HaLRGB workflow
Processing L, R, G, B and Ha exposures from a monochrome camera, from stacking to export. Ha strengthens the nebula in the red channel and in the luminance without distorting colours; RGB provides natural colours and star colours.
Overview
| Image | Role |
|---|---|
R, G, B | colour and star colours |
Ha | deep exposure of the Hα emission – strengthens the nebula in the red channel (step 7) and in the luminance (step 8) |
L | star field, dust structures, broadband structure |
HaRGB | colour image after blending in Ha |
L_neu | luminance from L and Ha |
1 Calibrate and integrateLinear
Goal: five aligned masters.
ToolsChains calibration, cosmetic correction, weighting, registration, normalisation and integration into a reproducible pipeline – from individual subs to aligned master lights. Open fundamentals →
One flat and one light group per filter, CFA off (Chains calibration, cosmetic correction, weighting, registration, normalisation and integration into a reproducible pipeline – from individual subs to aligned master lights. Open fundamentals →); one common registration reference for all filters, ideally the best Ha frame. Drizzle, if at all, the same for all filters.
2 CropLinear
Goal: remove stacking borders and stray light.
Set DynamicCrop on one master and apply the same crop to all others so they stay aligned. Crop away artefacts that do not appear in the rejection maps (Chains calibration, cosmetic correction, weighting, registration, normalisation and integration into a reproducible pipeline – from individual subs to aligned master lights. Open fundamentals →).
3 Gradient correction per channelLinear
Goal: remove background gradients while each channel is separate.
ToolsModels large-scale brightness and colour gradients (light pollution, moon, residual vignetting) and subtracts them – structures are kept out of the model by protection masks. Open fundamentals →Corrects gradients using a reference map of the sky (MARS) and can therefore distinguish real large-scale signal from gradients. For broadband data; requires SPFC. Open fundamentals →
Whyper channel and before combining?
Gradients differ per filter; moonlight is stronger in blue, light pollution is not spectrally neutral. After combining they would be merged into colour gradients.
Correct each master separately with Models large-scale brightness and colour gradients (light pollution, moon, residual vignetting) and subtracts them – structures are kept out of the model by protection masks. Open fundamentals →. For L, R, G, B alternatively Corrects gradients using a reference map of the sky (MARS) and can therefore distinguish real large-scale signal from gradients. For broadband data; requires SPFC. Open fundamentals → if extended emission lies in the field; GradientCorrection for Ha.
4 Colour image and colour calibrationLinear
Goal: a colour-calibrated RGB image as the basis for nebula and star colours.
ToolsCalibrates the colours of a broadband or OSC image by comparing thousands of stars with Gaia spectra – objective and reproducible, including background neutralisation. Open fundamentals →
- ChannelCombination:
R,G,B→RGB. - ImageSolver, if the solution does not come from WBPP.
- 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 → with sensor and RGB filter curves, narrowband mode off, white reference Average Spiral Galaxy, region of interest on a nebula-free preview; in nebula-filled fields lower the upper limit (+1.0 to +1.5). Dark nebulae are unsuitable as reference.
Whynow, before deconvolution?
SPCC measures the stars photometrically and needs unaltered star profiles for that.
5 DeconvolutionLinear
Goal: correct star shapes and sharpen structures – with matching star profiles in RGB and Ha.
ToolsAI-based deconvolution for linear data – corrects star shapes, makes stars smaller and sharpens nebula structure. Open fundamentals →
AI-based deconvolution for linear data – corrects star shapes, makes stars smaller and sharpens nebula structure. Open fundamentals → on RGB, L and Ha, each first Correct Only, then normal mode (Sharpen Stars 0.25–0.5, halos 0 to −0.1, Sharpen Nonstellar 0.5–0.9; Ha takes more, RGB less).
6 Linear denoisingLinear
Goal: remove noise while it is evenly distributed.
ToolsAI-based noise reduction with separate strength for fine- and large-scale noise and for brightness and colour. Several light passes keep more detail than one strong pass. Open fundamentals →
| Image | Setting |
|---|---|
RGB – least integration per channel, most noise | AI-based noise reduction with separate strength for fine- and large-scale noise and for brightness and colour. Several light passes keep more detail than one strong pass. Open fundamentals →: HF 0.5–0.6 / 0.8–0.9, LF 0.3–0.4 / 0.6–0.8 |
L | AI-based noise reduction with separate strength for fine- and large-scale noise and for brightness and colour. Several light passes keep more detail than one strong pass. Open fundamentals →: HF 0.5–0.7, LF 0.3–0.4 |
Ha | HF 0.4–0.6, LF 0.2–0.3; with long integration also without frequency separation, a single value of 0.4–0.5 |
7 Blending Ha into the red channelLinear
Goal: add pure emission signal to the red channel – red nebula structures without pink stars.
