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.FITS file extension

To open .FITS files on Windows, install and open SAOImage DS9 (a common FITS viewer), then use File → Open to load the .FITS file.

To open a .FITS file, use an astronomy/scientific imaging app such as SAOImage DS9, or open it programmatically with Python using astropy.io.fits. If your default image viewer can’t open it, don’t rename the extension—use a FITS-capable tool.

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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 .FITS files

Use these platform-specific instructions to open .FITS files safely.

Windows

  1. Install and open SAOImage DS9 (a common FITS viewer), then use File → Open to load the .FITS file.
  2. If you use Python, install Astropy and open the file with astropy.io.fits (e.g., load HDUs and inspect headers/data).
  3. If the file still won’t open, verify it is a real FITS file (not mislabeled) and try another FITS-capable tool on a desktop OS.
Full Windows guide

Mac

  1. Open SAOImage DS9 and use File → Open to load the .FITS file for viewing/analysis.
  2. For analysis in Python, use Astropy (astropy.io.fits) to read the HDU list and inspect the header and data arrays.
  3. If Finder/Preview can’t open it, that’s normal—use a FITS-aware app rather than a general photo viewer.
Full Mac guide

Linux

  1. Open SAOImage DS9 and load the .FITS file using File → Open.
  2. For command-line/programmatic work, use Python with Astropy (astropy.io.fits) to read images/tables and headers.
  3. If you suspect file corruption, try reading it with a different FITS reader/library (many tools are built on CFITSIO).
Full Linux guide

iOS

  1. iOS typically won’t preview FITS natively; if you need to view/analyze it, transfer the file to a desktop (Windows/macOS/Linux) and open it with DS9 or Python/Astropy.
Full iOS guide

Android

  1. Android typically lacks standard FITS support in default viewers; for reliable viewing/analysis, move the file to a desktop and open it with DS9 or Python/Astropy.
Full Android guide

Security notes

  • FITS files are data files (headers + data blocks) and do not typically contain active content like macros, but they can still be risky if opened with vulnerable parsers—use well-maintained tools and libraries (e.g., established FITS viewers and Astropy/CFITSIO-based readers).
  • Be cautious with FITS files from untrusted sources: extremely large dimensions or many extensions can trigger heavy memory/CPU use (a practical denial-of-service risk) when opened or when display scaling is applied.
  • If you script with FITS data, treat header values and metadata as untrusted input (e.g., don’t pass header strings into shell commands or file paths without validation).

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Can't open this file?

These are the most common causes and fixes when .FITS files fail to open.

Common reasons

  • The file won’t open in my normal photo viewer
  • Opens, but looks black/blank or the contrast is wrong
  • Error reading FITS: corrupted/truncated file or invalid header
  • The FITS file has multiple extensions and I can’t find the image/table I need

Fix steps

  1. Open it with a FITS-capable viewer such as SAOImage DS9.
  2. If you need to process the data, use Python with astropy.io.fits to read the HDUs and extract the image array.

OS-specific troubleshooting

What is a .FITS file?

FITS (Flexible Image Transport System) is a standardized format for transporting and archiving astronomical data. A FITS file is organized into one or more Header/Data Units (HDUs): a human-readable ASCII header of keyword/value metadata followed by the associated data block (such as an image array or a table). FITS is designed to be self-describing so the file includes the information needed to interpret the data.

Background

FITS is widely used in astronomy for sharing, storing, and processing observations because it combines data and metadata in a consistent, standardized structure. It commonly stores 2D images (and higher-dimensional arrays), but it can also contain tabular data and multiple “extensions” in one file, enabling complex datasets to travel as a single artifact.

In practical workflows, FITS files are often produced by telescopes, observatories, and scientific instruments, then inspected visually in specialized viewers and analyzed with scientific software. The format’s headers are plain text, which makes it easier to inspect metadata such as instrument settings, coordinate systems, and calibration information.

Many applications rely on established FITS libraries for reliable reading/writing; CFITSIO is a widely used library in the astronomy ecosystem. For scripting and analysis, Astropy provides a common Python interface for working with FITS HDUs, headers, images, and tables.

Common MIME types: image/fits

Further reading

Authoritative resources for more details on the .FITS format.

Common .FITS issues

The file won’t open in my normal photo viewer

FITS is a scientific data format and is not broadly supported by consumer image apps, even though it can store images.

  1. Open it with a FITS-capable viewer such as SAOImage DS9.
  2. If you need to process the data, use Python with astropy.io.fits to read the HDUs and extract the image array.

Opens, but looks black/blank or the contrast is wrong

Astronomical FITS images often contain high dynamic range data, non-8-bit pixel types, or values that require scaling/stretching to display properly.

  1. In a FITS viewer (e.g., DS9), adjust the display scale/stretch (common for astronomical images).
  2. Check the header keywords and data type by reading the file with astropy.io.fits to understand scaling and value ranges.

Error reading FITS: corrupted/truncated file or invalid header

FITS readers expect a strict structure (HDU headers and data blocks). Interrupted downloads or partial transfers can break that structure.

  1. Re-download or re-copy the file from the original source and compare file sizes if possible.
  2. Try opening with another FITS reader/library; many tools rely on CFITSIO, which may provide clearer error messages.
  3. If you have access to the producer software/instrument pipeline, regenerate or re-export the FITS file.

The FITS file has multiple extensions and I can’t find the image/table I need

A single FITS file can contain multiple HDUs (primary HDU plus extensions like images or tables). Some viewers open only the first HDU by default.

  1. In DS9, browse/select different extensions (HDU list) to view the desired image/plane.
  2. In Python, use astropy.io.fits to list HDUs and select the correct extension by index or name.

FAQ

What does a .FITS file usually contain?

Often an astronomical image (2D or multi-dimensional array), but it can also contain tables and multiple extensions (HDUs) in a single file, along with detailed metadata in the headers.

Is .fits the same as .fit?

They are commonly used for the same FITS format in practice, but your software may prefer one extension. If a file won’t open, don’t rename it blindly—use a FITS-aware tool that detects the format from the file content.

What are the correct MIME types for FITS downloads/uploads?

Registered media types include image/fits and application/fits (per IANA registrations and RFC 4047). Which one is used depends on the context and what the file contains.

Can I convert FITS to a normal image like PNG or JPEG?

Yes, but conversion usually requires choosing scaling/contrast because FITS data is often scientific/high dynamic range. Open the FITS in a viewer (e.g., DS9) or load it in Python (Astropy) and export after applying an appropriate stretch.

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