How many subs do I need?
Pick a deep-sky target, your smart telescope and your sky. Get the total integration time and the number of sub-exposures for a quick look, a solid image, or a showpiece.
Calculator
How it works
- Each target's brightness per square arcsecond, from its catalogue magnitude and size, sets how many photons land on each pixel.
- Your Bortle class sets the skyglow photons that land on the same pixel. The two compete; the sky usually wins from town.
- Your scope's aperture, pixel scale, read noise and sub length fill in the rest. The model solves for the time that reaches a target signal-to-noise. Full method and assumptions.
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Telescopes
Questions people ask
What is a sub?
A sub, or sub-exposure, is one of the short frames a smart telescope takes and stacks. A Seestar takes 10-second subs by default; 360 subs make an hour of total integration. The stacked image improves with the square root of total time, so four times the subs gives twice the signal-to-noise.
Why does the sky matter more than the telescope?
Skyglow adds noise to every pixel. Going from a Bortle 8 city sky to a Bortle 4 rural one cuts the sky background roughly ten-fold, and for faint targets that shortens the time needed for the same result by a similar factor. No affordable telescope upgrade gets you that.
Are more subs always better?
Yes, with diminishing returns. Doubling total time improves signal-to-noise by about 41%. The figures here mark sensible stopping points for three quality levels; there is no hard ceiling, only your patience and the weather.