How to open FITS files

FITS files (Flexible Image Transport System) are the standard format for astronomical images and data. If you've received a FITS file and aren't sure how to open it, this guide walks you through every option — from quick online viewers to professional desktop software.

Option 1: Open FITS files online (quickest)

The fastest way to open a FITS file is to use a free online FITS viewer. No software installation, no account required.

1. Go to fitsviewer.com
2. Click “Open FITS file” or drag your .fits, .fit, or .fts file onto the page
3. The image loads instantly in your browser
4. Use colormaps, stretch, zoom, and region tools to explore

Best for: Quick checks, sharing with colleagues, classroom demonstrations, or when you don't have admin rights to install software.

Option 2: SAOImage DS9 (most popular desktop viewer)

DS9 is the most widely used FITS viewer in professional astronomy. It's free and available for Windows, Mac, and Linux.

1. Download DS9 from the official SAO website
2. Install and open the application
3. Go to File → Open and select your FITS file
4. Use the Scale, Color, and Zoom menus to adjust the display

Best for: Professional analysis, working with very large files, advanced features like region analysis and catalog overlay.

Option 3: Aladin Sky Atlas

Aladin is a interactive sky atlas from the Strasbourg astronomical Data Center (CDS). It's excellent for overlaying FITS images on survey data and catalogs.

1. Download Aladin from the CDS website
2. Open the application and load your FITS file via File → Open
3. Navigate, zoom, and overlay catalog data

Best for: Cross-matching with astronomical catalogs, multi-wavelength comparisons, survey data visualization.

Option 4: FITS Liberator

FITS Liberator (by NOIRLab) is designed for creating color images from FITS data. It excels at stretching and combining multiple FITS files into visual images.

Best for: Astrophotography processing, creating publication-quality color images from FITS data.

Option 5: Python with Astropy

For programmatic access, the Astropy library in Python provides full FITS support:

Best for: Automated processing pipelines, custom analysis scripts, batch processing of many FITS files.

Which option should you choose?

Related resources

• What is a FITS file?
• Online FITS viewer
• Frequently Asked Questions