View source for FITS

{{short description|File format}}
{{Other uses}}
{{Infobox file format
| name                   = FITS
| icon                   = 
| logo                   = 
| screenshot             = 
| caption                = 
|_noextcode              = on
| extension              = {{code|.fits}}, {{code|.fit}}, {{code|.fts}}
| mime                   = image/fits<br>application/fits<ref>[http://fits.gsfc.nasa.gov/rfc4047.txt MIME Sub-type Registrations for Flexible Image Transport System (FITS)], rfc4047.txt</ref>
| type code              = 
| uniform type           = 
| magic                  = 
| owner                  = IAU FITS Working Group<ref>{{cite web|title=IAU FITS Working Group|url=http://fits.gsfc.nasa.gov/iaufwg/iaufwg.html|publisher=[[NASA]]}}</ref>
| released               = {{Start date and age|1981}}
| latest release version = 4.0
| latest release date    = {{Start date and age|2016|07}}
| genre                  = [[Image file formats|image format]], [[structured data]]
| container for          = 
| contained by           = 
| extended from          = 
| extended to            = 
| standard               = 
| free                   = 
| url                    = {{URL|fits.gsfc.nasa.gov}}
}}

'''Flexible Image Transport System''' ('''FITS''') is an [[open standard]] defining a digital [[file format]] used for storage, transmission and processing of data: formatted as multi-dimensional arrays (for example a  2D image), or tables.<ref>{{cite web|title=Flexible Image Transport System: a new standard file format for long-term preservation projects?|url=http://www.vaticanlibrary.va/moduli/Allegrezza_EWASS2012.pdf|publisher=[[Vatican City]]|date=2012-07-05|access-date=2013-03-06|archive-date=2013-05-30|archive-url=https://web.archive.org/web/20130530040016/http://www.vaticanlibrary.va/moduli/Allegrezza_EWASS2012.pdf|url-status=dead}}</ref> FITS is the most commonly used digital [[file format]] in [[astronomy]]. The FITS standard was designed specifically for astronomical data, and includes provisions such as describing [[photometry (optics)|photometric]] and spatial calibration information, together with image origin metadata.

The FITS format was first standardized in 1981;<ref>{{cite journal |last1=Wells |first1=D. C. |last2=Greisen |first2=E. W. |last3=Harten |first3=R. H. |title=FITS: A Flexible Image Transport System |journal=Astronomy and Astrophysics Supplement Series |volume=44 |pages=363–370 |date=June 1981 |bibcode=1981A&AS...44..363W}}</ref> it has evolved gradually since then, and the most recent version (4.0) was standardized in 2016.  FITS was designed with an eye towards long-term archival storage, and the maxim ''once FITS, always FITS'' represents the requirement that developments to the format must be [[backward compatible]].

[[Image metadata]] is stored in a human-readable [[ASCII]] header. The information in this header is designed to calculate the byte offset of some information in the subsequent data unit to support direct access to the data cells. Each FITS file consists of one or more headers containing ASCII [[card image]]s (80 character fixed-length strings) that carry keyword/value pairs, interleaved between data blocks. The keyword/value pairs provide information such as size, origin, coordinates, binary data format, free-form comments, history of the data, and anything else the creator desires: while many keywords are reserved for FITS use, the standard allows arbitrary use of the rest of the name-space.

FITS is also often used to store non-image data, such as [[Electromagnetic spectrum|spectra]], [[photon]] lists, [[data cube]]s, or [[structured data]] such as multi-table [[database]]s. A FITS file may contain several extensions, and each of these may contain a data object. For example, it is possible to store [[x-ray]] and [[infrared]] exposures in the same file.

== Composition ==

The earliest and still most commonly used type of FITS data is an image header/data block.{{fact|date=December 2019}} The term 'image' is somewhat loosely applied, as the format supports data arrays of arbitrary dimension—normal image data are usually 2-D or 3-D, with the third dimension representing for example time or the color plane.  The data themselves may be in one of several integer and floating-point formats, specified in the header.

