BackgroundNeutralization
BackgroundNeutralization adjusts the colour channels so that the background of a colour image becomes neutral (R ≈ G ≈ B). Unlike a colour calibration, it does not set a white balance for the object.
Purpose
| Task | Application |
|---|---|
| neutralise the background of a false-colour image (HOO, SHO) | instead of a colour calibration |
| fix a colour cast in the background afterwards | with a background preview as reference |
Whynot SPCC for false colour?
SPCC derives the white balance from star colours and assumes natural broadband colours. In false colour there is no “correct” star colour; only the background should be neutral. BackgroundNeutralization is enough for that.
Principle
The background value of each channel is determined from a reference area – ideally pure background – and the channels are shifted so these values are equal. Pixels above the Upper limit (stars, nebula) are ignored for this.
Parameters
| Parameter | Effect | Typical choice |
|---|---|---|
| Reference image | area from which the background is determined | preview on nebula-free background |
| Lower limit | values below are ignored | default |
| Upper limit | values above are ignored | just above the background median, so only background is captured |
Operation
- Place a preview (Alt+N) on an area without visible nebula, ideally with few bright stars, about 300–400 px on a side, at least 50–100 px from the image edge (edges may contain remnants of stacking borders).
- Select the preview as Reference image.
- Set the Upper limit, apply.
- Readout at several background spots: R ≈ G ≈ B.
Typical applications
False-colour image after combining
After assembling an HOO or SHO image, before shaping the colours.
Checking the background for neutrality
Readout at two or three star-free spots, at least one far from coloured structures. R, G and B nearly equal.
- A colour impression next to strongly coloured structures is often an optical illusion (simultaneous contrast) – the measured values count.
- For a real cast: BackgroundNeutralization with a background preview, or a minimal channel correction.
Background area in colour calibration
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 → uses the same principle for its background neutralisation: without a defined area (region of interest) the background is determined from the whole image; with extended nebula, nebula components flow in and faint outer parts turn partly grey. Choose a nebula-free preview there as well.
What a good result looks like
Typical errors
| Observation | Cause | Remedy |
|---|---|---|
| Faint nebula grey | reference contains nebula | preview on a nebula-free area; lower the Upper limit |
| Cast in one corner | gradient, not a global colour shift | gradient correction first |
| Background looks coloured, values neutral | simultaneous contrast | trust the measurements |
| Edges of the result stand out | preview too close to the edge | 50–100 px distance |
When to use it – and when not
| Good for | Better another tool |
|---|---|
| false colour: neutral background, freely shaped colours | natural colours → 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 → |
| a small remaining cast in the background | colour gradient across the image → gradient correction |
| greenish blotches in objects without green → 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 → |
HaLRGB workflow · Narrowband workflow (HOO/SHO) · OSC workflow with a quad band-pass filter · SpectrophotometricColorCalibration (SPCC)