Mono and narrowband: LRGB, SHO and HOO

A mono camera sees no color at all

Every pixel measures light, and none sits behind a colored filter it does not need. The color comes from shooting the same target through several filters and combining them afterwards. That combining step is usually where people open a second program and a tutorial.

In Akastroid it is one screen: choose the files, check the channel each landed in, pick a palette, and compose.

The palettes

A whole filter-wheel folder in one go

You do not have to stack each filter yourself first. Drop the folder straight from the capture: the frames are grouped by filter, each filter is judged against its own frames (a blue sub holds fewer stars than a red one and is not a worse frame for it), stacked separately, and combined. A filter shot on another night that will not match the others is stacked on its own and aligned afterwards.

When the folder holds both narrowband and red, green and blue, you choose the picture before it runs: natural color with Hα (the true-color image, with the hydrogen frames deepening the nebula's red), or the Hubble palette or HOO. Natural color is combined from the linear stacks so a star keeps one color however bright it is; the false-color palettes are balanced channel by channel, which is what evens out three emission lines an order of magnitude apart.

Filters read from the files

Each file's filter is read from its header or its name, in every spelling capture software uses (Ha, H-alpha, OIII, O3, SII, R, G, B, L and dual-band names like Ha-OIII), so an Hα, OIII and SII set usually lands in the right slots before you touch anything. Anything it cannot tell is left unassigned for you to choose rather than guessed. Change any of them with one click; the palette then reassigns the whole set.

Registered on their stars

Filters are shot hours or nights apart, and the stacks rarely line up exactly. They are aligned on their stars before anything is combined, different sizes onto the largest. A file that cannot be matched is left out and named rather than blended in half a pixel wrong. If your stacks are already aligned, say so and that step is skipped.

On the public M42 LRGB dataset, measured on 3 October 2026 against Siril 1.4.4, the median misalignment between color channels was 0.17 px in Akastroid and 0.18 px in Siril, and star width on the mono stack was 1.64 px against 1.67 px half-width. Star signal-to-noise on the mono stack was lower, 0.90 to 0.92 of Siril's; Akastroid kept 46 of the 51 frames, where Siril stacked all 51.

When a channel is missing

Two filters make a real image. When a color channel has no filter behind it, the app says so: a missing green is interpolated between the bands you have, a missing red or blue borrowed from the nearest, and saturation held back so the picture does not split into two complementary colors. Filters with no red, green or blue equivalent are ordered by wavelength, and you are told the hues represent wavelength rather than what the eye would see.

Dual-band on a color camera

If you shoot a one-shot-color camera through an L-eXtreme, L-Enhance, ALP-T, NBZ or similar filter, you do not need the composer: Deep Sky recognizes the filter from its name, and treats data whose stars come out far from neutral the same way. The star colors are left alone instead of balanced to white, which is what usually turns a dual-band image muddy; only the background is neutralized.

What each line is and which targets suit it are covered in the guide to narrowband filters. The stacks themselves come from the deep-sky workflow.