Astrophotography Processing Guide: PixInsight vs Siril Tutorial for Beginners

Image Credit: NASA/ESA/Hubble – The Whirlpool Galaxy (M51). Advanced processing techniques can extract similar detail from amateur data. Public Domain.

Why Advanced Processing Matters

The difference between a good astrophoto and a great one is often not the data — it’s the processing. Two astrophotographers can capture identical data, and one produces an image that looks noisy and flat while the other reveals intricate detail and vibrant color. The secret is mastering advanced processing tools and understanding the science behind each step.

PixInsight: The Professional’s Choice

PixInsight (PI) is the most powerful astrophotography processing software available. It’s subscription-based (~$230 for a perpetual license) and has a steep learning curve, but it offers capabilities no other software matches.

Key PixInsight Processes

• WBPP (Weighted Batch Pre-Processing): PI’s automated calibration and stacking script. Handles darks, flats, bias, registration, and integration with sophisticated rejection algorithms (sigma clipping, winsorized sigma) that eliminate satellite trails, aircraft, and cosmic rays.
• DBE (Dynamic Background Extraction): The gold standard for gradient removal. Place sample points on the background and PI models and subtracts the gradient, leaving a perfectly flat background. Far more precise than simple polynomial fitting.
• SPCC (Spectrophotometric Color Calibration): Uses star catalogs and your filter/sensor profile to achieve scientifically accurate colors. A major upgrade from simple white-balance-based methods.
• BlurXTerminator: An AI-powered deconvolution process (third-party, by Russell Croman) that sharpens star and nebula detail with stunning results. It has become essential in modern astrophotography workflows.
• NoiseXTerminator: AI noise reduction by the same developer. Produces cleaner results than traditional noise reduction while preserving fine structure.
• StarXTerminator / Starnet++: AI star removal allowing independent processing of stars and background nebulosity, then recombination. This technique has transformed how astrophotographers handle dynamic range.
• HDRMultiscaleTransform: Recovers detail in bright cores (like the center of M42) without blowing out the rest of the image.
• HistogramTransformation: PI’s stretching tool — far more precise than Photoshop’s levels/curves for astronomical data.

Siril: The Free Powerhouse

Siril has matured enormously and now rivals PixInsight for many tasks. As of 2025-2026, it handles the complete workflow from calibration through final processing.

Key Siril Features

• Scripted preprocessing: Siril’s built-in scripts automate the entire calibration and stacking pipeline. One click processes lights, darks, flats, and bias.
• Background Extraction: Siril’s gradient removal has improved dramatically — it now supports polynomial models and manual sample placement similar to PI’s DBE.
• Photometric Color Calibration: Plate-solves your image and uses star catalogs for accurate color. Similar to PI’s SPCC.
• Asinh Stretch: A mathematically elegant stretch that preserves color ratios — star colors remain natural instead of saturating to white.
• GHS (Generalized Hyperbolic Stretch): A newer stretching method offering precise control over which tonal ranges to expand.
• Starnet integration: Siril has built-in Starnet++ support for star removal.
• Star recomposition: After processing background and stars independently, Siril can recombine them cleanly.

Advanced Processing Workflow

1. Calibrate and stack (WBPP in PI or scripts in Siril)
2. Crop stacking artifacts from edges
3. Background extraction (gradient removal)
4. Color calibration (SPCC or photometric)
5. Deconvolution/sharpening (BlurXTerminator or Richardson-Lucy in Siril) — do this while the image is still linear for best results
6. Noise reduction (NoiseXTerminator or Siril’s denoise) — also best done in linear
7. Stretch (histogram transformation, GHS, or asinh)
8. Star removal (Starnet++/StarXTerminator)
9. Non-linear adjustments: Curves, saturation, local contrast enhancement on the starless image
10. Star restoration: Process stars separately (reduce size, adjust color), then add back
11. Final polish: Minor curves, saturation, crop, and export

Tips for Better Results

• Process in 32-bit: Always work with 32-bit floating-point data until final export. This preserves the full dynamic range.
• Less is more: Over-processing is the #1 beginner mistake. Subtle, gentle adjustments produce more natural-looking results.
• Take breaks: After hours of processing, you lose perspective. Save your work, step away, and return with fresh eyes.
• Learn from others: YouTube channels like Nico Carver, Nebula Photos, and Astro Backyard provide excellent processing tutorials for both PI and Siril.

Next in our series: The James Webb Space Telescope: Latest Discoveries and What They Mean