What is the FITS file format?
.fits files are the universal standard for storing and exchanging astronomical data, in continuous use since 1981. Developed by the IAU FITS Working Group and endorsed by NASA, the format follows the principle of "once FITS, always FITS" - no backward-incompatible changes are ever made, so files created in 1981 remain readable today.
A .fits file is structured as a sequence of Header and Data Units (HDUs). Each HDU contains a plain ASCII header made up of 80-character keyword=value records padded to 2880-byte blocks, followed by binary data. The primary HDU usually holds the main image or data array; extension HDUs can store supplementary images, binary tables of source catalogs, or spectra. All multi-byte binary values use big-endian byte order.
Unlike everyday image formats, .fits handles far more than simple pictures. Astronomers store data cubes (for example, radio survey data with RA, Dec, and velocity axes), spectral time series, and large source catalogs within a single file. Pixel values can be 8-bit unsigned integers, 16/32/64-bit integers, or 32/64-bit IEEE floating-point - the BITPIX keyword specifies the type. Optional CHECKSUM and DATASUM keywords provide integrity verification.
.fits is not suited for everyday photography: there is no built-in lossy compression, files are large, and software support is narrow outside astronomy. NASA's Hubble, Chandra, and JWST archives distribute data in this format. The extensions .fit and .fts are interchangeable names for the identical format.
Security & safety
RISK: LOWFITS is a passive scientific data file containing pixel arrays and ASCII metadata - no executable code. Opening one is safe. The only pitfall is using wrong software (a FITS astronomy file won't open in an image editor without FITS support, and will appear as an error or blank).
Format details
in a nutshellPrograms that open FITS files
Windows4 apps
GIMP
Open-source GIMP can open FITS files natively (simple images); adjust levels/curves after import to make the image visible. 
SAOImageDS9
Open-source The standard free astronomy viewer; File > Open the .fits to display, scale, and analyse. Supports multi-extension FITS, WCS, catalogs.
NASA FITS Liberator
Free Open FITS and export to 16-bit TIFF for astrophotography processing in Photoshop/GIMP. 
XnView MP
Freemium Free viewer for quick visual inspection of FITS images (not for science analysis). 
macOS3 apps
Linux3 apps
SAOImageDS9
Open-source The primary FITS viewer on Linux; open via File > Open or command-line 'ds9 file.fits'.
Aladin Lite
Free Browser-based astronomy sky viewer; can load FITS images and overlay sky survey data. Technical details
deep spec| Format type | Scientific image and multi-dimensional data table container |
| Encoding | Binary data with ASCII header blocks |
| Byte order | Big-endian (network byte order for all binary data values) |
| Header structure | 80-character keyword=value records, padded to 2880-byte blocks |
| Container | Sequence of HDUs (Header and Data Units); self-describing, no external schema needed |
| Compression | None (standard); optional tiled image compression in BINTABLE extension (FITS convention) |
| Pixel depth | 8-bit unsigned, 16/32/64-bit integer, or 32/64-bit IEEE float; BITPIX keyword specifies type |
| Maximum axes | Up to 999 axes (NAXIS keyword); each axis limited by 64-bit integer length |
| Typical file size | 100 KB - several GB (JWST/HST images routinely multi-GB) |
| Integrity check | Optional DATASUM and CHECKSUM keywords (1's-complement checksums per RFC 4047) |
| MIME type | image/fits (also application/fits; both registered in RFC 4047) |
| Alternate extensions | .fit, .fts (identical format; all three interchangeable) |
| Associated OS | Windows, macOS, Linux, web (Aladin Lite) |
| Released | 1981 (first IAU standard) |
| Latest version | FITS Standard 4.0 (2016) |
| Open standard | Yes · royalty-free |
| Specification | fits.gsfc.nasa.gov |
FITS conversions
Community Q&A
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