Simple FITS Viewer

A Python/Tkinter desktop application for viewing and exploring FITS astronomy image files.

Features

• Open single files or entire directories of FITS images
• Frame navigation — step through multi-frame FITS files or a directory of images using playback controls
• Zoom controls — Zoom In, Zoom Out, Fit to window, and 1:1 pixel view
• Stretch modes — Linear, ZScale, and other contrast stretches
• Colormaps — gray, heat, viridis, and more
• Interactive histogram — drag black/white level markers or use sliders; Auto Stretch button
• WCS coordinate readout — live X, Y, ADU, RA, and Dec display as you move the cursor
• FITS Header viewer — browse the full header in a resizable window
• Toolbar menus — File, View, Image, and Help (User Guide, Revision History, About)

Requirements

• Windows 10/11
• Python 3.10+ with the following packages:
  • astropy
  • astropy_iers_data
  • numpy
  • matplotlib
  • Pillow

Installation

pip install astropy astropy-iers-data numpy matplotlib Pillow
python fits_viewer.py

Building the Executable

Requires PyInstaller:

pip install pyinstaller
pyinstaller fits_viewer.spec

The onedir build will be created in the dist/ folder.

Note: The PyInstaller spec must collect data files for astropy, astropy_iers_data, asdf, and asdf_astropy — the ASDF JSON schemas are required at runtime.

Usage

1. Click Open to load a single FITS file, or Open Dir to load all FITS files in a folder.
2. Use the Stretch and Colormap dropdowns to adjust the display.
3. Adjust contrast with the histogram sliders or click Auto Stretch.
4. Use the playback controls at the bottom to navigate between frames or files.
5. Hover over the image to see live pixel coordinates and WCS (RA/Dec) in the toolbar.
6. Click Header to inspect the full FITS header.

Version History

Version	Notes
1.2.0	Fixed 1 fps playback (threaded loader); trough-click slider override; header window stays open; toolbar RA/Dec readout; Help menu
1.1.x	Histogram controls; Auto Stretch; colormap selector
1.0.0	Initial release

License

MIT License — © Arthur T. Trail 2026. See LICENSE for details.