FITS file format: what it is and how to use it

FITS is astronomy's data standard since 1981: n-dimensional arrays plus keyword headers recording exactly how the telescope saw the sky. Learn when to use it and what to choose instead.

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What is a FITS file?

A .fits file is scientific data in the Flexible Image Transport System, the standard endorsed by the IAU and used by NASA, ESA and every observatory since 1981: ASCII keyword headers (SIMPLE, BITPIX, NAXIS…) describing n-dimensional data arrays and extensible HDUs that can hold images, tables and spectra in one file. The header is the point: it records how, when and where the data was taken.

When should you use FITS?

  • Astronomy data products: the mandated format of observatories and archives (SDSS, Hubble, JWST).
  • Metadata-critical science: instrument context lives inside the file, readable by machine.
  • Multi-extension datasets: image + table + spectrum in one container.

When is another format a better fit?

For showing someone the picture, stretch and export PNG or JPG, since browsers do not speak FITS. For medical imaging the analogous role belongs to DICOM. For flat analysis storage TIFF works, but in astronomy FITS is not optional.

The short version: FITS is astronomy's filing system, the scientific record. Publish pictures out of it, never instead of it.

How to open a FITS file

On Windows

Double-click the file, or open it with SAOImage DS9.

On macOS

Aladin opens FITS files on macOS.

In your browser

Open FITS files right in a browser tab or with the dexrr viewer. No install, no upload.

Also opens in

  • SAOImage DS9
  • Astropy
  • Aladin
  • FITS Liberator

View or convert FITS files

A quick preview or a new format. No install, no upload.

View FITS files Preview and inspect FITS files Convert to FITS Turn files into FITS FITS to DICOM Digital Imaging and Communications in Medicine FITS to EXR OpenEXR FITS to TIFF Tagged Image File Format

FITS technical details

Full name
Flexible Image Transport System
Extensions
.fits .fit .fts
MIME type
image/fits, application/fits
Magic bytes
53 49 4D 50 4C 45 (SIMPLE)
Compression
None (uncompressed)
Developer
IAU / NASA / FITS Working Group
Specification
fits.gsfc.nasa.gov

FITS questions, answered

How do I open a .fits file?

SAOImage DS9, astropy, GIMP (plugin) and astronomy tools read it; general viewers do not. dexrr displays and converts fits files directly in your browser.

Can I convert FITS files online?

Yes, dexrr converts fits locally in your browser to png or jpg, applying a stretch so the data is actually visible.

Is FITS better than TIFF?

For astronomy, decisively. The keyword header is standardized and every analysis tool understands it. TIFF is fine for ordinary images, not observation data.

Why is my FITS image almost black?

Pixel values are raw flux counts, not display brightness. Apply a log or asinh stretch (or the viewer's auto-scale) to reveal the picture.

Is the FITS format outdated?

It is 40+ years old and still mandated. 'Once FITS, always FITS' is the community's own joke about its permanence.