How to open .FIT files on Windows
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.
Step-by-step instructions
- 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).
Common 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.
Security note
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.