HOO Palette Workflow
Field Guide · #1
How to capture and process a HOO palette image from a one-shot color camera with a dual-band filter — from stacked master to finished image.
What This Guide Covers
- How the HOO palette works — H→Red, OIII→Green+Blue
- Why a dual-band filter rejects light pollution at the source
- The OIII formula and why it matters for OSC cameras
- Star separation: why it's done and how Siril handles it
- GraXpert background extraction — the step that changes everything
- The automated Siril HOO palette script, step by step
- GIMP finishing: curves, saturation, export
- Reading your result and knowing when to go back for more data
16 minutes of integration. A Bortle 9 backyard. One image that proved the sky is no obstacle — if you use the right tools.
The Rosette Nebula is enormous — more than twice the width of the full moon — and almost entirely invisible from a Bortle 9 backyard without the right filter. Broadband imaging sees only a faint, gradient-drowned smudge. Narrowband imaging sees a wall of hydrogen gas surrounding a young star cluster, rendered in reds and teals that no human eye has ever witnessed directly. The difference is a 7-nanometer window in the spectrum.
This Field Guide documents the complete HOO palette workflow that produced that image: from a stacked master FITS file to a finished, publication-ready result in approximately 15 minutes of processing time. All software is free. The workflow is repeatable from any backyard.
What HOO Actually Is
HOO is a color palette assignment, not a capture technique. When you image with a dual-band filter, your camera collects two distinct signals simultaneously: hydrogen-alpha (Hα, 656nm) and doubly-ionized oxygen (OIII, 500nm). HOO assigns those signals to color channels in the final image — Hα to Red, and OIII to both Green and Blue.
The result is a false-color image where hydrogen-rich regions render warm red-orange, oxygen-rich regions render teal-blue, and areas with both signals blend toward golden yellow. It is not what your eye would see — it is something richer: a map of chemistry and physics, rendered as color.
Why not SHO?
SHO (the Hubble palette) adds a third signal — sulfur-II (SII, 672nm) — for a more complex false-color rendering. An OSC camera with a dual-band filter captures Hα and OIII only; SHO requires a dedicated monochrome camera with separate narrowband filters. HOO is the right palette for this equipment.
Why It Works at Bortle 9
A 7nm dual-band filter passes less than 2% of the visible spectrum — precisely the two slices where emission nebulae are brightest, and where almost no artificial light source emits. LED streetlights, sodium vapor lamps, the ambient glow of Chicago: the filter rejects nearly all of it. What reaches your sensor is overwhelmingly signal, not noise.
Emission nebulae are also intrinsically bright in these wavelengths. Unlike galaxies — which reflect starlight across the full broadband spectrum and suffer badly under light pollution — emission nebulae actively generate their own Hα and OIII light. They are playing on the same frequency the filter is tuned to. That is why 16 minutes from a Bortle 9 backyard produces a result that would have been unthinkable with broadband imaging.
What's in the PDF
The Field Note documents the complete workflow with worked examples from the Rosette Nebula session: the OIII formula derivation, the star separation architecture, every step of the Siril HOO script, and the GIMP finishing sequence. It also covers how to read your result — what strong Hα with weak OIII looks like, what it means, and when it's worth going back for more integration time.
What's in the PDF
HOO palette theory, OIII formula derivation, star separation architecture, GraXpert background extraction, Siril scripting, GIMP finishing, and target suitability guidance for light-polluted skies. 7 pages. The complete workflow, nothing omitted.
Clear skies / Pete // bortle9astro.com
Part of the Field Guides series · Introduction — Painting with Light You Can't See · Field Guide #2 — Palette Comparison
6 pages - free PDF download