ToolsCombines images pixel by pixel with formulas – for combining channels, blending luminances or recombining stars. Open fundamentals →
WhyHa into the red channel?
Hα lies at 656 nm, within the passband of the red filter. The Ha image is a much deeper exposure of exactly this signal; it strengthens the nebula where it is weak in RGB without distorting the colour.
Whycontinuum subtraction?
Besides the emission line, the Ha filter also passes continuum light, mainly from stars. If the whole Ha image were added, all stars would get excess red – pink stars. The subtraction removes the continuum share using the red channel, in which the continuum dominates.
Whylinear?
Only in the linear state are pixel values proportional to flux; adding and subtracting channels is then physically correct.
Simple, without control over the subtraction:
| Field | Setting |
|---|---|
| Target | RGB |
| Channel R | Ha |
| NB Bandwidth | bandwidth of the Ha filter (e.g. 3, 5 or 7 nm) |
| Broadband Bandwidth R | bandwidth of the red filter (typically about 100 nm) |
| Scale | 1.0–3.0 (strength of the Ha boost) |
| G, B | empty |
Check the result: stars neutral, nebula clearly red.
Full control in three steps.
1. Extract the red channel: ChannelExtraction (RGB) on the BXT-processed RGB, R only → R_cal.
2. Subtract the continuum (Combines images pixel by pixel with formulas – for combining channels, blending luminances or recombining stars. Open fundamentals →), Create new image, ID Ha_rein, rescale off:
Ha - q * (R_cal - med(R_cal))q depends on filter bandwidths, SPCC scaling and exposure times and is determined from the data; typically 0.1–0.3 after SPCC.
Starting value: on three or four medium-bright, unsaturated stars read core value and background in Ha and R_cal; q = (Ha_star − Ha_background) / (R_star − R_background), take the mean.
Fine-tuning: preview in the Ha image on several medium-bright stars with little nebula; drag PixelMath with Replace target image onto the preview tab (the main image stays unchanged); vary q in steps of 0.01–0.02, Ctrl+Z / Ctrl+Y. For judging use a gentler STF than the auto-STF (auto-STF with Ctrl held) – the auto-STF is too steep, values near the optimum would be indistinguishable.
Target: faint and medium stars largely disappear; remaining residues about equally often bright and dark; nebula unchanged. Mostly dark residues → lower q, mostly bright → raise it; ±0.01 is enough.
Appearance in Ha_rein | Cause | Remedy |
|---|---|---|
| almost all stars as filled black discs | q too high | lower q clearly |
| almost all stars as bright dots | q too low | raise q |
no difference between clearly different q | auto-STF too steep | gentler stretch |
crescent shape on bright stars, regardless of q | sub-pixel offset | tolerable, step 3 catches it |
| rings, different for each star | star profiles of Ha and R_cal differ | match BXT star parameters (step 5) |
| white dot at the centre of bright stars | saturated cores | tolerable |
A single q never removes all stars perfectly: Ha lies at the red end of the red filter, cool stars stay slightly bright, hot ones are slightly over-subtracted.
3. Add to the red channel – PixelMath on a copy of RGB, Use single RGB/K expression off, rescale off:
R/K: $T[0] + max(0, Ha_rein - med(Ha_rein)) * f
G: $T[1]
B: $T[2]
Symbols: f = 1.0max(0, …) ensures that only additional Ha signal is added and nothing is ever subtracted – negative residues would create dark holes and later cyan star cores. Start f at 1.0, for a faint nebula 1.5 – 2 – 3; too high: nebula garish orange-red, background with a red cast.
Optional: a small Ha share in the blue channel ($T[2] + max(0, Ha_rein - med(Ha_rein)) * 0.1) shifts the red slightly towards pink, as Hβ emission would naturally do.
8 Luminance from L and HaLinear
Goal: a luminance with nebula from Ha and star field from L.
ToolsA luminance built from existing channels, so that structure and colour can be processed separately and merged again with LRGBCombination. Open fundamentals →
Whycombine L and Ha?
