What is a .fits file?
Flexible Image Transport System · Data · NASA / IAU · 1981
FITS is the standard file format of astronomy. It stores telescope images and data tables together with a human-readable text header of metadata, and it has been used for space data for over forty years.
What is a .fits file used for?
A FITS file begins with an ASCII header, the first keyword being SIMPLE, that describes the data: dimensions, coordinates, instrument and observation details. The header is followed by the binary image or table data.
Its longevity is a design goal: FITS files from decades ago still read today. Astronomers open them in tools like SAOImageDS9, or in Python with Astropy.
How to open a .fits file
The .fits format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list. For a step-by-step walkthrough, see how to open a .fits file.
Windows
- SAOImageDS9
- Python (astropy)
- FITS Liberator
macOS
- SAOImageDS9
- Python (astropy)
Linux
- SAOImageDS9
- Python (astropy)
- ds9
How to convert a .fits file
You can convert a .fits file to png, tiff, csv using a conversion tool or the export menu of an app that opens it.
.fits file signature (magic bytes)
Programs recognise a .fits file by the bytes at the start of the file, not by its name. These are the signatures for FITS. For a full breakdown, see the FITS file signature page.
| Hex signature | Offset | Note |
|---|---|---|
| 53 49 4D 50 4C 45 | 0 | 'SIMPLE' header keyword |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .fits file?
FITS is the standard file format of astronomy. It stores telescope images and data tables together with a human-readable text header of metadata, and it has been used for space data for over forty years.
How do I open a FITS file?
Astronomers use SAOImageDS9 to view the images, or read them in Python with the Astropy library. General image viewers usually cannot open FITS.
What is a FITS file used for?
Storing astronomical images and data tables from telescopes and space missions, along with a rich metadata header describing each observation.
Why is FITS still used after 40 years?
Backward compatibility is a core principle: old FITS files must keep working. That stability is priceless for archives of irreplaceable astronomical data.
Related formats
Sources
Details on this page were checked against authoritative references: