.FIT file extension
To open .FIT files on Windows, confirm the file is FITS: it often has a companion extension .fits; .fit is also used for FITS in astronomy.
To open a .FIT file, use software that supports FITS (for example, astronomy/scientific tools that can read FITS images and headers). If your default photo viewer can’t open it, the file is likely FITS data and needs a FITS-aware application.
Last updated: April 30, 2026 · Reviewed by Julian Stricker
Open on your device
Choose your operating system for a dedicated step-by-step opening guide.
How to open .FIT files
Use these platform-specific instructions to open .FIT files safely.
Windows
- Confirm the file is FITS: it often has a companion extension .fits; .fit is also used for FITS in astronomy.
- Open the file in a FITS-capable application (an astronomy/scientific tool that supports FITS import). If you have such an app installed, use: Right-click → Open with → choose the app.
- If you don’t have a FITS-capable app installed, transfer the file to a machine/environment where you can use FITS tools (many workflows use FITS libraries like CFITSIO via scientific software).
Mac
- Control-click the .FIT file → Open With → choose a FITS-capable application (astronomy/scientific software that can import FITS).
- If no compatible app is listed, use a FITS-aware toolchain on your Mac (many scientific apps rely on CFITSIO) or open it on a desktop setup where your astronomy tools are installed.
Linux
- Use your file manager: Right-click → Open With → select a FITS-capable viewer/analysis tool if installed.
- If you work in a scientific environment, open the file using software that reads FITS via common libraries such as CFITSIO (typical in astronomy toolchains).
iOS
- iOS typically won’t preview FITS files in the Photos/Quick Look pipeline; share the file to a specialized scientific app if you have one, or move the file to a desktop FITS environment for viewing/analysis.
Android
- Android usually lacks built-in FITS viewing; try opening it only with a specialized scientific/astronomy app if available, otherwise transfer the file to a desktop FITS-capable workflow.
Security notes
- FITS is typically data-only, but it is complex enough that malformed files can trigger bugs in parsers; prefer well-maintained FITS software and keep it updated.
- Be cautious with FITS files from untrusted sources when using tools that perform extensive parsing or automatic processing (batch pipelines), since unexpected headers/structures can cause crashes or unsafe behavior in poorly hardened software.
- If a download is served over the web, note that FITS has registered MIME types (image/fits and application/fits); mismatched types or forced downloads are not automatically malicious, but can be a sign of a misconfigured source.
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Can't open this file?
These are the most common causes and fixes when .FIT files fail to open.
Common reasons
- It won’t open in Photos or the default image viewer
- The file opens, but looks blank or “wrong”
- Software says the FITS file is corrupted or invalid
- Confusion between .fit and other unrelated formats
Fix steps
- Open it with a FITS-capable astronomy/scientific application (one that explicitly supports FITS import).
- If you only need a quick inspection, use a FITS-aware toolchain commonly found in astronomy workflows (often built on CFITSIO).
OS-specific troubleshooting
What is a .FIT file?
.FIT is a common filename extension for FITS, an open standard data format used in astronomy. FITS files store data (often images) together with a structured header that describes the contents; the standard is maintained by the IAU FITS Working Group and is widely used in NASA/astronomy workflows. FITS has registered Internet media types including image/fits (and also application/fits).
Background
FITS (Flexible Image Transport System) is the standard data format used in astronomy and is endorsed by major scientific organizations (including NASA and the IAU). While many people think of it as an “image format,” FITS is best understood as a scientific data container: it typically includes one or more image arrays and a metadata-rich header describing how to interpret the data.
You will commonly encounter .fit or .fits files when downloading telescope observations, working with astronomical surveys, or using science/engineering tools that rely on FITS libraries (such as the widely used CFITSIO software library). Because FITS is designed for data interchange and long-term scientific use, it is not usually supported by consumer photo apps.
On the web and in APIs, FITS is formally registered with IANA/IETF. The MIME subtype image/fits is defined in RFC 4047; RFC 4047 also registers application/fits for cases where the content is not strictly an image.
Common MIME types: image/fits
Further reading
Authoritative resources for more details on the .FIT format.
Common .FIT issues
It won’t open in Photos or the default image viewer
FITS is a scientific format and is not broadly supported by consumer photo apps, even though it can contain image data.
- Open it with a FITS-capable astronomy/scientific application (one that explicitly supports FITS import).
- If you only need a quick inspection, use a FITS-aware toolchain commonly found in astronomy workflows (often built on CFITSIO).
The file opens, but looks blank or “wrong”
FITS images can be high bit-depth and may require scaling/normalization; some tools may display the raw data without appropriate stretch or may open a different HDU/extension than you expect.
- Check whether the file contains multiple HDUs/extensions and select the correct one in your FITS-capable software.
- Adjust display scaling/stretch in your FITS viewer/analysis tool to map scientific pixel values to visible brightness.
Software says the FITS file is corrupted or invalid
The file may be incomplete (failed download/transfer) or not actually FITS despite the .FIT extension.
- Re-download or re-transfer the file and compare file size to the original source if possible.
- Verify the source: FITS should conform to the FITS standard; try opening with an alternative FITS-capable tool to rule out a single-app parsing issue.
Confusion between .fit and other unrelated formats
The .fit extension is used for FITS in astronomy, but file extensions can be reused in other contexts; relying on the extension alone can be misleading.
- Check where the file came from: astronomy/telescope/survey data strongly suggests FITS.
- If unsure, inspect the file with a trusted tool that can identify FITS content by structure/header rather than by extension.
FAQ
Is .FIT the same as .FITS?
Often yes: .fit and .fits are both commonly used filename extensions for FITS (Flexible Image Transport System) files in astronomy.
What MIME type should a FITS file use?
RFC 4047 registers FITS media types including image/fits (commonly used for FITS images). RFC 4047 also registers application/fits for cases where the content is not strictly an image.
Can I convert a .FIT file by renaming it to .PNG or .JPG?
No. Renaming only changes the filename. To make a standard image, use a FITS-aware tool to export or convert the data to a common format.
Why does the image look very dark or very bright when opened?
FITS pixel values often represent scientific measurements and may need a display stretch/scaling to look correct. Use a FITS viewer/analysis tool that lets you adjust the display scaling.
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