SHO Palette Workflow
Field Guide #2 · SHO Palette Workflow
How to produce the SHO gold-and-blue aesthetic from OSC dual-band data — and how to choose between SHO and HOO for any target.
The Hubble palette is one of the most recognized visual vocabularies in astronomy — gold and yellow-green hydrogen shells, blue-purple oxygen cavities, the color language of the iconic Space Telescope images of the Eagle Nebula, the Pillars of Creation, the Crab. It was designed for monochrome cameras with three dedicated narrowband filters: SII, H-alpha, and OIII. If you are imaging with a one-shot color camera and a single dual-band filter, you have two of those three channels. This Field Guide covers how to derive the third synthetically, and how to use the result to produce a full SHO palette image from the same data that drives your HOO workflow.
The technique is called synthetic SII. The outer shell of most emission nebulae is dominated by hydrogen-alpha emission; the inner, hotter regions produce the strongest OIII signal. Since SII emission also traces the outer hydrogen shell — physically, both SII and H-alpha arise from ionized hydrogen gas at the nebula's outer boundary — scaling the H-alpha channel produces a reasonable proxy for SII. The degree of scaling is controlled by a single tunable parameter: the SII boost coefficient.
What the SHO Palette Is
In the standard SHO mapping, SII drives the Red channel, H-alpha drives Green, and OIII drives Blue. Red + Green = Yellow; a strong SII and strong H-alpha in the same region produce the characteristic gold-yellow tones of the outer hydrogen shell. Where OIII dominates — the hotter inner cavity — the result tends toward blue and blue-purple. Where all three signals overlap, the colors blend into the familiar orange-green-blue palette you have seen in every Hubble press release of an emission nebula.
With dual-band OSC data, the channel assignments are:
| Output Channel | Source | Formula | What it shows |
|---|---|---|---|
| Red (SII) | Synthetic | r × boost |
Ha-proxy — warm yellow/gold outer shell |
| Green (Ha) | Red pixels | r directly |
Full H-alpha emission — the nebula's main structure |
| Blue (OIII) | Green + Blue pixels | g × 0.5 + b × 0.5 |
Oxygen emission — inner zones, hotter gas |
The SII Boost Coefficient
The boost coefficient is the key creative variable in the SHO workflow. A value of 1.0 makes the synthetic SII exactly equal to the H-alpha signal. Values above 1.0 amplify the warm outer tones, producing stronger gold in regions of dense hydrogen emission. Values below 1.0 cool the result, letting the OIII blue dominate more of the image.
| Boost Value | Effect | Best for |
|---|---|---|
| 0.80 | Cooler — less dominant gold | Targets with strong OIII relative to Ha |
| 1.00 | Neutral — SII equals Ha signal | Starting point; faithful to the data |
| 1.10 | Slightly warmer gold outer shell | Balanced result, good default |
| 1.25 | Strong gold — classic Hubble look | Recommended for the Rosette Nebula |
| 1.50+ | Very warm — can oversaturate | Use with care on fainter targets |
Rosette result
For NGC 2237 at Bortle 9 with 16 minutes of integration, a boost of 1.25 produced the most balanced result — strong gold in the outer hydrogen shell while preserving the blue OIII inner cavity. This is the recommended starting value for similar targets.
HOO or SHO — How to Choose
Both palettes come from the same data. The choice is creative and scientific, not technical. HOO is the most direct mapping for dual-band OSC data — the channel assignments follow naturally from the filter physics, and the result is predictable and forgiving. SHO requires the synthetic SII step but produces the iconic Hubble aesthetic that is immediately recognizable and visually dramatic.
A few guidelines:
- Choose HOO for your first narrowband image — it is the most predictable and the easiest way to judge data quality.
- Choose HOO when your target has strong, spatially distinct Ha and OIII zones — the teal-and-salmon color separation will be dramatic and physically meaningful.
- Choose SHO when you want the classic Hubble gold-and-blue aesthetic — it reads immediately as "professional" to most viewers.
- Choose SHO when presenting your image alongside Hubble or professional telescope imagery of the same target.
- Process both when publishing a comparison — each palette tells a different part of the same story, and the juxtaposition is itself a Field Note.
GIMP Finishing for SHO
The key GIMP step for SHO is warming the gold tones. After the standard midtone curves lift, go to Colors > Hue-Saturation and select the Yellow channel. Boost Yellow saturation by 20–40% and shift the Yellow hue slightly warmer (+5 to +10) to deepen the gold. The Blue channel can be desaturated slightly if the inner nebula looks too purple. For the Rosette, the target look is a warm amber-gold outer ring around a cool blue-violet interior — if both regions are the same temperature of color, the image has lost its story.
The Dual Palette Workflow GUI
Both HOO and SHO are available in the same Siril Python script — the Dual Palette Workflow GUI v2.0.0. Select your palette via a tab at the top of the dialog; all controls are shared. The same master.fits input file produces either output with no manual file management. The script is available as a free download on the Field Notes landing page.
What's in the PDF
SHO palette theory, synthetic SII derivation, the SII boost coefficient table, channel assignment formulas, HOO vs. SHO selection guidance, GIMP finishing tips for each palette, and the complete Dual Palette Workflow GUI controls reference. 9 pages. All three palettes — classic Hα, HOO, and SHO — demonstrated from the same 16-minute Rosette dataset.
Clear skies / Pete // bortle9astro.com
Part of the Field Guides series · Introduction — Painting with Light You Can't See · Field Guide #1 — HOO Palette Workflow
6 pages - free PDF download