📸 Deep Sky Exposure Planner

How long a single subframe needs to be before the target signal clears the read and sky noise, and how many of them you need to stack. Includes the f-ratio squared penalty that makes fast scopes so much more efficient.

Enter your setup.
Formulas

Focal ratio f = FL / aperture; sky signal per pixel scales as (aperture / FL)², which is why a fast f/4 scope reaches a given sky level in a quarter of the time of an f/8. The minimum subframe is set by requiring the sky signal to exceed the read noise by a target factor — conventionally 3× the read noise in quadrature, giving tₘᵢₙ ≈ (3·RN)² / sky rate. Stacked signal-to-noise grows as √N, so doubling your total time buys only 41% more depth. Dark current and tracking limits are not modelled here.