How Long Should My Subs Be?

Sub length is the most argued-about setting in the hobby, and the argument ended years ago — most people just didn't get the memo. Here's the actual physics, in plain English, and the defaults that fall out of it.

Every time your camera reads out a frame it adds a fixed dose of electronic noise: read noise. Stack a hundred short subs and you've paid that dose a hundred times; stack ten long subs and you've paid it ten times. That's the entire argument for long subs. But there's a second noise source that grows with time: the sky itself. Photons from light pollution and airglow arrive randomly, and their shot noise grows with exposure. Once the sky noise in a single sub is several times bigger than the read noise, the read noise is 'swamped' — and making subs longer buys you essentially nothing.

What that means in practice

The sky sets your sub length. Under a bright suburban sky with a modern low-read-noise CMOS camera and a fast scope, the sky swamps read noise in tens of seconds — 30–60 s subs capture virtually everything 300 s subs would, with far less risk. Under a genuinely dark sky through a 3 nm narrowband filter, photons trickle in so slowly that you legitimately need 3–10 minutes per sub to climb over the read noise. Same camera, same math, opposite answers.

Meanwhile every extra minute of sub length raises the stakes: one wind gust, one guiding hiccup, one satellite through the core, one cloud — and you lose the whole sub. Losing one 60-second sub is a shrug; losing one 600-second sub is one percent of your night. Shorter subs also keep bright stars from saturating, which is why star colors survive better in short-sub stacks.