Once you've captured your subs, the real work begins. Choosing the right image processing software can make or break your results — and the three dominant options couldn't be more different from each other. PixInsight is the professional's workhorse. Siril is the free and increasingly capable challenger. Astro Pixel Processor (APP) sits between them as the friendliest route to high-quality stacks.
This comparison cuts through the noise with a clear breakdown of what each tool actually delivers, who it's best suited for, and whether the price is worth it.
Quick Summary
| Software | Price | OS | Learning Curve | Best For |
|---|---|---|---|---|
| PixInsight | €230 one-time | Win/Mac/Linux | Steep | Advanced imagers, full control |
| Siril | Free | Win/Mac/Linux | Moderate | Beginners, budget setups, OSC |
| Astro Pixel Processor | €75/yr or €160 life | Win/Mac/Linux | Gentle | Fast quality results, mosaics |
PixInsight
PixInsight 1.8 Ripley
PixInsight has been the gold standard in astrophotography processing for over a decade. It's not just software — it's an entire ecosystem of algorithms specifically built for astronomical imaging: Blink for frame rejection, Weighted Batch Preprocessing (WBPP) for automated calibration and stacking, BlurXTerminator and NoiseXTerminator for AI-driven deconvolution and noise reduction, and a complete tone-mapping toolchain from gradient removal through final export.
- Unmatched precision and control
- Huge tutorial ecosystem (YouTube, LightVortex, etc.)
- Best-in-class deconvolution tools
- Excellent narrowband workflow support
- Script/process scripting for automation
- Active development, frequent updates
- Steep, sometimes frustrating learning curve
- UI is dated and unintuitive
- €230 is a significant upfront cost
- Some basic tasks require 5-step workflows
- Overwhelming for beginners
The turning point for most PixInsight users is understanding its process icons and scripting system. Once you build a workflow you like, you can save it and replay it on future datasets in minutes. This makes it incredibly powerful for imagers who shoot the same targets repeatedly or have a consistent post-processing style.
The AI add-ons — BlurXTerminator (deconvolution) and NoiseXTerminator (noise reduction) from RC-Astro — cost extra but have become almost mandatory for serious PixInsight users. They're available in the PixInsight Resource Exchange and cost around $40–80 each. Budget for them.
Who should use PixInsight: Imagers who've been doing this for a year or more, shoot narrowband, want the absolute best output, and are willing to invest time learning. If you shoot Hubble Palette or do high-resolution planetary compositing, PixInsight is in a class of its own.
Siril
Siril 1.x
Siril has gone from a niche Linux tool to a legitimate PixInsight alternative in the span of a few years. It handles the entire pipeline: calibration frames (darks, flats, bias), registration, stacking with sigma clipping, and a growing suite of processing tools including gradient removal, colour calibration, deconvolution, and even starless extraction. The 2024/2025 releases added a script system that makes batch processing practical.
- Completely free, forever
- Excellent OSC (colour camera) workflow
- Good documentation and active forum
- Fast stacking even on modest hardware
- Graxpert integration for gradient removal
- Cross-platform, runs on Linux
- Processing tools still maturing vs PI
- Fewer advanced deconvolution options
- Mosaic support less robust than APP
- UI can be confusing to navigate
- Fewer tutorials than PixInsight
Siril's biggest win over PixInsight is the price: zero. For someone just starting out with a Seestar S50 or an entry-level refractor and OSC camera, Siril gets you from raw subs to a shareable image in an evening without spending a cent. Its comet stacking mode is better than most, and its OSC demosaicing pipeline is well-regarded in the community.
Who should use Siril: Beginners, anyone on a tight budget, Linux users, and OSC colour camera shooters. Also great as a free stacking back-end if you do your processing in Photoshop or Affinity Photo.
Astro Pixel Processor (APP)
Astro Pixel Processor 2.x
APP occupies a sweet spot that PixInsight and Siril don't cover: it's genuinely easy to use, produces excellent stacks quickly, and handles mosaics better than almost anything else in its price range. The workflow is largely automated — load your subs, point it at your calibration frames, hit Integrate, and APP handles normalisation, registration, and stacking with minimal hand-holding. It also handles OSC camera stacking and narrowband with equal competence.
- Best mosaic stitching in the category
- Fastest route from subs to clean stack
- Great colour calibration tools
- Handles large datasets efficiently
- Dedicated support and frequent updates
- Works brilliantly for both OSC and mono
- Subscription model frustrates some users
- Post-stack processing tools less mature than PI
- Smaller tutorial community
- Not free (though lifetime option helps)
Where APP really shines is mosaics. If you're shooting a two-panel or four-panel mosaic of a large object like the Andromeda Galaxy or the Veil Nebula, APP's mosaic integration engine handles gradient matching and stitching almost automatically. PixInsight's mosaic workflow is powerful but considerably more complex to set up.
Who should use APP: Intermediate imagers who want professional results without spending weeks on a learning curve. Ideal for mosaic shooters, anyone switching from a smart telescope to traditional imaging, and users who find PixInsight overwhelming.
What About Photoshop and Lightroom?
Photoshop remains a popular tool for the final touches on astrophotography: colour grading, selective sharpening, noise masking, and compositing. It's rarely used for calibration or stacking (it lacks the astronomical algorithms needed). Many imagers use PixInsight or APP for the stack and gradient removal, then jump to Photoshop for the creative processing. Lightroom is useful for batch exporting and some colour work, but its limited layer and masking system makes it a weak choice for astrophotography specifically.
The 2026 Emerging Option: Graxpert + AI Stack
Graxpert is a free, open-source gradient removal and background extraction tool that many imagers now use alongside both Siril and PixInsight. Its AI-powered gradient correction often outperforms PI's Dynamic Background Extraction on difficult widefield data. Pair it with Siril for stacking and you have a surprisingly capable free pipeline. Keep an eye on it — the development pace has been rapid.
Recommended Workflows by Setup
| Your Setup | Recommended Stack |
|---|---|
| Smart telescope (Seestar, Dwarf III) beginner | Siril (free) → Photoshop/Affinity |
| OSC camera, first year imaging | Siril or APP → Photoshop |
| Mosaic projects, intermediate | APP → PixInsight for post-stack |
| Narrowband mono, advanced | PixInsight end-to-end with BlurXTerminator |
| Budget-focused, any level | Siril + Graxpert (100% free) |
Bottom Line
Start with Siril — it's free, capable, and will tell you a lot about what you actually need from processing software. If you outgrow it and want mosaic power, move to APP. If you want total control and the best possible narrowband output, invest in PixInsight and budget a few months for the learning curve.
The cameras and mounts on AstroCompare are all capable of generating data that any of these tools can process beautifully. The limiting factor is almost always time spent learning — not the software itself.