Image stacker
Combine a burst of night-sky exposures into one clean image. Sensor noise is different in every frame while the stars are not, so averaging many frames cancels the grain and leaves the sky. Runs entirely in your browser — your photos are never uploaded.
About this astro stacker
A single night-sky exposure is mostly noise. Sensor noise is random and different in every frame, while the stars land in the same place each time — so averaging many frames cancels the noise and leaves the signal. Stacking sixteen frames roughly quarters the noise, which is often the difference between a grainy smudge and a visible galaxy.
This tool aligns your frames automatically before combining them, correcting the drift between exposures that comes from an untracked mount or a slightly bumped tripod. It reports how much drift it corrected, which is a useful read on your setup.
Your photos are never uploaded. Alignment and stacking run inside your browser tab. Unlike the other tools here, this one also does not download a model — the arithmetic is written from scratch, so nothing is fetched and nothing is sent.
Common questions
- How many photos do I need to stack?
- Noise falls roughly with the square root of the frame count, so four frames halve it and sixteen quarter it. Even three or four frames make a visible difference; more is better until your frames start drifting too far apart to overlap.
- Why is my stacked image so dark?
- Astro stacks are naturally very dark, with the interesting signal sitting just above the noise floor. Auto-stretch is on by default and rescales the histogram so faint detail becomes visible. Turn it off if you plan to stretch it yourself in an editor.
- Which combine mode should I use?
- Average gives the best noise reduction and is the right default. Median is slightly noisier but throws out anything that appears in only a few frames, which removes satellite trails and aircraft lights. Brightest keeps the maximum value per pixel, which is how you make star trails.
- Do my photos need to be the same size?
- Yes. All frames must share the same dimensions, since they are combined pixel for pixel. Shoot the whole set on the same camera at the same resolution and do not crop them beforehand.
- Does it correct for field rotation?
- No. Alignment corrects translation — the drift you get from an untracked mount over a few minutes. Field rotation needs a different and much slower approach. For wide-field shots over short sequences, translation is what dominates.
- How many frames can the browser handle?
- Up to 25 frames, within an overall pixel budget, because every frame stays decoded in memory for the whole run. Full integration runs across hundreds of full-resolution frames need a desktop app.