What Is a FITS File? The Complete Guide for Astrophotographers

Every serious astrophotograph begins as a FITS file. If you've downloaded an astrophotography dataset, received data from a remote telescope session, or browsed public observatory archives, you've encountered files with a .fits or .fit extension. Here's exactly what they are and how to use them.

FITS: The Universal Language of Astronomical Imaging

FITS stands for Flexible Image Transport System. Developed in 1981 by the International Astronomical Union and NASA, it's the universal format for scientific astronomical data — used by the Hubble Space Telescope, ESO's VLT, ALMA, and every serious observatory on Earth and in orbit.

Think of a FITS file as the astronomical equivalent of a camera RAW file — but with far more dynamic range, richer metadata, and no proprietary encoding. Unlike JPEG or PNG, FITS was designed for science, not display. Its goal is to preserve every photon count, exactly as recorded by the detector.

What Makes FITS Different from Regular Image Formats

Bit depth: Consumer cameras output 8-bit images (256 brightness levels per channel). Astronomy cameras produce 16-bit FITS files — 65,536 levels, or 256× more tonal resolution. This is essential for capturing both the blazing core of a nebula and its faint outer wisps in the same frame.
No lossy compression: Every photon count is preserved exactly as measured. FITS never discards data for file size.
Linear pixel values: FITS data is not gamma-corrected. Raw pixel values directly represent photon counts. This is essential for accurate calibration, photometry, and meaningful scientific measurements.
Embedded metadata headers: Every FITS file contains a human-readable block of text describing when, where, how, and with what the image was captured.
Multi-dimensional support: FITS stores 2D images, 3D data cubes (spatial + wavelength), and time series — making it equally useful for spectroscopy, interferometry, and imaging.

Reading a FITS Header

Open any FITS file in a viewer and you'll find a structured header like this:

SIMPLE  =                    T / Standard FITS format
BITPIX  =                   16 / 16-bit unsigned integers
NAXIS1  =                 9576 / Image width in pixels
NAXIS2  =                 6388 / Image height in pixels
EXPTIME =                  300 / Exposure time in seconds
FILTER  =              'Ha    '/ Filter used
DATE-OBS= '2026-01-20T02:14:33'/ UTC date of observation
INSTRUME= 'QHY600M '           / Camera model
OBJECT  =              'M42   '/ Target name
GAIN    =                   26 / Camera gain
AIRMASS =                1.234 / Atmospheric airmass at midpoint

This metadata tells your processing software exactly what it's working with — no guessing, no manual entry required.

Software to Open and View FITS Files

FITS files cannot be opened in Lightroom, Photoshop, or macOS Preview. You need dedicated software:

For Viewing

AstroImageJ: Free, Java-based. Excellent for quick inspection, basic photometry, and viewing FITS headers. Download from the University of Louisville astronomy site.
DS9: NASA's professional FITS viewer. Supports coordinate overlays, multi-extension FITS, and complex stretch functions. Free.
FITS Liberator: ESA/NASA free tool designed specifically for producing display-quality images from scientific FITS data. Beginner-friendly.

For Processing

Siril: Free, open-source, cross-platform. Handles the complete pipeline from calibration through stacking to finished image. The best free option for astrophotography FITS processing. Excellent documentation.
Astro Pixel Processor (APP): Paid (~€75 perpetual). User-friendly with good automation. Popular choice for beginners who want results quickly.
PixInsight: The professional standard. Unmatched processing power. Steep learning curve. €230 perpetual license. Worth it for serious work.

Calibration Frames: What They Are and Why You Need Them

Raw FITS light frames from a telescope contain more than just your target's signal — they include thermal noise, dust on the sensor, optical vignetting, and read noise. Calibration frames measure and remove these artifacts:

Dark frames: Taken with the lens cap on, same exposure duration and camera temperature as your lights. Captures the thermal noise pattern specific to your camera and temperature. Subtract from lights to remove hot pixels and glow.
Flat frames: Short exposures of a uniformly lit surface (flat panel or twilight sky). Captures vignetting, dust motes, and pixel sensitivity variation across the sensor. Divide lights by flats to correct.
Bias frames: Zero-second exposures (or the shortest possible shutter speed). Captures the electronic read noise offset the camera adds to every pixel. Combined with darks to improve accuracy.

Never skip calibration when processing premium data. Skipping calibration from a professional 16-bit dataset is like developing a medium-format film negative with cheap chemicals — you're throwing away what you paid for.

All SkyShare Astro datasets and booking sessions include pre-made master calibration frames — you apply them in one step, no manual calibration capture required.

The FITS Processing Pipeline Step by Step

Load calibration masters: Import your master dark, master flat, and master bias into Siril or PixInsight
Calibrate lights: The software subtracts the dark from each light frame and divides by the flat — removing thermal noise and optical artifacts
Debayer (color sensors only): Convert the raw Bayer color matrix to separate RGB channels
Register: Align all calibrated frames precisely by calculating the transformation that maps star positions between frames. Atmospheric turbulence shifts every frame slightly.
Integrate: Stack the aligned frames using sigma-clipped mean or Winsorized rejection. This dramatically reduces random noise while preserving signal — the fundamental math of why more frames equals a better image.
Stretch: Apply a non-linear stretch to bring faint signal into the visible range. The raw stacked image looks almost entirely black — the data is there, compressed into the lowest few percent of the histogram.
Process: Noise reduction, color calibration, deconvolution/sharpening, star treatment, and aesthetic adjustments.

Where to Get FITS Data

SkyShare Astro datasets: Our pre-captured dataset store offers calibrated FITS stacks from our Bortle 1 Atacama observatory, ready for final processing. The calibration and stacking is done — you start at step 6 (stretching).
Book a telescope session: Rent imaging time and receive your own raw sub-frames from a professional instrument. $50/session.
NASA Hubble Legacy Archive: Free public access to Hubble Space Telescope FITS data at hla.stsci.edu — world-class data for learning and practice.

FITS files are the gateway to real astronomical imaging. Once you understand the pipeline — calibrate, register, integrate, stretch — you can produce images that rival professional observatories. Every photon that traveled millions of light-years to reach the sensor is waiting for you in those files.