Learn · Imaging
What is live stacking?
Live stacking means the telescope keeps taking short exposures, aligns them on the stars and averages them while you watch. Each new frame lowers the noise, so faint detail slowly appears. Four times as many frames give about half the noise.
Why not one long exposure?
A small telescope on an alt-azimuth mount cannot take a single exposure of an hour: the stars would trail, the field would rotate and one passing car or satellite would ruin it. Short exposures of 10 to 30 seconds avoid all of that. On their own they are noisy, but there are many of them.
What happens to every new frame
- The telescope finds the stars in the new frame and shifts and turns it to match the stack so far.
- Frames with trails, clouds or bad focus are rejected automatically on most models.
- The new frame is averaged into the stack and the result is stretched so faint parts become visible on your screen.
Why the noise goes down
Noise is random: in one frame a pixel is a bit too bright, in the next a bit too dark. The light from the nebula is the same every time. When you average N frames the signal stays and the noise shrinks by the square root of N. Four frames halve the noise, a hundred frames reduce it tenfold. That is why the image improves fast in the first minutes and more slowly after that.
The same nebula after more and more frames
How long should you stack?
Bright objects such as the Orion Nebula or the Ring Nebula look good after 10 to 20 minutes. Faint galaxies and large, dim nebulae are worth one to three hours, ideally spread over several nights. Light pollution adds noise, so from a city you need more time for the same result. The example below and our photo simulator show the difference.
Live stacking versus processing later
The live stack is a quick result for the screen. If your telescope saves the individual frames, you can stack them again later on a computer with free software such as Siril and get more out of the same data: better rejection of bad frames, gradient removal and careful colour processing.
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