Stacking Frames Tracking Photo

How Many Frames to Stack for a Target SNR Gain

Stack astrophotos and the signal climbs in step with how many frames you have, but the random noise only climbs with the square root of that count. So the signal-to-noise ratio ends up improving as SNR gain = √N, where N is the number of stacked frames. Flip the equation around and you get the frame count for whatever gain you're after: N = (desired gain)². Want 5× the SNR? That's 5² = 25 frames. Want 10×? Now you're at 100.

Since a square root is doing the work here, the payoff fades fast. Jumping from 25 to 100 frames means four times the shooting time just to double the SNR one more time. Total integration time comes out to N × exposure per sub, which makes a 25×120 s plan run 50 minutes and a 100×120 s plan stretch past three hours. And even then atmospheric seeing, sensor heat and tracking errors all put a ceiling on what you actually gain.

Applications

It helps you judge whether a faint target is worth the extra hours. Set the SNR gain you want over a single frame and the tool tells you how many subs that takes, which is really telling you how much total time you're signing up for.

FAQ

Why does doubling frames not double SNR? Noise adds in quadrature, which is why the improvement tracks √N instead of N. To double the SNR you have to quadruple the frames.

Is there a point of diminishing returns? There is. Somewhere between a few dozen and a couple hundred frames, each new frame buys you almost nothing once you weigh it against the added time and the hard limits of seeing and thermal noise.

Do more frames replace longer exposures? Only up to a point. Stacking averages away the random noise, yet very faint signal still needs enough exposure per sub to climb above the read noise floor in the first place.