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Deep-Sky Stacking & Processing

Siril, DeepSkyStacker, GraXpert, PixInsight — calibrate, register, integrate, and stretch deep-sky data.

The "200 sub-exposures into one finished image" pipeline. Calibrate (subtract darks, divide by flats, subtract bias), register (align stars), integrate (combine — usually sigma-rejected average), then post-process (gradient removal, stretching, noise reduction, deconvolution, star removal, color calibration). The OSS stack is genuinely strong in 2026 — Siril + GraXpert + StarNet++ does the whole pipeline for free. PixInsight + Russell Croman's "X" suite remains the paid gold standard for serious work.

For solar / lunar / planetary stacking (totally different pipeline) see Solar / Lunar / Planetary Stacking; for star removal / AI tools see Star Removal & AI Processing; for the upstream capture see Image Acquisition; for general raster post-processing of the finished image see Photo RAW Editors and Image Editing; for cataloging the masters see Photo DAM & Management and Self-Hosted Photos & Media.

Free / OSS — the modern OSS pipeline

  • ★ ★ Siril — GPL, Win/macOS/Linux + ARM. The 2026 OSS deep-sky pipeline reference. Calibration, registration (star, comet, mosaic), stacking (winsorized sigma clip, generalized extreme studentized deviate, average), photometric color calibration, gradient removal, deconvolution, star removal (built-in via plug-in), starless processing, scriptable. Pyxis-based scripts (OSC_Preprocessing, Mono_Preprocessing) get you from raw subs to stretched master in minutes. Replaced DeepSkyStacker as the OSS default for most users in 2024–25.
  • ★ ★ GraXpert — GPL, Win/macOS/Linux. AI-based gradient removal + stretching + denoise, free. Drop a TIFF in, get a flat-field, denoised, stretched image out. The 2024–26 free-tools surprise hit; rivals PixInsight DBE / NoiseXTerminator on many images. Bundled into Siril's plug-in menu.
  • ★ ★ StarNet++ V2 — free (binary; non-commercial). Neural-net star removal; lets you process galaxies / nebulae without star bloat. GPU-accelerated. Bundled into Siril and N.I.N.A. See Star Removal.
  • ★ DeepSkyStacker (DSS) — free Win-only. The classic; still actively maintained; great for DSLR shooters; simple UI; the gentlest learning curve. Less aggressive sigma rejection than Siril but reliable.
  • Sequator — free Win-only; the nightscape stacker (Milky Way + foreground). Auto-segments sky/foreground, stacks the sky, freezes the foreground; perfect for landscape astro. Sibling to Starry Landscape Stacker (Mac). Use Sequator before reaching for Siril if your subject is "Milky Way over a mountain."
  • Starry Landscape Stacker — paid (~$40) macOS-only; same role as Sequator on Mac.
  • Astrosharp / AstroBin tools — free webapp for quick measurements; not a stacker.
  • ★ ★ PixInsight (Pleiades Astrophoto) — paid (~$280 perpetual one-time + free updates for 1 year, then ~$100/yr for continued updates). The professional reference for deep-sky processing for 15+ years; immense process library (Dynamic Background Extraction, MultiscaleLinearTransform, ABE, ACDNR, MUREDenoise, PixelMath, Range Selection, …); script-friendly (JavaScript); steep learning curve; not optional for serious narrowband / mosaic / multi-night work. Cross-platform. Honest take: expensive and steep, but most published "wow" images go through it.
  • ★ ★ Russell Croman's "X" suite — paid (~$200 each, often bundled): BlurXTerminator (PSF-deconvolution sharpening; closed-source neural; the single biggest sharpness gain available in 2024–26), NoiseXTerminator (SOTA noise reduction), StarXTerminator (star removal — better than StarNet++ V2 on most images). Sold as PixInsight processes and Photoshop plug-ins. Closed-source; no free trial; community consensus is they pay for themselves on the first project. The biggest shift in deep-sky processing since Pixinsight itself.
  • AstroPixelProcessor (APP) — paid (~€150 perpetual or €50/yr). Easier than PixInsight, especially mosaic / multi-night / multi-camera; great quality-of-life for hobbyists who don't want to learn PI's whole vocabulary. Cross-platform. The strongest paid alternative to PixInsight for "I want quality without 100 hours of YouTube."
  • StarTools — paid (~$60) Win/Mac/Linux. Idiosyncratic "tracking-data-aware" pipeline (knows what each step did to noise); cult following; UI not for everyone.
  • AffinityPhoto with astro persona — paid (~$70 perpetual). Astrophotography-aware persona ships in Affinity Photo 2; gradient removal, star reduction, Bayer demosaic for FITS. Not a complete pipeline but a strong companion to Siril for finishing.
  • Adobe Photoshop — paid subscription. Universal finishing app; not a stacker; reach for it for masking, color tweaks, layered composites, after Siril/PI/APP have produced a master TIFF. See Image Editing.

