What is the FIT file format?
A .fit file is used to store astronomical data captured through telescopes for research and scientific purposes. .fit is an alternate extension for the Flexible Image Transport System - more commonly saved as .fits - a format endorsed by both NASA and the International Astronomical Union (IAU) as the standard for scientific imaging and data exchange.
Unlike typical raster formats such as .tiff or .png, a .fit file stores data in a combination of plain-text ASCII header blocks and binary data arrays. Each file can hold multiple Header+Data Units (HDUs), allowing one .fit file to contain images, tables, and spectra together. The ASCII headers use fixed 80-character keyword cards, the first of which is always SIMPLE = T at byte offset 0. Processed .fit files can be opened, analyzed, and exported with applications such as SAOImageDS9 or the NASA FITS Liberator.
Security & safety
RISK: LOWA FITS .fit file is passive scientific data (an image/table), not an executable, so opening one is safe. The usual pitfall here isn't malware but using the wrong app: a FITS astronomy file won't open in a fitness viewer, and a Garmin activity .fit won't open in an astronomy viewer. Identify the file by its source (telescope vs watch) and pick the matching tool.
Format details
in a nutshell- Garmin FIT (Flexible & Interoperable Data Transfer) activity file - Compact binary fitness/GPS workout log written by Garmin and other sports watches/bike computers; opened in Garmin Connect/Strava or converted to GPX/TCX with GPSBabel.
Programs that open FIT files
Windows3 apps
SAOImageDS9
Open-source The standard free astronomy viewer; File > Open the .fit/.fits to display and analyse the image.
NASA FITS Liberator
Free Free tool to open FITS and export to TIFF/16-bit for astrophotography processing. 
XnView MP
Freemium Free image viewer that opens FITS images for a quick look (not for science analysis). 
macOS1 app
Linux1 app
Technical details
deep spec| Magic identifier | ASCII text `SIMPLE = T` at byte offset 0; no binary magic number |
| Header card format | Fixed 80-character keyword-value-comment cards, packed 36 per 2880-byte block |
| Block size | 2880 bytes per block - a legacy multiple of 36 × 80 characters inherited from the magnetic-tape era |
| Structure unit | Header+Data Unit (HDU); a single file may hold multiple HDUs containing images, tables, and spectra |
| MIME type | application/fits |
| Alternate extensions | `.fits` (canonical), `.fts`; all three extensions refer to the same format |
| Byte order | Big-endian (network byte order) for all binary array data |
| Supported integer depths | 8, 16, 32, and 64-bit signed integers; depth controlled by the BITPIX keyword |
| Supported float depths | 32-bit and 64-bit IEEE 754 floating-point (BITPIX values −32 and −64) |
| Maximum array axes | Up to 999 axes via the NAXIS keyword (NAXIS1 through NAXIS999) |
| World Coordinate System (WCS) | Built-in header keywords encode sky coordinates (RA, Dec, equinox, projection) for astrometric calibration |
| Table support | Binary table (BINTABLE) and ASCII table extensions store catalogs and spectra alongside image data |
| Tile compression | Optional lossless and lossy tile compression via the FITS tiled-image compression convention, stored as BINTABLEs |
| Null pixel values | Undefined pixels flagged with the BLANK keyword for integers or IEEE NaN for floating-point arrays |
| Standard authority | IAU FITS Working Group with NASA/GSFC technical support; conformance defined by the FITS standard document |
| Released | 1981 (IAU standard) |
| Open standard | Yes · royalty-free |
| Specification | fits.gsfc.nasa.gov |
FIT conversions
Community Q&A
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