L contributes the star field and dust structures, Ha the nebula filaments with a far better signal-to-noise ratio. max() takes the brighter channel at each point: nebula from Ha, stars from L – without adding stars from both images.
- Copy
Ha_LF, A luminance built from existing channels, so that structure and colour can be processed separately and merged again with LRGBCombination. Open fundamentals → toL. - A luminance built from existing channels, so that structure and colour can be processed separately and merged again with LRGBCombination. Open fundamentals →:
max(L, Ha_LF); softer:L*0.6 + Ha_LF*0.4.
9 StretchingNon-linear
Goal: stretch colour image and luminance separately and match them.
ToolsStretches linear data so that contrast is created around a chosen brightness value – in several small passes, alternating with a black point. Open fundamentals →Measures at fixed spots whether a separate luminance and the colour image match in background, midtones and highlights, and adjusts the luminance with a curve – so the LRGB combination is neither pale nor oversaturated. Open fundamentals →
Whystretch separately?
The luminance carries structure and sharpness, the colour image colour. Separately, the colour image can be stretched in Colour mode (saturation kept) and the luminance optimised for maximum structure. In LRGBCombination both only need to match in brightness.
HaRGBwith Stretches linear data so that contrast is created around a chosen brightness value – in several small passes, alternating with a black point. Open fundamentals →, Stretches linear data so that contrast is created around a chosen brightness value – in several small passes, alternating with a black point. Open fundamentals →.L_neuwith the same scheme; target: background and filament brightness similar to the luminance ofHaRGB.- Measures at fixed spots whether a separate luminance and the colour image match in background, midtones and highlights, and adjusts the luminance with a curve – so the LRGB combination is neither pale nor oversaturated. Open fundamentals →: ChannelExtraction on
HaRGB→HaRGB_L; measuring pointsDark,Mid,Bright. Typical pattern: only Mid differs, becauseL_neucontains Ha structure → midtones only, with a Measures at fixed spots whether a separate luminance and the colour image match in background, midtones and highlights, and adjusts the luminance with a curve – so the LRGB combination is neither pale nor oversaturated. Open fundamentals →.
10 Separating the starsNon-linear
Goal: be able to process nebula and stars separately.
ToolsSplits an image into a starless image and a star image, so that nebula and stars can be processed separately and recombined at the end with a screen blend. Open fundamentals →
| Image | Generate star image | Unscreen stars | Large overlap |
|---|---|---|---|
HaRGB | enabled | enabled | for dense star fields |
L_neu | off | – | for dense star fields |
The stars come from HaRGB – only it contains star colours (Splits an image into a starless image and a star image, so that nebula and stars can be processed separately and recombined at the end with a screen blend. Open fundamentals →).
11 Processing the starless imagesNon-linear
Goal: colour and colour contrast in the colour image, structure in the luminance.
ToolsRemoves 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 →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 →AI-based noise reduction with separate strength for fine- and large-scale noise and for brightness and colour. Several light passes keep more detail than one strong pass. Open fundamentals →Raises local contrast – structures that differ only slightly from their immediate surroundings stand out without shifting the global tonal values. Open fundamentals →Creates a mask from a brightness range – for example "everything brighter than the background" – so that a following process acts only there. Open fundamentals →
Whynow and in this order?
Without stars, contrast and saturation act only on the nebula. First clean up the colour, then contrast, then saturation, finally denoise – otherwise colour errors would be amplified, and noise removed beforehand would reappear with the contrast boost.
11.1 Colour image
- SCNR before the curves: 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 →, Amount 0.5–0.8. Extended greenish blotches would otherwise stand out after contrast and saturation. Harmless, because the nebula contains no real green (no OIII); check whether yellowish areas tip towards orange.
- Contrast: 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 →.
- Saturation: 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 →; optionally ColorSaturation for one particular hue.
- Denoising: AI-based noise reduction with separate strength for fine- and large-scale noise and for brightness and colour. Several light passes keep more detail than one strong pass. Open fundamentals →.
11.2 Luminance
- Raises local contrast – structures that differ only slightly from their immediate surroundings stand out without shifting the global tonal values. Open fundamentals →: 64–128 for whole filaments, nebula areas and dark cloud edges, 16–32 for fine structure within the filaments.
- Optional: Creates a mask from a brightness range – for example "everything brighter than the background" – so that a following process acts only there. Open fundamentals → if faint areas are too dark after LHE.
