Sub-exposure Length: Why Shorter Isn't Always Safer
Short subs protect you from clouds and satellites, but they are a safety net rather than a strategy. Here's where the sweet spot actually sits.
AstroBear Team
Every astrophotographer hears the same advice early on: keep your sub-exposures short so you throw away less when a satellite, a gust of wind or a drifting cloud ruins a frame. It is good advice. It is also incomplete.
What a sub-exposure actually buys you
A longer sub-exposure does not add signal to your stack by itself. What it does is lift the faintest parts of the target above the read noise of your camera. Once the sky background comfortably dominates read noise, extra length buys very little.
Where the sweet spot sits
A useful rule of thumb is to expose until the sky background in a single frame is roughly ten times the read noise of your sensor. In light-polluted skies that arrives in seconds. From a dark site with a narrowband filter it can take several minutes.
Why very short subs hurt
Thousands of very short frames mean a lot of read noise in the final stack, very large files, and processing that takes most of the following week. Short is a safety net, not a strategy.
Why very long subs hurt
Long subs saturate stars, waste an entire frame when anything goes wrong, and demand guiding that many setups simply cannot hold for that long.
The practical approach
Estimate the length from your camera's read noise and your sky brightness, round down to something your mount guides reliably, and accept that the number changes when the moon returns or when you change filter. Recalculate when the conditions change, not once a year.