Signal-to-noise climbs with the square root of frame count, so the curve flattens as the stack grows. Axes are in frames (with shoot time) and SNR relative to a single sub.
Calibration frames — practical counts
Darks30
Flats20–30
Even light through the same optics, focus & rotation. A few minutes total.
Bias50–100
Shortest exposure the camera allows, capped. Seconds total — shoot plenty.
Dark-flats20–30
Darks taken at your flat exposure. Use these instead of bias on most CMOS sensors.
Flats, bias and dark-flats are set by your optics and camera, not how many lights you shoot — the ranges above are the practical sweet spot at any session length. Darks roughly track your light count but level off near 50; beyond that, extra darks barely move the noise and your time is better spent on more lights. Best of all, build a reusable dark/bias library at fixed gain & temperature and you can skip re-shooting them.
What these frames do
Light frames
Your actual exposures of the target. Everything below exists to clean these up before they're stacked.
Dark frames
Same exposure length, ISO/gain and sensor temperature as your lights, but with the scope capped. They map thermal signal, hot pixels and amp glow so those can be subtracted from every light.
Flat frames
An evenly lit frame through the same optics, focus and camera rotation — twilight sky, a flat panel, or a white screen. They cancel vignetting and dust shadows, the uneven brightness across the frame.
Bias frames
The shortest exposure your camera can take, capped. They record the sensor's read-noise floor and electronic offset — the zero point that lets darks and flats subtract cleanly.
Stacking averages out random noise: signal-to-noise improves by √N for N equal sub-exposures. Quadrupling your frames only doubles SNR — worth knowing when you decide how long to stay out.