- AI-based noise reduction with separate strength for fine- and large-scale noise and for brightness and colour. Several light passes keep more detail than one strong pass. Open fundamentals →.
12 Combining and backgroundNon-linear
Goal: merge structure and colour.
ToolsMeasures at fixed spots whether a separate luminance and the colour image match in background, midtones and highlights, and adjusts the luminance with a curve – so the LRGB combination is neither pale nor oversaturated. Open fundamentals →Replaces the brightness of a colour image with a separate luminance – structure and sharpness from the luminance, colour from the colour image. Open fundamentals →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 →Gleicht die Kanäle so an, dass der Hintergrund neutral grau wird – ohne die Farben des Objekts zu kalibrieren. Für Falschfarben und als Nachkorrektur. Open fundamentals →
- Match again: ChannelExtraction on
HaRGB_starless→HaRGB_starless_L; the typical pattern now is Measures at fixed spots whether a separate luminance and the colour image match in background, midtones and highlights, and adjusts the luminance with a curve – so the LRGB combination is neither pale nor oversaturated. Open fundamentals → → gentle S-curve. Reason: the colour image gained contrast through S-curve and saturation, the luminance changed only locally through LHE. - Combination: Replaces the brightness of a colour image with a separate luminance – structure and sharpness from the luminance, colour from the colour image. Open fundamentals → with
L_neu_starlessas L onHaRGB_starless. If dust structures without a measuring point are lost, Replaces the brightness of a colour image with a separate luminance – structure and sharpness from the luminance, colour from the colour image. Open fundamentals → and adjust Saturation accordingly. - Background if needed 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 →; Gleicht die Kanäle so an, dass der Hintergrund neutral grau wird – ohne die Farben des Objekts zu kalibrieren. Für Falschfarben und als Nachkorrektur. Open fundamentals →.
13 Processing the starsNon-linear
Goal: natural star colours, smaller stars.
ToolsRemoves 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 →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 →Makes stars smaller instead of merely dimming them – faint stars recede, bright ones remain as accents. Easiest in a separated star image. Open fundamentals →
On a copy of HaRGB_stars:
- 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 →, Amount 0.5–0.8, 1.0 if a green cast remains.
- Check saturation: orange and blue already clear → do not raise further; too colourful after recombination → lower channel S slightly (0.5 → 0.45). Do not remove completely – the colours show the stars’ temperature distribution.
- Makes stars smaller instead of merely dimming them – faint stars recede, bright ones remain as accents. Easiest in a separated star image. Open fundamentals →: Selection 0.2–0.3, Amount 0.5–0.8, 5×5 circular.
14 Recombining the starsNon-linear
Goal: put the stars back softly.
ToolsSplits an image into a starless image and a star image, so that nebula and stars can be processed separately and recombined at the end with a screen blend. Open fundamentals →Combines images pixel by pixel with formulas – for combining channels, blending luminances or recombining stars. Open fundamentals →
Splits an image into a starless image and a star image, so that nebula and stars can be processed separately and recombined at the end with a screen blend. Open fundamentals → on a duplicate of Starless_final, Replace target image:
~((~Starless_final) * (~(HaRGB_stars * s)))s = 1.0 after reduction.
15 Finishing and exportNon-linear
Goal: overall check, last small corrections, saving.
- Optional small correction with 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 → – with stars in the image only cautiously.
- Optional AI-based noise reduction with separate strength for fine- and large-scale noise and for brightness and colour. Several light passes keep more detail than one strong pass. Open fundamentals →, only for visible noise.
- Export: XISF 32-bit (archive), TIFF 16-bit (further editing), JPEG sRGB (publication). Save process icons, save the project.
Common problems
HaLRGB-specific. General errors are listed under “Typical errors” of each fundamentals article.
| Symptom | Cause | Remedy |
|---|---|---|
| Pink stars | Ha added without continuum subtraction | variant B with Ha_rein |
| Cyan star cores | negative residues of Ha_rein subtracted | max(0, …) in the expression |
Rings or holes in Ha_rein | star profiles of Ha and RGB differ | same BXT star parameters (step 5) |
| Nebula garish orange, background reddish | f too high | lower f |
| Faint nebula grey after SPCC | background neutralisation without a region of interest | nebula-free preview, lower the upper limit |
| Dust structures gone after LRGB | luminance lowered there, no measuring point | lower Lightness, add a measuring point |