Electron Count & SNR Visualizer

For a given camera, target and sky, this estimates the per-pixel electrons recorded from the signal, sky glow, thermal (dark) current and read noise, how each contributes to the total noise variance, and how the stacked SNR varies with sky brightness and sub-exposure length. Vertical grey lines mark the Bortle sky-darkness scale. Equations are at the bottom.

Sensor and Optics Parameters

Camera (sets pixel, QE & dark)

ISO (sets read noise R)

Pixel size p (µm)

Quantum Efficiency QE (%)

Read noise R (e⁻)

Dark current D (e⁻/px/s @ 25°C)

Telescope/lens f-ratio f

Bandwidth B (nm)

Target & exposure

Target (sets integrated mag & size)

Target integrated mag

Major axis (arcmin)

Minor axis (arcmin)

Sub-exposure t_sub (s)

Total exposure T (hrs)

Sky brightness µ (mag/arcsec²) — Bortle slider (left = bright, right = dark)

Bortle 7 · µ 18.30 custom µ

Stacked SNR vs. sky brightness & sub-exposure time

Electron count vs. sky brightness (per sub-exposure)

SignalReadSkyThermalTotal

Noise variance contribution (per sub-exposure)

SignalReadSkyThermal

Equations

Per-pixel electron rates P (e⁻/s); T = total integration time, t_sub = sub-exposure length, so the stack has T/t_sub frames. Constants: f₀ = 10⁶ photons cm⁻² s⁻¹ arcsec⁻², B_ref = 90 nm. QE as a fraction, p pixel size (µm), w,h target major × minor axis (arcmin), R read noise (e⁻), D dark current (e⁻/px/s).

Stacked SNR

SNR = \frac{P_{tgt} T}{\sqrt{P_{tgt} T + P_{sky} T + D T + {(R \sqrt{\frac{T}{t_{sub}}})}^{2}}}

Optical factor

G = \frac{π \cdot 206265^{2}}{4 f^{2}}

Signal rate

P_{tgt} = \frac{QE \cdot G \cdot f_{0} \cdot \frac{100}{90} \cdot 10^{- 0.4 m_{tgt}} \cdot {(\frac{p}{10^{4}})}^{2}}{3600 \cdot w \cdot h}

Sky rate

P_{sky} = QE \cdot G \cdot f_{0} \cdot \frac{B}{B_{ref}} \cdot 10^{- 0.4 μ_{sky}} \cdot {(\frac{p}{10^{4}})}^{2}

Optimal sub-exposure

t_{opt} = \frac{10 R^{2}}{P_{sky}}

(sky shot-noise variance = 10 × read-noise variance)

Electron Count & SNR Visualizer v20260613

Sensor and Optics Parameters

Target & exposure