FITS image headers can contain information about one or more scientific [[coordinate system]]s that are overlaid on the image itself.  Images contain an implicit [[Cartesian coordinate system]] that describes the location of each pixel in the image, but scientific uses usually require working in 'world' coordinates, for example the [[celestial coordinate system]]. As FITS has been generalized from its original form, the world coordinate system (WCS) specifications have become more and more sophisticated: early FITS images allowed a simple scaling factor to represent the size of the pixels; but recent versions of the standard permit multiple nonlinear coordinate systems, representing arbitrary distortions of the image. The WCS standard includes many different [[spherical projection]]s, including, for example, the [[HEALPix]] spherical projection widely used in observing the [[cosmic microwave background radiation]].<ref>{{cite journal | last1 = Greisen | first1 = E. W. | last2 = Calabretta | first2 = M. R. | title = Representations of world coordinates in FITS  | journal = Astronomy and Astrophysics | volume = 395 | issue = 3 | pages = 1061–1075 | date = December 2002 | bibcode = 2002A&A...395.1061G | doi = 10.1051/0004-6361:20021326|arxiv = astro-ph/0207407 | s2cid = 3248582 }}</ref>

FITS also supports tabular data with named columns and multidimensional rows. Both binary and ASCII table formats have been specified. The data in each column of the table can be in a different format from the others. Together with the ability to string multiple header/data blocks together, this allows FITS files to represent entire [[relational database]]s.

== Adoption ==
[[file:SAOImage-DS9.png|thumb|SAOImage DS9 in [[FVWM|FVWM2]]]]

FITS support is available in a variety of programming languages that are used for scientific work, including [[C (programming language)|C]],<ref name="FITSIO">{{cite web|title=HEASARC Software|url=http://heasarc.gsfc.nasa.gov/docs/software.html|publisher=[[NASA]] [[Goddard Space Flight Center]]}}</ref> [[C++]], [[C Sharp (programming language)|C#]], [[Fortran]],<ref name="FITSIO"/> [[IGOR Pro]], [[Interactive data language|IDL]], [[Java (programming language)|Java]], [[Julia (programming language)|Julia]],<ref name="FITSIO.jl">{{cite web|title=FITSIO.jl| website=[[GitHub]] | date=15 February 2022 |url=https://github.com/JuliaAstro/FITSIO.jl}}</ref> [[LabVIEW]], [[Mathematica]], [[MATLAB]], [[Perl]], [[Perl Data Language]] (PDL), [[Python (programming language)|Python]], [[R (programming language)|R]], and [[Tcl]]. The FITS Support Office at [[NASA]]/[[Goddard Space Flight Center|GSFC]] maintains a list of libraries and platforms that currently support FITS.<ref>{{cite web|title=FITS I/O Libraries|url=http://fits.gsfc.nasa.gov/fits_libraries.html|publisher=[[NASA]] [[Goddard Space Flight Center]]|access-date=2011-09-29}}</ref>

Image processing programs such as [[ImageJ]], [[GIMP]], [[Photoshop]], [[PhotoLine]], [[Chasys Draw IES]], [[XnView]] and [[IrfanView]] can generally read simple FITS images, but frequently cannot interpret more complex tables and databases. Scientific teams frequently write their own code to interact with their FITS data, using the tools available in their language of choice. The [[FITS Liberator]] software is used by imaging scientists at the [[European Space Agency]], the [[European Southern Observatory]] and [[NASA]].<ref>{{cite web|title=The ESA/ESO/NASA FITS Liberator 3|url=http://www.spacetelescope.org/projects/fits_liberator/|publisher=[[ESA]]}}</ref> The SAOImage DS9 Astronomical Data Visualization Application<ref>{{cite web | url=https://sites.google.com/cfa.harvard.edu/saoimageds9 | title=SAOImageDS9 }}</ref> is available for many OSs, and handles images and headers.<ref>{{cite web|title=SAOImage DS9: Astronomical Data Visualization Application|url=http://hea-www.harvard.edu/RD/ds9/|publisher=[[Harvard–Smithsonian Center for Astrophysics]]}}</ref>

