SCNR
SCNR (Subtractive Chromatic Noise Reduction) limits one colour channel, usually green, to the level of the other two. This corrects greenish colour errors – but can also distort real signal if the object actually contains green components.
Purpose
| Task | Application |
|---|---|
| remove a greenish colour error in the background | green, Average Neutral, with a mask if needed |
| correct greenish stars | in the star image |
| shape the SHO palette | remove green so that gold and blue emerge |
| remove magenta fringes | in the inverted image |
Principle
SCNR changes only pixels whose share of the selected colour lies above a protection limit; all other pixels stay unchanged.
| Protection method | Limit for green |
|---|---|
| Average Neutral | mean of R and B |
| Maximum Neutral | maximum of R and B – stronger effect |
Parameters
| Parameter | Effect | Typical choice |
|---|---|---|
| Color to remove | colour to limit | Green |
| Protection method | protection limit (see above) | Average Neutral |
| Amount | strength | 0.3–1.0 depending on the application |
Operation
- Select the image, set the parameters, apply.
- Compare before/after with Ctrl+Z / Ctrl+Y at an affected spot (1:1 or 2:1).
- Readout on a greenish pixel: after SCNR, G at most at the level of the protection limit.
Typical applications
Green cast in the background
- Object without green components (e.g. a pure Ha nebula): Amount 0.5–0.8 on the whole image, before raising contrast and saturation – these would otherwise make greenish blotches clearly visible. Check yellowish areas (lots of red and green) to see whether they tip towards orange; then Amount 0.5–0.7.
- Object with OIII: only for a real green cast in the background, Amount 0.3–0.5, with an Creates a mask from a brightness range – for example "everything brighter than the background" – so that a following process acts only there. Open fundamentals → that protects the nebula. Compare before/after at an OIII spot: the teal must be kept.
Star image
Greenish stars are colour errors (channel misalignment, noise, Bayer interpolation); natural stars practically never appear green. In a star-only image SCNR is harmless because it contains no nebula with real green. Amount 0.5–0.8, 1.0 if a green cast remains.
Shaping the SHO palette
In an SHO image the strongest signal (Ha) lies in the green channel. SCNR removes the green excess; areas with SII (red) turn yellow/gold, areas with OIII (blue) turn cyan/blue. The typical gold-blue palette is thus created by removing green, not by saturation.
- Amount 1.0: classic gold/blue look, hardly any green.
- Amount 0.5–0.7: some green remains, more natural transitions.
- Several passes with a lower Amount are easier to control than one at 1.0. Maximum Neutral acts more strongly.
Removing magenta
Magenta cannot be selected directly. In the inverted image magenta becomes green:
- Image → Invert.
- SCNR (Green), Amount 0.5–1.0.
- Image → Invert again.
Example: magenta fringes around stars in an SHO image, because stars, as broadband emitters, appear stronger in SII (red) and OIII (blue) than in Ha (green).
What a good result looks like
Typical errors
| Observation | Cause | Remedy |
|---|---|---|
| OIII areas blue or grey instead of teal | SCNR applied to the nebula | SCNR only in the star image or masked in the background |
| Yellowish areas tip towards orange | Amount too high | Amount 0.5–0.7 |
| Star stays greenish | Amount too low or wrong window | Amount 1.0; check the target image |
| SHO image flat after SCNR | saturation not yet raised | raise saturation afterwards via Remaps tonal values along a freely shaped curve – for contrast, saturation, hue and for shifting individual tonal ranges. Acts globally; locally only with a mask. Open fundamentals → |
When to use it – and when not
| Good for | Better another tool |
|---|---|
| green cast without real green signal | overall colour balance → Calibrates the colours of a broadband or OSC image by comparing thousands of stars with Gaia spectra – objective and reproducible, including background neutralisation. Open fundamentals → or BackgroundNeutralization |
| gold/blue palette in the SHO image | fine hue shifts → Remaps tonal values along a freely shaped curve – for contrast, saturation, hue and for shifting individual tonal ranges. Acts globally; locally only with a mask. Open fundamentals → |
| magenta fringes via inversion |
BackgroundNeutralization · HaLRGB workflow · MorphologicalTransformation · Narrowband workflow (HOO/SHO) · NoiseXTerminator (NXT) · OSC workflow with a quad band-pass filter · PixelMath · RangeSelection (Range Mask)