Optimal sub-exposure
Expose long enough—not longer than the sky allows.
Balance read-noise swamping against bright-star headroom to find a defensible sub-exposure range.
Purpose and method
What this calculator does
How long should each subframe be? Balance read-noise swamping against highlight saturation.
In words: Expose until background variance sufficiently exceeds read-noise variance, subject to saturation.
Change a value and the result updates immediately. A measured background electron rate is more reliable than a Bortle-only estimate. The recommendation is a planning bound, not a guarantee against saturation across the whole field.
Working geometry
Exposure envelope
The useful exposure lies after the background-noise threshold and before the saturation ceiling.
Inputs
Camera and measured sky
Calculated now
Exposure envelope
- Minimum useful exposure
- 107.14 s
- Saturation ceiling
- 80.00 s
- Recommended sub-exposure
- 80.00 s
- Current limiting factor
- Star saturation
Governing equation
In words: Minimum time equals the selected sky-noise multiple times read-noise variance, divided by the combined sky and dark-current rate.
Show the working
The calculator applies the equation above to the values currently entered, without rounding until this display.
- Minimum useful exposure
- 107.14 s
- Saturation ceiling
- 80.00 s
- Recommended sub-exposure
- 80.00 s
- Current limiting factor
- Star saturation
A measured background electron rate is more reliable than a Bortle-only estimate. The recommendation is a planning bound, not a guarantee against saturation across the whole field.