The Hubble Palette: Why SHO Starts Green and Ends Gold

You have a night of Hα, a night of OIII and a night of SII. You stack each filter, combine them in the Hubble palette, and the picture comes out green. Not a little green - a nebula the colour of pond water.

Nothing is broken. That green is exactly what the palette produces before its final step, and every SHO image you have admired went through it.

What the palette actually maps

SHO names the order the filters go into the colour channels:

• SII into red
• Hα into green
• OIII into blue

It is a false-colour palette. Both SII (671.6nm) and Hα (656.3nm) are deep red to the eye; OIII (500.7nm) is blue-green. The palette spreads three lines that the eye would see as two colours across all three channels, so that where each gas sits becomes visible. The colours mean which element, not what you would see. There is more on the lines themselves in narrowband filters explained.

Why the raw result is green

Hydrogen is by far the most abundant gas in an emission nebula, so Hα is almost always the brightest of the three stacks - often several times brighter than SII. The palette puts Hα in green. So the brightest signal in the image lands in the green channel, and the nebula comes out green.

The North America Nebula and the Wizard are good examples: both are mostly hydrogen, and both come out of a straight SHO combination looking like moss.

The step that makes it gold

The gold-and-teal look comes from one more move, made after the channels are combined: the green that belongs to hydrogen is shifted toward red and yellow, and whatever green is left is pulled back.

Done by hand, it is usually a green-removal tool (SCNR) followed by a hue shift or curves. Removing green on its own is not enough - Hα is the green channel, so taking the green out takes the nebula’s brightest signal with it and leaves a dim brown. The surplus has to move, not vanish: hydrogen-bright areas go gold, OIII-bright areas stay teal-blue, and where the two mix you get the pale cream between them.

That is what Akastroid does automatically when Hα sits in the green channel. Green’s surplus over the average of red and blue is moved into red, and a little of it out of green, so a hydrogen pixel turns orange-gold with its brightness kept. Where green is not in surplus - the neutral sky, the stars, the OIII regions - nothing moves.

Magenta stars are the same palette’s other side

Stars are broadband: most of their light misses every narrowband filter. What gets through tends to be bright in SII and OIII and comparatively faint in Hα - and SII and OIII are red and blue. Red plus blue without green is magenta, so an SHO field fills with violet stars.

The correction is the mirror of the gold one: where green sits below both red and blue, it is lifted to the weaker of the two. It has to be both - in HOO, green and blue are the same OIII data, so any red hydrogen pixel has green below the red-blue average without being magenta at all, and a correction keyed to the average turns a red nebula brown.

HOO and the other orders

HOO puts Hα in red and OIII in both green and blue. It needs only two filters, it is the natural output of a dual-band filter, and it looks closer to what the nebula actually emits: red hydrogen against teal oxygen. No gold step is needed, because hydrogen is already red.

HSO puts Hα in red and SII in green - a warmer variant from the same three files.

When you also shot red, green and blue

Many filter-wheel sessions carry both: narrowband for the nebula and short RGB frames for the stars. From version 1.3.2, when a folder holds both, Akastroid asks which picture you want before it stacks - natural colour with the Hα added, or SHO or HOO from the narrowband. Natural colour is the default, because a first look at a target is usually a look at what it is. See adding Hα to RGB for how that one is built.

The short version

• SHO maps SII, Hα, OIII to red, green, blue. Hα is the brightest, so the raw result is green. That is correct.
• The Hubble gold comes from moving hydrogen’s green toward red and gold afterwards - not from deleting it.
• Magenta stars are the palette’s other artefact and are fixed by lifting green where it sits below both red and blue.
• HOO needs two filters and no gold step; it is what dual-band data wants.
• The colours in SHO mean which gas, not what the eye sees. Say so when you share one.