FITS

From Academic Kids

FITS or Flexible Image Transfer System is the most commonly used file format in astronomy.

FITS is often used to store also non-image data, such as spectra, photon lists, data cubes, and much more. A FITS file may contain several extensions, and each of these may contain a data object. For example, it is possible to store images made in X-rays and infrared in the same FITS file.

A major advantage of FITS for scientific data is that the header information is human readable ASCII, so that an interested user can examine the headers to investigate a file of unknown provenance. Each FITS file consists of one or more headers containing ASCII card images (80 character fixed-length strings) that carry keyword/value pairs, interleaved between data blocks. The keyword/value pairs provide metadata such as size, origin, 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.

Contents

Images

The earliest and still most commonly used type of FITS data is an image header/data block. The term 'image' is somewhat loosely applied, as the format supports data arrays of arbitrary dimension -- normal image data are generally 2-D or 3-D (with the third dimension representing 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 systems that are overlain on the image itself. Images contain an implicit Cartesian coordinate system that describes the location of each pixel in the image, but scientific uses generally require working in 'world' coordinates. As FITS has been generalized from its original form, the world coordinate system 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.

Tables

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 databases -- a far cry from simple image data.

Using FITS files

FITS support via standard libraries is available in most languages that are used for scientific work, including C, FORTRAN, Perl, PDL, Numerical Python, and IDL. The FITS Support Office at NASA/GSFC maintains a list of libraries and platforms that currently support FITS.

Image processing programs such as the GIMP and photoshop 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.

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, and the Starlink libraries in C++.

See also

External links

fr:Flexible image transfer system

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