Integration Time Calculator: How Many Subs to Stack?

Stacking is how faint nebulae and galaxies come out of the noise, but the payoff is not linear. Enter your sub length, the gaps between frames and how long you can image tonight, and this works out how many usable subs you will get, how many nights your target total needs, and what each extra hour buys you. The rule that shapes every plan: noise falls with the square root of total exposure, so making an image twice as clean takes four times as long.

Why more hours always help, but less each time

Every sub contains the same faint signal plus random noise. Add frames together and the signal grows in step with the number of frames, while the random part only grows with its square root. The result is that the signal to noise ratio of a stack rises with the square root of the total exposure. Four hours is twice as clean as one hour; sixteen hours is twice as clean as four.

That is why the table in the result shows the gain against one hour rather than against a single frame. Going from one night to two is a clear visible step. Going from eight hours to ten is real but subtle, and on a UK winter with few clear nights it may be better spent starting a second target.

Sub length versus sub count

For the same total, fewer long subs and more short subs end up closer than most people expect, as long as each sub is long enough for the sky background to swamp the camera's read noise. Under a bright suburban sky that point arrives quickly, so short subs lose very little. Under a truly dark sky or through a narrowband filter the background is faint, which is why longer subs are the norm there. Short subs have practical advantages too: a gust of wind or a passing plane costs you seconds, not minutes, and tracking errors have less time to show.

Making the plan work on the night

• Use the real dark window. Our astronomical darkness calculator gives tonight's usable hours; in a British June there may be none at all.
• Watch the gap. Ten seconds of download and dither after a two minute sub costs about 8% of the night. After a ten second sub, the same gap would halve your time on target, so dither every few frames instead of every frame.
• Be honest about rejects. Check the subs after the first session and use your real rejection rate for the next plan.
• Keep calibration consistent. Darks need the same exposure and sensor temperature as the lights, and flats should be shot without moving the focus or the camera between sessions on the same target.