Astrophotography SNR Estimator
Enter your signal, sky background and camera noise to see how clean a stack will be, how much total time a target needs and the shortest sensible sub length.
Signal to noise
SNR for single subs and the full stack, and how much time a target needs.
Per pixel, from a test frame minus the background.
Per pixel. Higher under city skies and with a bright Moon.
Per pixel, from the camera's data sheet at your sensor temperature.
Stacked SNR
32.3
60 subs, 2 h of integration. Target SNR reached.
- SNR per sub
- 4.18
- Signal per sub
- 60 e⁻
- Noise per sub
- 14.37 e⁻
- Subs for target SNR
- 52
- Integration for target
- 1 h 44 min
- Sky-limited sub length
- 18.75 s
Noise in one sub
- Target shot noise
- 7.75 e⁻
- Sky shot noise
- 12.00 e⁻
- Dark current noise
- 0.49 e⁻
- Read noise
- 1.50 e⁻
Sky limited. Read noise is only 1.1% of the noise in each sub, so longer subs would add little. Spend the time on more subs instead.
A simplified model: it ignores noise from flats, bias and dark calibration frames, and assumes every sub is equally good. Use it to compare plans, not to predict exact numbers.
How this is calculated
- 1
SNR of one sub
SNR = S·t / √(S·t + B·t + D·t + R²), where S is the target signal, B the sky and D the dark current in e⁻/s per pixel, t the sub length and R the read noise. With the defaults: 60 / √206.5, about 4.18.
- 2
Stacking
Averaging N equal subs improves SNR by √N, so 60 subs give 4.18 × 7.75, about 32.3. Doubling SNR always needs four times the integration.
- 3
Subs for a target
The number of subs needed is (target SNR / SNR per sub)², rounded up. Reaching SNR 30 here needs 52 subs, about 1 h 44 min.
- 4
Sky-limited sub length
Once sky noise dominates, longer subs add little. A common rule is t = 10 × R² / B, the length at which sky shot noise variance is ten times the read noise variance. Here that is 18.8 s.
Frequently asked questions
What is a good SNR for astrophotography?
An SNR of about 10 in faint areas looks noisy but usable. Around 20 to 30 looks smooth, and 50 or more gives clean results that stand up to strong stretching.
Is it better to take more short subs or fewer long subs?
Once each sub is sky limited, total integration time matters far more than sub length. Shorter subs then lose almost nothing and you lose less to satellites, wind and guiding errors. Below the sky-limited length, read noise makes short subs inefficient.
How does stacking improve signal to noise?
The signal adds up in proportion to the number of subs, while random noise adds up only as the square root. So SNR grows with the square root of the number of subs: 4 subs double it and 16 subs quadruple it.
Does cooling the camera improve SNR?
It reduces dark current, which matters most on warm nights and in long subs. In a Pakistani summer an uncooled sensor can easily run above 30 °C, where dark current can become a noticeable part of the noise.