wells" <[email protected]>

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Author/Change controller:

"Steve Allen" <[email protected]>

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

5.2. Registration of image/fits

To: [email protected]
Subject: Registration of Standard MIME Media type image/fits

MIME media type name: image

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.

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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

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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

Non-astronomical FITS image display applications include:

netpbm http://netpbm.sourceforge.net/
gimp http://www.gimp.org/
IDL http://www.rsinc.com/
ImageMagick http://www.imagemagick.com/
Mathematica http://www.wolfram.com/
MatLab http://www.mathworks.com/
xv http://www.trilon.com/xv/xv.html

There are also two FITS plug-ins for Adobe Photoshop
(http://www.adobe.com/products/photoshop/), available at
http://astroshed.com/fitsplug/fitsplug.htm and
http://www.spacetelescope.org/projects/fits_liberator/

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

Additional information:

A FITS file described with the media type "image/fits" SHOULD have a
PHDU with positive integer values for the NAXIS and NAXISn keywords,
and hence SHOULD contain at least one pixel. Files with 4 or more
non-degenerate axes (NAXISn>1) SHOULD be described as
"application/fits", not as "image/fits". (In rare cases it may be
appropriate to describe a NULL image -- a dataless container for FITS
keywords, with NAXIS=0 or NAXISn=0 -- or an image with 4+ non-
degenerate axes as "image/fits" but this usage is discouraged because
such files may confuse simple image viewer applications.)

FITS files declared as "image/fits" MAY also have one or more
conforming XHDUs following their PHDUs. These extension HDUs MAY
contain standard, non-linear, world coordinate system (WCS)
information in the form of tables or images. The extension HDUs MAY
also contain other, non-standard metadata pertaining to the image in
the PHDU in the forms of keywords and tables.

A FITS file described with the media type "image/fits" SHOULD be
principally intended to communicate the single data array in the
PHDU. This means that "image/fits" SHOULD NOT be applied to FITS
files containing MEF (multi-exposure-frame) mosaic images. Also,
random groups files MUST be described as "application/fits" and not
as "image/fits".

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A FITS file described with the media type "image/fits" is also valid
as a file of media type "application/fits". The choice of
classification depends on the context and intended usage.

Recommendations for application writers:

An application that is intended to handle "image/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 writer MAY choose to ignore HDUs beyond the
PHDU, but even in this case the application SHOULD be able to present
the user with the keyword/value pairs from the PHDU.

Note that an application intended to render "image/fits" for viewing
by a user has significantly more responsibility than an application
intended to handle, e.g., "image/tiff" or "image/gif". FITS data
arrays contain elements which typically represent the values of a
physical quantity at some coordinate location. Consequently they
need not contain any pixel rendering information in the form of
transfer functions, and there is no mechanism for color look-up
tables. An application SHOULD provide this functionality, either
statically using a more or less sophisticated algorithm, or
interactively allowing a user various degrees of choice.

Furthermore, the elements in a FITS data array may be integers or
floating-point numbers. The dynamic range of the data array values
may exceed that of the display medium and the eye, and their
distribution may be highly nonuniform. Logarithmic, square-root, and
quadratic transfer functions along with histogram equalization
techniques have proved helpful for rendering FITS data arrays. Some
elements of the array may have values which indicate that their data
are undefined or invalid; these should be rendered distinctly. Via
WCS Paper I [WCS1] the standard permits "CTYPEnnn = 'COMPLEX'" to
assert that a data array contains complex numbers (future revisions
might admit other elements such as quaternions or general tensors).

Three-dimensional data arrays (NAXIS=3 with NAXIS1, NAXIS2 and NAXIS3
> 1) are of special interest. Applications intended to handle
"image/fits" MAY default to displaying the first 2D plane of such an
image cube, or they MAY default to presenting such an image in a
fashion akin to that used for an animated GIF, or they MAY present
the data cube as a mosaic of "thumbnail" images. Even in the absence
of WCS indication of a temporal axis the time-lapse movie-looping
display technique can be effective, and application writers SHOULD
consider offering it for all three-dimensional arrays.

An "image/fits" PHDU with NAXIS=1 is describing a one-dimensional
entity such as a spectrum or a time series. Applications intended to

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handle "image/fits" MAY default to displaying such an image as a
graphical plot rather than as a two-dimensional picture with a single
row.

An application that cannot handle an image with dimensionality other
than 2 SHOULD gracefully indicate its limitations to its users when
it encounters NAXIS=1 or NAXIS=3 cases, while still providing access
to the keyword/value pairs.

FITS files with degenerate axes (i.e., one or more NAXISn=1) MAY be
described as "image/fits", but the first axes SHOULD be non-
degenerate (i.e., the degenerate axes SHOULD be the highest
dimensions). An algorithm designed to render only two-dimensional
images will be capable of displaying such an NAXIS=3 or NAXIS=4 FITS
array that has one or two of the axes consisting of a single pixel,
and an application writer SHOULD consider coding this capability into
the application. Writers of new applications which generate FITS
files intended to be described as "image/fits" SHOULD consider using
the WCSAXES keyword [WCS1] to declare the dimensionality of such
degenerate axes, so that NAXIS can be used to convey the number of
non-degenerate axes.

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.

