A quicker way to colourise your narrowband frames

Create a bicolour image in Siril using data from just two narrowband filters

- Charlotte Daniels

Monochrome astro images taken with narrowband filters give us superb contrasts and details. To colourise them, we assign each filter's data to one or more of the main colour channels, red (R), green (G) and blue (B). While there are three main harrowband types, hydrogen-alpha (Ha), Oxygen (OIII) and sulphur (SII), we don't need all three for beautiful results. If we map Ha data to the R channel and Oll data to the green and blue channels, we make what's called a HOO-palette image.

Siril is a free image-processing program that, with its tools StarNet and Pixel Math, provides an end-to-end imaging workflow for this process, including stacking, removing stars and improving background gradients. Pixel Math is particularly interesting, as it allows you to carry out mathematical operations on each image pixel. This means that, among other things, you can add, subtract, multiply or divide the images created by your narrowband filters to control the strength of each in your RGB channels. Here we'll take you through the steps we followed using Pixel Math in Siril to combine and colourise narrowband data, to produce a HOO image of the North America Nebula.