Gradient removal in 2026: GraXpert and its alternatives

Gradients: the urban astrophotographer's number one enemy

Even under quality skies, your stacked images almost always exhibit a brightness gradient: a parasitic gradation caused by light pollution, the Moon, optical vignetting, or simply atmospheric scattering. This gradient masks faint details and distorts colors.

Removing it is one of the first processing steps. For a long time, it was also one of the most tedious. Then GraXpert came along and changed the game with its one-click AI mode.

But GraXpert is not alone. Here is a complete overview of the tools available in 2026.

GraXpert: the free reference

GraXpert is a free and open-source software available on Windows, macOS, and Linux. It offers two modes:

Manual mode

You place sampling points on "empty" sky areas (without objects), and GraXpert models the gradient from those points. The more you place, the more precise the model. This is the classic method, similar to DBE in PixInsight.

AI mode

A pre-trained neural network identifies and subtracts the gradient automatically. In practice, the result is comparable to a well-configured DBE, in a fraction of the time.

Strengths and limitations

Strengths:

• Free and cross-platform
• The AI mode is remarkably effective for 90% of cases
• Supports the XISF format for saving sampling points
• Simple and clean interface

Limitations:

• Sampling positions are lost if you restart (unless exported as XISF)
• No native batch processing
• May sometimes subtract extended nebulosity in AI mode

Siril: built-in gradient removal

Since version 1.4, Siril includes its own gradient removal, partially based on GraXpert's code. Two methods are available:

Polynomial

Models the gradient with a polynomial surface (degree 1 to 4). Fast and effective for simple, linear gradients.

RBF (Radial Basis Function)

Uses radial basis functions to model complex, non-linear gradients. More flexible than the polynomial method, but requires careful placement of sampling points.

Why this is interesting

Direct integration into Siril means you can remove the gradient without leaving your processing software. No back-and-forth between applications, no intermediate export/import. For a smooth workflow, it is a significant time saver.

PixInsight: three tools, three philosophies

PixInsight (perpetual license ~$260) offers three distinct approaches:

ABE (AutomaticBackgroundExtractor)

The simplest of the three. ABE automatically places sampling points and fits a surface. Quick and effortless, but it can struggle with complex or multi-directional gradients.

When to use it: simple gradients, quick processing, first passes.

DBE (DynamicBackgroundExtraction)

The historical reference tool. You manually place sampling points, giving you total control over the modeling. The results are excellent, provided you invest the necessary time.

When to use it: complex gradients requiring fine control, images with extended nebulosity that needs protection.

Drawback: it is the most time-consuming tool of the bunch. Correctly placing 30 to 50 points, adjusting parameters, checking the result... expect 10 to 20 minutes per image.

MGC (MultiscaleGradientCorrection) - the newcomer

Introduced in PixInsight 1.9.0, MGC is the most advanced tool on the market. It uses:

• Multiscale wavelet analysis to separate the gradient from astronomical structures
• The proprietary MARS database (~1.35 GB) that provides a reference of the real sky
• Gaia DR3 stellar data for calibration

The result is spectacular: MGC preserves extended nebulosity and faint gradients better than any other tool, in a single pass.

The catch: MGC does not work alone. It requires a chain of prerequisites:

1. ImageSolver (astrometric solving)
2. SpectrophotometricFluxCalibration (flux calibration)
3. MGC at last

For a beginner, this is a barrier to entry. For an experienced PixInsight user, it is the Holy Grail of gradient removal.

GradientXTerminator

GradientXTerminator ($59.95, RC Astro) is a plugin for Photoshop and Affinity Photo. It handles large-scale gradients, vignetting, and circular gradients with disarming ease.

Strengths

• Works on 8/16/32-bit images, RGB and grayscale
• Very intuitive for Photoshop users
• Excellent on circular gradients (often problematic for other tools)

Limitation

• Paid, and tied to the Adobe/Affinity ecosystem
• Not suited for linear data (it works on already stretched images)

AstroFlat Pro

AstroFlat Pro (ProDigital Software) is another Photoshop plugin. Its approach is radically simple: three sliders, a real-time preview, and that is it.

It can even serve as a substitute for real optical flats, useful when you forgot to take them in the field (it happens to everyone).

Comparison table

Which solution should you choose?

Starting out with Siril? Begin with Siril's built-in gradient removal. Supplement with GraXpert in AI mode if the result is not satisfactory.

Want the best free result? GraXpert in AI mode, combined with Siril for the rest of the processing. This is the combination we recommend in the Form'Astro guide.

Using PixInsight? Try MGC if you are comfortable with the calibration chain. Otherwise, DBE remains the safe bet.

Processing in Photoshop? GradientXTerminator is an investment that pays for itself in time saved from the very first week.

Conclusion

Gradient removal has never been more accessible. In 2026, even a complete beginner can achieve a clean sky background in just a few clicks thanks to GraXpert or Siril. Paid tools bring additional finesse and power, but are no longer essential for a very respectable result.

This is exactly the Form'Astro philosophy: impressive results, with free tools, without drowning in complexity.

Download GraXpert: graxpert.com