OSC workflow with a quad band-pass filter
Processing exposures from a colour camera (OSC) with a quad band-pass filter such as the Optolong L-QEF – from stacking to export. The result is a near-natural RGB image with emphasised emission lines, not a narrowband image.
Overview
Filter: quad band-pass filters pass Hβ, OIII, Hα and SII and suppress typical light-pollution lines. They behave like a broadband light-pollution filter with emphasis on the emission lines.
| Topic | Mono LRGB + Ha | OSC + quad band-pass |
|---|---|---|
| Channels | separate masters | one colour master |
| Blending in Ha, continuum subtraction | required | not needed |
| Luminance | captured, or L + Ha | synthetic from the colour image |
| SCNR on the nebula | harmless without OIII | critical – OIII is teal (G + B) |
| Colour noise | low | higher (debayering) |
Image names: the colour image RGB, the synthetic luminance L_syn.
1 Calibrate and integrateLinear
Goal: one colour master.
ToolsChains calibration, cosmetic correction, weighting, registration, normalisation and integration into a reproducible pipeline – from individual subs to aligned master lights. Open fundamentals →
Chains calibration, cosmetic correction, weighting, registration, normalisation and integration into a reproducible pipeline – from individual subs to aligned master lights. Open fundamentals →: flats and lights with CFA images, debayer VNG, optionally Bayer drizzle 1× (many dithered subs). Flats taken with the filter, one group per session.
2 CropLinear
Goal: remove stacking borders before gradient correction mistakes them for gradients.
DynamicCrop; with Autocrop enabled usually only a small correction. Crop away streaks that do not appear in the rejection maps.
3 Gradient correctionLinear
Goal: remove background gradients before colour calibration determines the background.
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 →
Whybefore colour calibration?
SPCC determines background and white balance from the image. A gradient would distort both – a different background colour in each corner.
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 → on the colour master; with OSC the correction acts on all channels together. Threshold 0.05–0.1 (fine filaments often run through large parts of the image), Amount 0.5.
Wide fields often show a colour gradient across the whole image → 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 →, degree 1.
An equivalent alternative on a copy when extended emission (Milky Way) lies in the field: ImageSolver → SPFC → 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 →.
With a quad band-pass filter the image differs spectrally from the broadband reference: 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 → (1536) and compare with 1024. An SPFC dispersion of 5–9 % is normal with this filter.
Bright star in the field: its halo must not be modelled as gradient – 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 →.
4 Colour calibrationLinear
Goal: near-natural white balance and a neutral background.
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 →
ImageSolver (if not from WBPP), then 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 →:
| Parameter | Setting |
|---|---|
| QE curve | the camera’s sensor |
| Filter R/G/B | colour filter curves of the OSC sensor |
| Additional filter | the quad band-pass filter, if listed |
| Narrowband mode | off |
| White reference | Average Spiral Galaxy |
| Background Neutralization | region of interest on a nebula-free preview; upper limit +1.0 to +1.5 if needed |
In Milky Way fields with extended nebula the region of interest is decisive: without it, outer filaments turn grey. Place the preview outside the nebula, not inside it (often faint emission there).
5 DeconvolutionLinear
Goal: correct star shapes, sharpen filaments.
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 →: first Correct Only, then normal mode with Sharpen Stars 0.25–0.5, halos 0 to −0.1, Sharpen Nonstellar rather restrained (0.5–0.7), because OSC has more colour noise.
6 Linear denoisingLinear
Goal: above all remove the colour noise from debayering.
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 →
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 → with both separations: HF 0.5–0.6 / 0.8–0.9, LF 0.3–0.4 / 0.6–0.8. OSC data has more colour noise because every colour value is interpolated from neighbouring pixels.
7 Synthetic luminanceLinear
Goal: a luminance so that structure and colour can be processed separately.
ToolsA luminance built from existing channels, so that structure and colour can be processed separately and merged again with LRGBCombination. Open fundamentals →
Whyfor OSC?
There is no separate luminance. A luminance extracted from the colour image allows separate processing: structure (LHE, denoising) on the luminance, colour on the colour image.