Mobile

  • AstroSharp (paid Android) — basic stacking on phone shots.
  • NightCap / Halide Mark II (paid iOS) — long-exposure capture; not stacking.
  • Generally do mobile stacking on a desktop afterwards via Sequator / Siril.

A typical OSS pipeline (Siril script flavor)

  1. Calibration: build masters (stack on bias/dark/flat), apply per sub.
  2. Cosmetic correction: hot/cold pixel removal.
  3. Debayer (one-shot color cameras only).
  4. Registration: global star alignment.
  5. Stacking: winsorized sigma-clip average → master light.
  6. Photometric color calibration (Siril) — sets white balance physically.
  7. Gradient removal — Siril or GraXpert.
  8. Stretch — generalized hyperbolic stretch (Siril) or asinh.
  9. Star removal (StarNet++ V2 or StarXTerminator) → starless and stars layer.
  10. Process starless for nebula contrast / saturation; recombine with stars softly.
  11. Denoise (GraXpert or NoiseXTerminator).
  12. Sharpen (BlurXTerminator if paid; deconvolution if not).
  13. Final color tweaks in Photoshop / Affinity Photo / GIMP.

What's changing in 2024–2026

  • GraXpert is genuinely closing the gap with PI's gradient removal and noise reduction; the gap between OSS and paid narrowed sharply.
  • Russell Croman's "X" suite is the new must-have if you can pay; community calls BlurX "magic."
  • AI denoise convergence — GraXpert AI Denoise vs NoiseXTerminator vs Topaz DeNoise; all close in quality, GraXpert free.
  • Siril plug-ins — GraXpert and StarNet++ are now first-class menu items; the OSS pipeline is one app for most users.
  • Smart-scope JPEGs — Seestar / Vespera / Dwarf produce stackable raw stacks that you can re-process in Siril for much better results than the in-app JPEG.

License / pricing summary

  • Free / OSS: Siril, DeepSkyStacker, Sequator (free Win), GraXpert, StarNet++ V2 (binary).
  • Paid (~$40–$70): Starry Landscape Stacker (Mac), StarTools, AffinityPhoto.
  • Paid (~$150–$280): AstroPixelProcessor, PixInsight.
  • Paid add-ons (~$200 each, often bundled ~$500): BlurXTerminator, NoiseXTerminator, StarXTerminator.

Pick this if…

  • Free, complete OSS pipeline: Siril + GraXpert + StarNet++ V2.
  • DSLR-only, simplest learning curve: DeepSkyStacker → Photoshop.
  • Nightscapes (Milky Way over landscape): Sequator (Win) or Starry Landscape Stacker (Mac).
  • Pro / serious narrowband / mosaic, paid: PixInsight + Russell Croman's "X" suite.
  • Paid but easier than PI: AstroPixelProcessor + RC X suite.
  • Best-bang-for-buck $200 paid plug-in: BlurXTerminator (works in PI or Photoshop).
  • Smart-scope owner who wants to re-process the data: dump RAW stack → Siril.

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