/subexposure

/subexposure <bortle> [guiding]

Suggests a practical sub-exposure length RANGE for TRACKED astrophotography (a properly polar-aligned mount actively tracking, e.g. a GEM) — a range from common astrophotography practice, not a precise calculation, for when you don’t have camera-specific numbers handy. For an actual formula-based calculation instead, see /subexposureprecise.

Usage: /subexposure bortle: 4 (if you don’t already know your Bortle class, the lightpollutionmap.app link in /sky lp shows one for your location) — optionally add guiding to narrow the range: unguided setups are capped by periodic error regardless of how dark your sky is, while excellent guiding with tight polar alignment lets you push toward the sky-limited ceiling instead.

Treat the result as a sensible starting point to refine from, not a target to hit exactly — your specific camera’s read noise and gain setting shift the real optimum in ways this can’t account for without that data.

Parameters

Name	Required	Description
bortle	Yes	Your sky's Bortle class, 1 (darkest) to 9 (inner-city) -- find yours via the /sky lp map link
guiding	No	Your tracking/guiding setup (defaults to Guided if not specified) Options: Unguided (tracking only), Guided (standard autoguiding), Excellent (guided + tight polar alignment).