Nocturne

Tools / Hα/OIII

Toolbar tool

Hα/OIII

Your Seestar’s light-pollution filter records two gases on one sensor. Hα/OIII separates them, hydrogen from the red pixels and oxygen from the green and blue ones, and stacks both at once.

Nocturne's Hα/OIII extractor grading a folder of raw subs, with separate Hα and OIII files enabled
Point it at the same folder you would hand to Stack. It grades every frame, then stacks both gases in one pass. Click to enlarge.

What it does

A dual-band filter lets Hα and OIII through together. Hydrogen lands almost entirely on the camera’s red pixels and oxygen on the green and blue ones, so the two gases can be told apart before the frame becomes a colour image. Hα/OIII reads those raw pixels directly, builds a hydrogen frame and an oxygen frame from every sub, and stacks each.

The result is one FITS, still linear, with Hα in red and OIII in green and blue. From there it is an ordinary stack — crop it, remove the background, stretch it, and everything else in the pipeline works as usual.

It needs raw subs, not a stack

Hα/OIII needs the raw sub-exposures, the same folder you would give Stack. It cannot work from a stacked master or from any colour image, because by then the two gases are mixed together in every pixel.

How to use it

  1. Click Hα/OIII in the toolbar and choose your folder of raw subs.
  2. Pick a Strictness and an Integration method, exactly as you would for a normal stack — the frames are graded the same way, and poor ones are rejected before either gas is built.
  3. Optionally tick Also write separate Hα and OIII files, which saves each gas as its own FITS alongside the master.
  4. Extract. The finished two-gas master loads straight into the editor.

Hα/OIII does not choose your colours. It separates the gases; the palette is chosen later, in Narrowband, after the stretch.

What it leaves to Narrowband

Combine — when you already have the channels

If you already have a hydrogen file and an oxygen file, from this tool or from anywhere else, Combine builds the same two-gas master from them without going back to the subs.

Nocturne's Combine dialog with an Hα file and an OIII file loaded and the balance control set to matched
Two files in, one linear master out — with the balance between the gases under your control. Click to enlarge.

Balance decides how bright the oxygen is next to the hydrogen. At one end the oxygen stays exactly as measured, which keeps the true ratio between the gases but often leaves the oxygen faint. At the other end it is brightened to match the hydrogen, which is what Hα/OIII’s own master does and where the slider starts. Anywhere in between, the ratio you choose is the ratio you get.

Tips

Both give you a linear master: stretch it, then use Narrowband to choose the colours. The guide shows where each step fits.

Download Nocturnefree · macOS, Linux & Windows Try it on sample data

New to Nocturne? The guide walks through every step in order.