Why does light pollution set the optimal sub-exposure?

Sky background light adds shot noise that grows with the square root of exposure time. Once that sky noise is several times the camera's fixed read noise, read noise becomes negligible. The brighter the sky, the faster it dominates, so light-polluted sites need shorter subs to be read-noise limited.

What does swamping the read noise mean?

It means exposing long enough that the random shot noise from the sky background is much larger than the camera's read noise, so stacking many subs gives nearly the same result as one long exposure. A common rule is to make sky-noise at least 3 to 5 times the read noise.

How is the optimal sub time calculated?

Set sky shot noise equal to a multiple of read noise: sqrt(sky_rate times t) = factor times read_noise. Solving gives t = (factor times read_noise) squared divided by sky_rate. A higher swamp factor or lower sky rate means longer subs.

Why not just take one very long exposure?

Long single exposures saturate bright stars, are ruined by satellites, planes, wind gusts, and guiding errors, and cannot be dithered. Many shorter subs that already swamp read noise give the same depth with far more flexibility, which is why the optimal sub is a minimum, not a target to exceed greatly.

How do I measure my sky background rate?

Take a short test sub, measure the median background level in ADU on an empty area, subtract the bias offset, multiply by the camera gain in electrons per ADU, and divide by the exposure time. That gives sky background electrons per pixel per second for that filter and sky.

Astrophotography Light Pollution Exposure Calculator

The most common astrophotography question from a light-polluted backyard is “how long should my subs be?” The answer is set by physics: expose just long enough that the noise from the sky background overwhelms your camera’s read noise, and no longer. This calculator finds that optimal sub-exposure from your sky rate and read noise.

How it works

Read noise is a fixed cost paid once per sub-exposure. Sky background contributes shot noise that grows as the square root of time. You want the sky shot noise to be several times the read noise so that read noise stops mattering:

sky_noise = sqrt(sky_rate * t)
goal:  sky_noise = factor * read_noise

Solving for the sub-exposure time:

t_optimal = (factor * read_noise)^2 / sky_rate

A “swamp factor” of 3 means sky noise is 3x read noise, which already pushes total read noise contribution to under 6% — a widely used target. Factor 5 is more conservative.

Why brighter skies want shorter subs

Because t is inversely proportional to the sky rate, doubling the sky brightness halves the optimal sub time. A Bortle 8 city sky might be read-noise limited in 30–60 seconds with a modern low-read-noise CMOS camera, while a dark Bortle 2 site needs several minutes.

Tips

Modern cooled CMOS cameras have very low read noise (1–2 e-), so optimal subs are short and many — perfect for dodging satellites and dithering between frames. Measure your sky rate per filter: narrowband filters cut the sky dramatically, pushing optimal subs into the multi-minute range even from a city. Treat the result as the minimum useful sub length.