Stacking is the foundation of astrophotography processing. You combine dozens or hundreds of short exposures to produce a single image with far higher signal-to-noise ratio than any single frame. The software you use for this step matters, but the good news is that excellent free options exist.

This guide compares the two most popular free tools — Siril and DeepSkyStacker — and briefly covers the paid alternatives, so you can choose the right tool for your workflow.

Why stacking matters

A single astrophotography exposure is noisy. The signal (light from the target) is buried under random noise from the camera sensor, sky background, and thermal effects. Stacking multiple exposures averages out the noise while preserving the signal, improving the signal-to-noise ratio by approximately √N (where N is the number of frames).

Frames stacked Relative SNR improvement
1 1×
4 2×
16 4×
64 8×
100 10×

More frames = cleaner data. But the stacking software must align and combine frames correctly for this to work.

DeepSkyStacker (DSS)

Overview

DeepSkyStacker is a free, Windows-only stacking application that has been a staple of amateur astrophotography for over 15 years. It is purely a stacking tool — it does not do post-processing.

Strengths

  • Simple interface — drag and drop your files, configure settings, and click "Stack"
  • Reliable calibration — handles darks, flats, bias, and dark flats correctly
  • Multiple stacking methods — average, median, sigma clipping, kappa-sigma, and more
  • Large community — extensive documentation and forum support
  • Low learning curve — you can produce good results on your first attempt

Limitations

  • Windows only (no macOS or Linux)
  • No post-processing — you must use a separate tool (Photoshop, GIMP, Siril, etc.) for stretching and colour work
  • Slower with large datasets — performance can lag with thousands of frames or very large sensor files
  • Development pace — updates have been infrequent in recent years

Best for

Beginners who want a dedicated, simple stacking tool and are comfortable doing post-processing elsewhere.

Siril

Overview

Siril is a free, open-source astrophotography processing suite available on Windows, macOS, and Linux. Unlike DSS, it handles the entire workflow: calibration, registration, stacking, and post-processing.

Strengths

  • Full pipeline — calibration, stacking, stretching, colour calibration, background extraction, and more in one application
  • Cross-platform — works on Windows, macOS, and Linux
  • Scriptable — you can automate the entire calibration-to-stack pipeline with built-in scripts
  • Active development — regular updates, new features, and responsive developers
  • Photometric colour calibration — automatically calibrates star colours using catalogue data
  • Good performance — handles large datasets efficiently

Limitations

  • Steeper learning curve — the interface is more complex than DSS
  • Documentation is improving but still developing — some features are not fully documented
  • Script syntax requires learning — automation is powerful but not immediately intuitive

Best for

Imagers who want a single free tool for the entire workflow, or who use macOS/Linux.

Head-to-head comparison

Feature DeepSkyStacker Siril
Price Free Free (open source)
Platform Windows Windows, macOS, Linux
Calibration ✅ ✅
Registration (alignment) ✅ ✅
Stacking ✅ (multiple methods) ✅ (multiple methods)
Post-processing ❌ ✅ (stretching, colour, noise reduction)
Scripting/automation ❌ ✅
Photometric colour calibration ❌ ✅
Background extraction ❌ ✅
Ease of use ⭐⭐⭐⭐⭐ ⭐⭐⭐
Community/support Large, mature Growing, active

Paid alternatives

PixInsight

The gold standard for advanced astrophotography processing. Extremely powerful, deeply configurable, and used by many serious amateurs and professionals.

  • Price: ~€230 (one-time licence)
  • Strengths: Unmatched processing depth, excellent noise reduction, advanced statistical tools, huge script ecosystem
  • Limitations: Very steep learning curve, complex interface, high price for beginners
  • Best for: Dedicated astrophotographers who want maximum control and are willing to invest time learning

Astro Pixel Processor (APP)

A newer paid tool focused on making the calibration-to-stack pipeline more intuitive, with a strong emphasis on mosaic panel alignment.

  • Price: ~€150 (one-time licence)
  • Strengths: Excellent mosaic support, intuitive interface, integrated processing
  • Limitations: Smaller community than PixInsight, less post-processing depth
  • Best for: Imagers who shoot mosaics regularly or want a simpler paid workflow

Who should use what?

Situation Recommendation
Complete beginner, Windows DeepSkyStacker (stack) + GIMP or Photoshop (post-process)
Beginner wanting one free tool Siril
macOS or Linux user Siril
Intermediate imager wanting automation Siril (scripted pipeline)
Advanced imager wanting maximum quality PixInsight
Mosaic specialist Astro Pixel Processor or PixInsight
Budget-conscious but serious Siril (free) or Siril + GIMP

Practical tips regardless of software

  1. Always calibrate. Apply darks, flats, and bias frames before stacking. This removes fixed-pattern noise and vignetting. See the Calibration Frames guide for details.
  2. Use sigma-clipping rejection during stacking to remove satellites, cosmic rays, and aircraft trails.
  3. Stack in 32-bit (or at least 16-bit) to preserve dynamic range.
  4. Do not stretch before stacking. Stacking works best on linear data.
  5. Save your stacked master as FITS or TIFF — avoid JPEG at this stage.

FP Softlab context

FP Softlab's deep-sky gallery features images processed through various stacking and processing pipelines. The classic software tools in the products section represent an earlier era of visualisation that complements modern astrophotography workflows.

Further reading