/subexposureprecise
/subexposureprecise <sky_background_rate> [camera] [read_noise] [noise_tolerance]
Calculates a precise sub-exposure length for TRACKED
astrophotography using Robin Glover’s (SharpCap) real formula,
confirmed directly from Glover’s own posts on SharpCap’s
official forum: sub-exposure = C × read noise² ÷ sky
background rate. For a rough starting-point range instead,
needing no camera-specific numbers, see /subexposure.
Usage: /subexposureprecise camera: [saved profile] sky_background_rate: 0.6 — or read_noise: 1.5 instead of
camera if you haven’t saved one. Get sky_background_rate
(the Sky Background Electron Rate, in e⁻/pixel/s — a different
thing from “sky brightness” in the Bortle/SQM sense, since this
one also depends on your optics and camera, not just the sky)
from [SharpCap’s free Sky Background Calculator]
(https://tools.sharpcap.co.uk) — deliberately not derived from
Bortle class alone here, since that would need combining
Bortle/SQM with your focal ratio, pixel size, and quantum
efficiency together, a relationship SharpCap’s own tool
computes but doesn’t publish as an open formula. Add
noise_tolerance if you want tighter (2%, more subs needed)
or looser (10%, fewer/longer subs) than the 5% default.
Narrowband filters (e.g. a 3nm Ha filter) don’t need a
different formula or a separate multiplier — get a fresh
sky_background_rate from SharpCap’s calculator FOR that
filter (entering its bandwidth in nm) rather than reusing a
broadband value, and this same formula naturally produces the
much longer sub-exposure narrowband needs, since a narrowband
filter’s effect is entirely captured by how much it lowers the
measured sky background rate. Glover himself walked through
exactly this case on the SharpCap forum: a 3nm filter measured
around 0.14 e⁻/pixel/s versus a typical broadband value an
order of magnitude higher, which is why the required
sub-exposure jumps so much. Select Monochrome in SharpCap’s
calculator for a narrowband filter even on a mono camera in a
filter wheel — his own reasoning is that essentially all the
light passing a narrowband filter falls in one channel anyway.
(A “×25 for narrowband” rule of thumb circulates online, but it
traces to one forum poster’s own guess, self-flagged as “maybe
incorrect” — not confirmed by Glover the way the core formula
was, so it’s not used here.)
Treat the result as a genuine minimum, not a target to hit exactly — many imagers go 2-3× longer on a well-guided mount and get excellent results.
Parameters
| Name | Required | Description |
|---|---|---|
sky_background_rate | Yes | Sky Background Electron Rate in e-/pixel/s, from SharpCap's free Sky Background Calculator |
camera | No | A saved camera profile with read noise (see /camera) -- alternative to read_noise |
read_noise | No | Camera's read noise in electrons -- alternative to camera, overrides a saved camera's value |
noise_tolerance | No | How much read noise to accept (defaults to Standard/5%) Options: Tight (2% noise, more subs needed), Standard (5% noise, balanced) [default], Loose (10% noise, fewer/longer subs). |