What is a FITS file?
FITS (Flexible Image Transport System) is the standard data format used in astronomy for storing, transmitting, and processing scientific images and data. Defined in 1981 and recognized by the International Astronomical Union, FITS has been the backbone of astronomical data exchange for over 40 years.
Why do astronomers use FITS?
Unlike JPEG or PNG, which are designed for visual display, FITS is designed for scientific data. It preserves the full dynamic range of the original observation — every pixel value is stored as a precise number, not rounded to 8-bit integers. This makes FITS essential for scientific analysis where accuracy matters.
FITS also stores extensive metadata alongside the image data. This metadata, called the FITS header, records everything about the observation: exposure time, telescope, camera, filters, coordinates, timestamps, and much more.
FITS file structure
A FITS file consists of one or more Header Data Units (HDUs). Each HDU has a header (containing metadata as keyword-value pairs) and optionally a data array (containing image pixels or tabular data).
- Primary HDU — The first HDU, usually containing the main image
- Image extensions — Additional image data (e.g., different filters)
- Binary table extensions — Catalog data, event lists, or other structured data
- ASCII table extensions — Text-based tabular data
What is a FITS header?
The FITS header is a collection of 80-character keyword records. Each record has a keyword name, a value, and an optional comment. Some common FITS header keywords include:
FITS file extensions
FITS files use several file extensions, all referring to the same format:
.fits— The standard extension.fit— Shortened version, common in some software.fts— Alternative shortened version
All three extensions are functionally identical. FITS Viewer supports opening any of them.
FITS vs JPEG vs PNG
| Feature | FITS | JPEG | PNG |
|---|---|---|---|
| Bit depth | 8, 16, 32, 64-bit | 8-bit only | 8 or 16-bit |
| Metadata | Extensive FITS headers | Basic EXIF | Basic text chunks |
| Compression | Lossless (optional) | Lossy | Lossless |
| Scientific use | Standard in astronomy | Not suitable | Limited |
How to open a FITS file
You can open FITS files with desktop software like SAOImage DS9, Aladin, or FITS Liberator. Or, for quick viewing without installing anything, use a free online FITS viewer that runs entirely in your browser.
Read our step-by-step guide on opening FITS files →
Frequently asked questions
Can I open FITS files on my phone?
Yes. FITS Viewer runs in any modern browser, including mobile Safari and Chrome on Android. However, very large files may be slow on mobile devices due to memory constraints.
Are FITS files the same as FIT files?
In astronomy, yes. Both .fits and .fit extensions refer to the Flexible Image Transport System format. However, .fit is also used by Garmin for fitness activity data, which is a completely different format.
What software creates FITS files?
FITS files are created by astronomical camera software (NINA, SGP, SharpCap), stacking software (DeepSkyStacker, Siril, PixInsight), and directly by scientific cameras and observatories.