Read-noise limited
Very short subs repeatedly pay the camera's read-noise penalty. Darker skies, narrow filters and high-read-noise CCDs push the required exposure longer.
Astro Lab
Camera noise & sky-background planner
Sub-exposure Optimizer
Estimate how long a single deep-sky exposure needs to be before camera read noise becomes a small part of the total background noise. Then balance that against highlight headroom, guiding and file count.
1. Camera & gain
Choose a preset, then select the gain/readout profile you actually use.
Gain 100
2. Optical system
For an extended sky background, focal ratio and pixel size are the key optical inputs.
f/
Include any reducer or Barlow
3. Sky brightness
Bortle is convenient but coarse. A measured SQM value gives a better planning estimate.
Editable — overrides the Bortle preset
4. Read-noise target
Choose how small the read-noise contribution should be after sky background is included.
Advanced assumptions
Throughput combines mirrors/lenses, window and filter transmission. The filter bandwidth is an equivalent planning bandwidth, not a full spectral integration.
How to interpret the result
The shortest scientifically efficient exposure is not automatically the exposure you must use.
Efficient range
Once sky shot noise dominates the read noise, longer individual subs add progressively less faint-detail benefit. Total integration remains the major driver of final SNR.
Highlights still matter
This tool cannot know the brightness of every star in your frame. If bright stars saturate, shorten the sub even if the noise calculation permits longer exposures.
Planning model, not a sensor lab
The sky-rate estimate uses focal ratio, pixel size, an approximate SQM/Bortle brightness, effective filter bandwidth, camera QE and optical throughput. Real light pollution spectra, moonlight, filter curves, sensor QE curves and local sky gradients vary. If you have measured SharpCap Sensor Analysis values, choose Custom / measured camera and enter them.