Many scientific computing environments make use of the coordinate system data in the FITS header to display, compare, rectify, or otherwise manipulate FITS images. Examples are the coordinate transform library included with PDL, the PLOT MAP library in the [[Solarsoft]] solar-physics-related software tree, the [[Starlink Project]] AST library in C, and the PyFITS package in Python, now merged into the [[Astropy]] library.<ref>{{cite web|title=PyFITS|url=http://www.stsci.edu/institute/software_hardware/pyfits/|publisher=[[Space Telescope Science Institute]]|access-date=2013-11-04|archive-url=https://wayback.archive-it.org/all/20150726152333/http://www.stsci.edu/institute/software_hardware/pyfits|archive-date=2015-07-26|url-status=dead}}</ref>

== Current status ==

The FITS standard version 4.0 was officially approved by the [[IAU]] FITS Working Group in July 2016.<ref>{{cite web | url=https://fits.gsfc.nasa.gov/standard40/fits_standard40aa.pdf | title=Definition of the Flexible Image Transport System (FITS) | author=FITS Working Group | date=2016-07-22 | access-date=2018-03-05}}</ref><ref>{{cite web | url=http://fits.gsfc.nasa.gov/fits_news.html | title=Recent FITS Activities and Issues | date=2016-07-22 | access-date=2018-03-05}}</ref>

{| class="wikitable"
|+ Release history
|-
! FITS version
! Support level
! Release date
! Notes
|-
| 4.0
| Current standard
| July 2016<ref name="StandardDocument">[http://fits.gsfc.nasa.gov/fits_standard.html FITS Standard Document], FITS Documentation Page, High Energy Astrophysics Science Archive Research Center</ref>
| Final 'language-edited' version formally approved on 13 August 2018<ref>{{Cite web |date=2018-08-13 |title=Definition of the Flexible Image Transport System (FITS) |url=https://fits.gsfc.nasa.gov/standard40/fits_standard40aa-le.pdf |access-date=2024-02-26 |website=[[NASA]] |format=PDF}}</ref>
|-
| 3.0
| Old standard; still supported
| July 2008<ref name="StandardDocument"/>
| -
|-
| 2.1b
| Old standard; still supported
| December 2005<ref name="StandardDocument" />
| Added support for 64-bit integer primary arrays and image extensions
|-
| NOST 100-2.0
| Old standard; still supported
| March 1999<ref name="StandardDocument" />
| -
|-
| NOST 100-1.0
| Old standard; still supported
| June 1993<ref name="StandardDocument" />
| -
|}

== See also ==

* [[Advanced Scientific Data Format]]
* [[FITS Liberator]]
* [[Hierarchical Data Format]]
* [[Common Data Format]]
* [[NetCDF]]
* [[Aperture Photometry Tool]]

== References ==

{{refs}}

== External links ==

* {{Official website|fits.gsfc.nasa.gov}}, NASA Goddard Support Office
* [https://fits.gsfc.nasa.gov/fits_resources.html A Primer on the FITS Data Format], NASA Goddard Support Office
* [http://fits.gsfc.nasa.gov/birthday.news FITS Birthday Announcement] - Historical note from Don Wells on the events leading up to the 'birth' of the FITS format in 1979.
* [http://fits.gsfc.nasa.gov/fits_libraries.html FITS I/O Libraries], a list of software for reading and writing FITS files for various languages
* [https://sites.google.com/cfa.harvard.edu/saoimageds9 SAOImage DS9]

{{Graphics file formats}}

[[Category:Astronomical imaging]]
[[Category:Graphics file formats]]
[[Category:High dynamic range file formats]]