Why Is My Stacked Image Green?

You stack a night’s frames, open the result, and the whole thing is green. Not slightly green - a flat, swampy, uniform green with a few white specks in it. It looks broken. The obvious conclusion is that something went wrong in the stacking.

Nothing went wrong. That green is what a colour camera actually records, and every stacked image from every one-shot-colour sensor starts out looking like that. Here is why, and what happens to it.

The sensor has twice as many green pixels

A colour camera does not record colour. It records brightness through a mosaic of tiny filters - red, green and blue - laid over the pixels in a repeating pattern. In almost every sensor that pattern is two green pixels for every red and every blue. The arrangement is named after its inventor, Bryce Bayer, and it copies the human eye, which is most sensitive to green.

So before anything else happens, half of your data is green. Then two more things push the same way. The green filter passes the widest band of wavelengths, so a green pixel collects more light per second than a red or blue one behind the same lens. And the sky itself - light pollution, airglow - is strongest in the green and yellow part of the spectrum.

Add those up and the green channel in a raw stack is routinely one and a half to two times brighter than red or blue. Displayed as-is, that is a green image.

Your camera hides this from you every day

A daylight photograph from the same camera is not green, because the camera applies a white balance before you ever see it: it multiplies the red and blue channels up until a grey card looks grey. That correction is baked into every JPEG and applied in every raw converter by default. You have never seen your camera’s uncorrected output, which is why the stack looks so alarming.

Astrophotography software works from the uncorrected data on purpose. The daylight white balance is wrong for a night sky - it was calibrated for sunlight reflected off things, not for starlight and airglow - so the correction has to be derived from the data itself.

What “removing the green” means

Two separate corrections, and it matters that they are separate.

Neutralising the background shifts each channel so the empty sky is the same brightness in all three. This is a subtraction, not a multiplication: it moves the sky to grey and leaves everything brighter than the sky exactly as it was. The green cast of the sky goes away; a green nebula would stay green.

White balancing multiplies the channels so that a known-neutral thing is neutral. The problem is finding the known-neutral thing. Daylight photography uses a grey card. Astrophotography uses stars - either the typical star in the field, on the argument that most stars are roughly sun-coloured, or, better, specific stars whose colours are in a catalogue. Akastroid does the first by default and the second when your frames record where the telescope was pointing. (Why the choice of reference star matters.)

Only after both does the image have colour that means anything.

Why Akastroid shows you the green

The split view in the Processing tab puts the raw stack on one side and the result on the other, and the raw side is green. That is deliberate, and an earlier version of the app quietly neutralised the raw side because the green looked like a mistake.

It was put back because hiding it hides the work. The green is real. It is the data. Removing it correctly - not just pushing a “less green” slider, but measuring what the sky and the stars actually are and correcting from that - is one of the more useful things the pipeline does, and a before-and-after that starts from a pre-cleaned image shows almost no difference.

If the green side bothers you, drag the divider. But it is not a fault, and a raw stack that is not green would be the thing to worry about.

The short version

• A colour sensor has twice as many green pixels, the green filter passes the most light, and the sky glows green. A raw stack is green.
• Your camera applies a daylight white balance to hide this; astrophotography works from the uncorrected data because the daylight balance is wrong for the sky.
• The fix is two steps: shift the sky to grey, then scale the channels so a reference star is neutral.
• The green “before” image in Akastroid is the honest starting point, not a bug.