A luminance built from existing channels, so that structure and colour can be processed separately and merged again with LRGBCombination. Open fundamentals → via ChannelExtraction (CIE L*a*b*, L only) → L_syn. Two moments:
| Route | Extraction | Consequences |
|---|---|---|
| A – simple (standard) | in step 8 from the stretched RGB | L_syn is already non-linear and identical to the colour image’s L*; stretching and matching L_syn are not needed |
| B – more control | now from the linear RGB | stretch L_syn separately (deeper black point, more structure); matching in 8.3 is mandatory |
8 StretchingNon-linear
Goal: make colour image and luminance visible, with saturation preserved.
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 →
8.1 Colour image
RGB with 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 → (Colour blend 1.000, Use RGB working space). For route A then ChannelExtraction → L_syn.
8.2 Luminance (route B only)
L_syn with the same scheme or via the Stretches linear data so that contrast is created around a chosen brightness value – in several small passes, alternating with a black point. Open fundamentals →; target: background and filament brightness similar to the luminance of RGB.
8.3 Matching (route B only)
Compare only now – a linear L_syn could not be compared with the stretched colour image. 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 RGB → RGB_L; measuring points Dark, Mid, Bright; usually only the midtones.
9 Separating the starsNon-linear
Goal: process nebula and stars separately – especially important in dense Milky Way fields.
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 |
|---|---|---|---|
RGB | enabled | enabled | enable for dense star fields |
L_syn | off | – | likewise |
Stars from RGB – only it contains star colours. Check the starless image for residual halos of bright stars; dampen remnants with a mask if needed.
10 Processing the starless imagesNon-linear
Goal: red/teal separation and colour contrast in the colour image, filament 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 colour, then contrast, then saturation, finally denoise – otherwise colour errors would be amplified, and noise removed earlier would reappear.
10.1 Colour image
- No blanket SCNR. OIII is teal (G + B); SCNR would shift it towards blue. Only for a real green cast in the background use 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 0.3–0.5; check the teal at an OIII spot.
- 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 →.
- Optional – red/teal separation: OIII stronger with ColorSaturation (cyan/blue-green range +0.1 to +0.2); Hα warmer 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 →. Keep the background neutral.
- 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 →; for OSC rather at the upper end of the values.
10.2 Luminance
- Raises local contrast – structures that differ only slightly from their immediate surroundings stand out without shifting the global tonal values. Open fundamentals →: pass 1 with 48–64 for filament bundles, pass 2 with 12–24 for individual fibres.
- Optionally a Creates a mask from a brightness range – for example "everything brighter than the background" – so that a following process acts only there. Open fundamentals → for faint outer parts.
- 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 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 →
- Matching: ChannelExtraction on
RGB_starless→RGB_starless_L; 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 → – typically less contrast in the luminance → gentle S-curve. Necessary for route A too, because both images have been processed differently since step 8. - 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_syn_starlessas L onRGB_starless. - 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 →.
12 Processing the starsNon-linear
Goal: tone down the dense star field, natural star colours.
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 →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 RGB_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 → – greenish stars are common with OSC (Bayer interpolation).
- Check saturation, do not remove it.
- Makes stars smaller instead of merely dimming them – faint stars recede, bright ones remain as accents. Easiest in a separated star image. Open fundamentals → – central in Milky Way fields, so that the nebula does not disappear behind the stars.
13 Recombining the starsNon-linear
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) * (~(RGB_stars * s)))14 Finishing and exportNon-linear
- Optionally a small 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 → correction, optionally 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 →.
- Export: XISF 32-bit (archive), TIFF 16-bit, JPEG sRGB. Save process icons, save the project.
Common problems
OSC- and filter-specific. General errors are listed under “Typical errors” of each fundamentals article.
| Symptom | Cause | Remedy |
|---|---|---|
| Green cast in the integrated master | expected with OSC | correction with SPCC |
| OIII areas blue or grey instead of teal | SCNR on the nebula | SCNR only in the star image or masked in the background |
| SPCC fits with large scatter, slope ≠ 1 | filter not modelled | expected; check visually |
| Outer filaments grey after SPCC | background neutralisation without a region of interest | nebula-free preview outside the nebula |
| Rings or patches around a bright star in the gradient model | halo captured as gradient | adjust protection; with DBE no samples in the halo |
| Colour noise, colour blotches | debayering | NXT with colour controls, Chrominance Noise Reduction |
| Stars dominate the image | dense Milky Way field | star reduction |