FITS Image Viewer & Converter

RGB Synthesis (0/3)

Understanding FITS Files

What Is a FITS File?

FITS (Flexible Image Transport System) is the international standard for astronomical image data, endorsed by NASA, ESA, and IAU. Developed in 1981, it stores raw pixel data alongside rich metadata headers — the universal language of ground and space telescopes.

FITS Standard v4.0

FITS Header & WCS Metadata

Every FITS file carries a human-readable ASCII header describing the instrument, observation time, coordinates, and wavelength. The World Coordinate System (WCS) keywords map pixel positions to celestial coordinates — essential for astrometry and photo plate solving.

FITS WCS Standard

Color Maps in Astronomy

Raw FITS data is monochrome intensity. Color maps (Viridis, Heat, Inferno) map pixel values to visible colors. SAOImage DS9 pioneered the standard color table conventions still used by this viewer — 11 perceptually uniform palettes for scientific visualization.

SAOImage DS9

FITS vs TIFF vs RAW

Unlike TIFF or camera RAW, FITS is natively multidimensional — a single file can hold multi-wavelength data cubes, time series, or polarization maps. It is the only archival format accepted by most observatories and astronomy journals.

NASA FITS Support Office

Frequently Asked Questions

What is a FITS file?
FITS (Flexible Image Transport System) is the standard file format for astronomical images used by NASA, ESA, and observatories worldwide. It stores image data, metadata headers, and can contain multiple data dimensions — from simple 2D images to complex 3D data cubes.
Do my FITS files leave my device?
No — all rendering is 100% local. FITS data never leaves your browser. Canvas output is generated client-side. You can use FITS Viewer completely offline after the initial page load.
What color maps are supported?
This viewer offers 11 color maps including Grayscale, Viridis, Heat, Inferno, Magma, Rainbow, Cubehelix, Twilight, and more. You can also adjust stretch modes (linear, log, sqrt) and fine-tune brightness/contrast with the histogram slider.
Can I combine multiple FITS files into a color image?
Yes. Import 3 FITS files, assign each to Red, Green, or Blue channel, and the viewer composites them into a single RGB image using the selected stretch for each channel. Ideal for multi-filter observations from the same target.

Want more processing power?

AstroSky is a full-featured Windows desktop app with intelligent auto-stretch, 12+ color maps, beauty/science dual mode, 8K wallpaper export, and WCS support — all GPU-accelerated and 100% offline.

Download from Microsoft StoreLearn more about AstroSky

All FITS image rendering runs 100% locally in your browser — no files are uploaded to any server. Canvas output is generated client-side. FITS data never leaves your device.

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All tools on fastool.io run entirely in your browser — zero data leaves your device. No personal data is collected, stored, or transmitted to any server. Solar calculations use SunCalc.js; lunar data uses JPL DE440 ephemeris; coordinate transforms use publicly documented EPSG/OGC standards. This site requires no signup, no account, and no cloud processing.