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Intended usage: Common

Persons to contact for further information:

"Steve Allen" <[email protected]>
"Don Wells" <[email protected]>

Author/Change controller:

"Steve Allen" <[email protected]>

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

6. References

6.1. Normative References

[Require] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[MIME1] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.

[MIME2] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.

[MIME4] Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: Registration
Procedures", BCP 13, RFC 2048, November 1996.

[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

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

[WCS1] Greisen, E. and M. Calabretta, "Representations of world
coordinates in FITS", Astronomy & Astrophysics, 395, p.
1061, 2002.

[WCS2] Calabretta, M. and E. Greisen, "Representations of
celestial coordinates in FITS", Astronomy & Astrophysics,
395, p. 1077, 2002.

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6.2. Informative References

[FITS] Wells, D., et al., "FITS: A Flexible Image Transport
System", Astronomy & Astrophysics Supplement, 44, p. 363,
1981.

[GROUPS] Greisen, E. and R. Harten, "An extension of FITS for
groups of small arrays of data", Astronomy & Astrophysics
Supplement, 44, p. 371, 1981.

[XTENSION] Grosbol, P., et al., "Generalized extensions and blocking
factors for FITS", Astronomy & Astrophysics Supplement,
73, p. 359, 1988.

[IAUFWG] McNally, D., ed., "Transactions of the IAU, Vol. XXB
1988, Proceedings of the Twentieth General Assembly
Baltimore 1988", Kluwer Academic, p. 52 (Resolution B2),
1988.

[TABLE] Harten, R., et al., "The FITS tables extension",
Astronomy & Astrophysics Supplement, 73, p. 365, 1988.

[IMAGE] Ponz, J., et al., "The FITS image extension", Astronomy &
Astrophysics Supplement, 105, p. 53, 1994.

[BINTABLE] Cotton, W., et al., "Binary table extension to FITS",
Astronomy & Astrophysics Supplement, 113, p. 159, 1995.

[Remark] Greisen, E., "FITS: A remarkable achievement in
information exchange" in "Information Handling in
Astronomy -- Historical Vistas", A. Heck, ed., Kluwer
Academic, p. 71, 2003.

[IVOA] The International Virtual Observatory Alliance,
http://www.ivoa.net/

[NVO] The US National Virtual Observatory, http://www.us-
vo.org/

[AstroGrid] The UK AstroGrid, http://www.astrogrid.org/

[AVO] The European Astrophysical Virtual Observatory,
http://www.euro-vo.org/

[ASU] Albrecht, M., et al., "Astronomical Server URL",
http://vizier.u-strasbg.fr/doc/asu.html, 1996.

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[SIAP] Tody, D., et al., "Simple Image Access Prototype
Specification", http://www.us-vo.org/pubs/, 2002.

[TIFF] Adobe Systems Incorporated, "TIFF Revision 6.0",
http://partners.adobe.com/asn/developer/pdfs/tn/TIFF6.pdf
1992.

[GeoTIFF] Ritter, N. and M. Ruth, "GeoTIFF Format Specification",
http://www.remotesensing.org/geotiff/geotiff.html, 2000.

7. Security Considerations

The security considerations of interchanging FITS files are discussed
above within the text of the IANA registration for each media type.

8. Contributors

Several individuals have made significant contributions to the
content and clarity of this text:

- Francois Ochsenbein (Observatoire Astronomique de Strasbourg)

- Clive Davenhall (Institute for Astronomy of the Royal
Observatory Edinburgh)

- Tom McGlynn (Laboratory for High Energy Astrophysics of
the NASA Goddard Space Flight Center)

- Lucio Chiappetti (Milan section of the Italian Istituto di
Astrofisica Spaziale e Fisica Cosmica)

- William Pence (NASA High Energy Astrophysics Science Archive
Research Center)

- Arnold Rots (High Energy Astrophysics Division of the
Smithsonian Astrophysical Observatory)

- Doug Tody (National Radio Astronomy Observatory)

- Bob Hanisch (Space Telescope Science Institute)

- Mark Calabretta (Australia Telescope National Facility)

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9. Acknowledgements

This document originated when William Joye of the Research and
Development Group at the Smithsonian Astrophysical Observatory High
Energy Astrophysics Division discovered many experimental and
unofficial MIME media types being used by various agencies.

Jeff Uphoff of the National Radio Astronomy Observatory (NRAO)
contributed the FITS entries for the magic number file that permits
the Unix-like "file" command on many systems to identify a FITS file.

Nelson Zarate verified that the fgread and fgwrite programs are able
to store hierarchical directories containing files with arbitrary
MIME media types within a HDU of a FITS file. The fgread and fgwrite
programs are part of the FITSUTIL IRAF external package (version
dated September 1999) written by N. Zarate, D. Tody, and R. Seaman at
National Optical Astronomy Observatory (NOAO).

Authors' Addresses

Steven L. Allen
UCO/Lick Observatory
University of California
Santa Cruz, CA 95064 USA

Phone: +1 831 459 3046
EMail: [email protected]

Donald C. Wells
National Radio Astronomy Observatory
520 Edgemont Road
Charlottesville, Virginia 22903-2475 USA

Phone: +1 434 296 0277
EMail: [email protected]

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Full Copyright Statement

Copyright (C) The Internet Society (2005).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

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Acknowledgement

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