OSC Dual-Band to the SHO Palette

Applies to: one-shot-colour data through a dual-band filter (L-Ultimate, L-eNhance, L-eXtreme), stacked elsewhere (APP, WBPP, Siril) and brought into PixInsight as a linear master.

Status: working reference. Every step has been checked against current vendor documentation, and anything that departs from vendor guidance is flagged in the Check boxes of the full guide.

The workflow at a glance

Stage 1: Linear repair

1. Load master + icon set.
2. STF autostretch. Screen only, survey gradient and edges.
3. DynamicCrop. Every stacking artefact gone.
4. GraXpert background extraction. Show background model, verify it.
5. BlurXTerminator. Defaults, linear only. Optional calibrated route: BXT (Correct Only), then SPCC narrowband, then BXT (sharpen).

Stage 2: The hinge

6. Stretch. HT Stretch Linked RGB, then reset STF.

Stage 3: Cleanup + split

7. NoiseXTerminator. Never before step 5.
8. StarXTerminator. Generate star image + unscreen.
9. GHS tweak on stars image. Optional, knocks back bright cores.

Stage 4: Colour

10. NarrowbandNormalization. Palette on HO, small OIII boost.
11. Global saturation curve. Mask prep, not a look decision.
12. Build yellow + blue masks. One MaskBlur pass each.
13. Blue mask, then curves. OIII to teal: blue up, green back.
14. Yellow mask, then curves. Ha to copper and gold, drop green shadow.

Stage 5: Finish

15. Contrast. HistogramTransformation, or gentle S-curve.
16. Recombine stars. Screen blend, never addition.
17. Final NoiseXTerminator. Catches chroma noise the curves raised.

The stars image parked at step 8 rejoins the chain at step 16.

The whole logic in one line

Everything that mathematically requires linear data happens before step 6, everything that is an aesthetic decision happens after it. If you remember one thing, remember where the stretch sits.

Non-negotiable

• BXT runs on linear data only, after crop and gradient removal, before any noise reduction.
• Crop stacking artefacts first. Ragged white borders break the BXT network outright.
• NarrowbandNormalization needs a stretched, starless image to give you its full toolset.
• Unscreen at extraction must pair with a screen blend at recombination.
• Palette stays on HO. It describes the data you hold, not the look you want.

Judgement call

• Whether to insert SPCC in narrowband mode between two BXT passes.
• How hard to push the OIII boost at step 10.
• Whether the bright stars need the GHS knockdown at step 9.
• Histogram versus S-curve for the contrast at step 15.
• How far to run saturation before the image starts to look manufactured.