The essentials in 30 seconds
VeraLux HMS is a free Siril script that stretches your images without destroying star colours. It installs in two clicks from Siril 1.4, is applied to the still-linear image (right after SPCC colour calibration), and gives good results with its default settings.
The problem: stars turning white
After stacking, your image is said to be linear: almost all the information is squeezed into the dark tones, and it looks nearly black. Stretching the histogram is what brings those details out.
The trouble is that classic methods (histogram, curves) damage the colours. The more you stretch, the more the stars saturate and turn white, and nebulae lose their subtle hues. You then spend time fixing what a better algorithm would have preserved from the start.
What VeraLux HMS does
VeraLux is a suite of free scripts for Siril, created by Riccardo Paterniti (GPL-3.0 licence). Its main module, HyperMetric Stretch (HMS), separates two things in every pixel:
- the luminance, meaning the light intensity, which gets stretched;
- the chromaticity, meaning the colour, which is kept exactly as it is.
The result: stars keep their natural tint (blue, yellow, orange) even with a strong stretch.
A nice extra: VeraLux knows the quantum efficiency of many sensors (Sony IMX585, IMX662...). You pick yours and the calculation adapts. If it isn't listed, a standard formula (Rec.709) takes over, which is still far better than a classic stretch.
How to use it
- Process your image in Siril as usual: stacking, gradient removal, deconvolution if needed.
- Run the SPCC colour calibration.
- Apply VeraLux HMS. The image must still be linear: if you've already stretched it, the result will be inconsistent.
- Finish with a few curves and a touch of saturation.
The default settings work in most cases:
| Setting | Role | Recommended value |
|---|---|---|
| Processing Mode | Aesthetic rendering (Ready-to-Use) or mathematical accuracy (Scientific) | Ready-to-Use |
| Target Background | Black level of the sky background | 0.12 to 0.15 |
| Protect b | Protects the brightest stars | Depends on star density |
| Color Grip | 1.0 = colours fully preserved | 1.0 |
StarComposer, to put the stars back
If you process the nebula without its stars ("starless"), the second module, StarComposer, blends them back in cleanly. It adjusts their intensity automatically, ignores leftover nebulosity caught in the star mask, and fixes purple or green halos.
The right habit: put the stars back right after the first stretch, before pushing contrast and saturation. Otherwise they no longer blend in naturally.
VeraLux versus other methods
| Method | Strength | Limitation |
|---|---|---|
| Histogram transformation | Fast, simple | Damages star colours |
| GHS (Generalized Hyperbolic Stretch) | Full control over the curve | Takes time to master |
| VeraLux HMS | Colours preserved, one-click result | Fewer fine-tuning options |
Installation
From Siril 1.4: Scripts menu, then Manage scripts, search for "VeraLux" and install it. The script then appears in the Scripts menu, ready to use.
My take
This is probably the biggest step forward for Siril users since GPU stacking. In a single step, VeraLux solves a problem that often took long manual touch-ups. If you process with Siril and haven't tried it yet, make it a priority.
Official repository: gitlab.com/free-astro/siril-scripts