NoiseXTerminator (NXT)
NoiseXTerminator (NXT) removes noise and, if desired, separates fine-scale from large-scale noise as well as brightness noise from colour noise. The defaults are usually too strong for astro images.
Purpose
| Task | Setting |
|---|---|
| reduce grain while keeping structure | HF Intensity higher, LF Intensity low |
| remove colour blotches in the background | LF Color higher |
| remove colour speckles | HF Color higher |
Principle
- In a linear image the noise is evenly distributed and easy to separate from the signal. After stretching it is disproportionately amplified in the dark areas.
- Several light passes at suitable points keep more detail than one strong pass.
- Fine-scale noise (HF) is grain and speckles of 1–3 pixels. Large-scale noise (LF) is brightness or colour variation over dozens of pixels – but that is also where faint, extended nebula components lie.
- The structure of a nebula lies mostly in brightness, not in colour. Colour noise can therefore be removed more strongly than brightness noise.
Parameters
| Control | Effect | Default |
|---|---|---|
| Intensity/Color Separation | separate strength for brightness and colour noise; colour images only | off |
| Frequency Separation | separate strength for HF and LF | off |
| Denoise HF Intensity | fine brightness grain | 0.90 |
| Denoise HF Color | fine colour speckles | 0.90 |
| Denoise LF Intensity | large-scale brightness variation – keep low, otherwise faint nebula components are flattened | 0.90 |
| Denoise LF Color | large-scale colour blotches – can be stronger | 0.90 |
| HF/LF Scale (pixels) | boundary between HF and LF | 5.0 |
| Iterations | number of runs | 2 |
Both separations can be enabled at the same time. For monochrome images the colour controls have no effect.
Operation
- Place a preview over an area with faint structure and adjacent background.
- Apply NXT to the preview and compare with the original (same STF).
- Adjust controls in steps of 0.1, then apply to the whole image.
- Judge at 1:1 – at 2:1 noise looks much stronger than in the finished image.
Typical settings
Linear, monochrome
Enable Frequency Separation.
| Image | HF Intensity | LF Intensity |
|---|---|---|
| luminance, strong channel (e.g. Ha) | 0.4–0.7 | 0.2–0.4 |
| weak channel (e.g. OIII) | 0.6–0.8 | 0.3–0.4 |
With little noise, alternatively without Frequency Separation, a single value of 0.4–0.5.
Linear, colour image
Enable both separations. OSC data has more colour noise than mono data because every colour value is interpolated from neighbouring pixels.
| Control | Value |
|---|---|
| HF Intensity / Color | 0.5–0.6 / 0.8–0.9 |
| LF Intensity / Color | 0.3–0.4 / 0.6–0.8 |
Non-linear, colour image before an LRGB combination
Focus on colour noise: in LRGBCombination the luminance replaces the brightness of the colour image; its brightness noise is covered, colour noise remains visible.
| Control | Value |
|---|---|
| HF Intensity / Color | 0.3–0.4 / 0.6–0.8 |
| LF Intensity / Color | 0.2–0.3 / 0.5–0.7 |
| HF/LF Scale | 5 (8–10 if medium-sized colour blotches remain; 3 if fine structures become too soft) |
Non-linear, luminance
Light: HF Intensity 0.2–0.3, LF Intensity 0.1–0.2. The luminance carries the structure of the final image; any smoothing is directly visible.
Finishing
Optionally a last, very light pass on the finished image, only if noise is still disturbing at 1:1.
What a good result looks like
Typical errors
| Observation | Cause | Remedy |
|---|---|---|
| Structures soft, dark nebula flat and featureless | LF Intensity too high | lower LF Intensity |
| Fine fibres gone | HF Intensity too high | lower HF Intensity |
| Colour blotches remain | LF Color too low or HF/LF Scale too small | raise LF Color, HF/LF Scale 8–10 |
| Waxy, artificial look | one pass too strong | several light passes |
| Noise visible again after stretching | too cautious in the linear stage, or a weak channel stretched strongly | another light pass after stretching |
| Colour controls have no effect | monochrome image | expected |
When to use it – and when not
| Good for | Better another tool |
|---|---|
| noise in the linear image after deconvolution | sharpening or shrinking stars → AI-based deconvolution for linear data – corrects star shapes, makes stars smaller and sharpens nebula structure. Open fundamentals → (before NXT) |
| colour noise of a colour image before the LRGB combination | colour cast, green blotches → Removes an excess of one colour, usually green, by limiting that channel to the level of the other two – a correction of colour errors or a deliberate creative tool. Open fundamentals → |
| a light pass after local contrast | colour noise directly at combination → Chrominance Noise Reduction in LRGBCombination |
BlurXTerminator (BXT) · HaLRGB workflow · LocalHistogramEqualization (LHE) · LRGBCombination · Narrowband workflow (HOO/SHO) · OSC workflow with a quad band-pass filter