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Astrophotography data guides

How do I process downloaded FITS data?

Process downloaded FITS data in four stages: calibrate, register, integrate and stretch. The eight steps below follow that order. On the tool side, Siril is free and PixInsight is a paid license, so check pixinsight.com for the current price (as of 2026-09).

Which software should a beginner use?

Siril. It is free and its workflow mirrors the four stages above (as of 2026-09). PixInsight is the common next step for advanced processing; it is a paid license, so check pixinsight.com for the current price (as of 2026-09).

Start with Siril, push one full dataset through it from calibration to the final image, and only then decide whether you need paid tools.

  1. Download the dataset and check the file list against the seller description.
  2. Inspect the FITS headers to confirm exposure, gain, sensor temperature and filter for every frame.
  3. Calibrate the light frames with darks, flats and bias or dark-flats.
  4. Register the calibrated frames, aligning the stars across all lights.
  5. Integrate the aligned frames into a master light.
  6. Remove gradients and light pollution from the stack.
  7. Stretch the master light to bring faint structure out of the histogram.
  8. Balance color and saturation, then export your final image.

What order do the steps go in?

Calibrate, then register, then integrate, then stretch. Gradient removal fits after integration and color work comes last. Skipping calibration is a common beginner mistake and it shows up in the final image.

What does calibration actually fix?

Calibration removes the camera's own signature from your lights. Darks fix the sensor pattern, flats fix the optical pattern, and bias or dark-flats make the darks usable. Without this, stacking averages your camera defects together with the sky signal.

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