Sky Background Electron Rate Calculator
Convert sky surface brightness or a photon-flux budget into electrons per pixel and per aperture, then quantify sky shot noise, read noise, dark current and a sky-limited exposure target.
Sky Background Electron Rate Calculator inputs
Choose either a calibrated electron zero point or a physical photon budget, then define pixel scale, aperture, exposures and detector-noise terms.
Sky Background Electron Rate Calculator results
How to use the sky background electron rate calculator
The sky background electron rate calculator converts surface brightness into electrons per second per square arcsecond and per pixel. It then adds sky, dark-current and read-noise variances across the selected aperture and stack.
Two physically different input paths
The sky background electron rate calculator supports a calibrated photometric zero point or a direct photon budget. In zero-point mode, the difference between sky surface brightness and the one-electron-per-second zero point is converted with the astronomical magnitude relation. In photon mode, collecting area, obstruction, throughput and detector quantum efficiency are applied explicitly.
Pixel scale and photometric aperture
Sky brightness is a surface rate, so the electron rate in one pixel scales with the square of the angular pixel scale. The aperture result multiplies the pixel rate by the number of pixels used for photometry. Enter the effective aperture size after masking, weighting or fractional-pixel treatment if the reduction software does not use a simple circular count.
Noise variance across a stack
Sky and dark-current shot noise follow Poisson statistics, while read-noise variance is added once per frame per pixel. The calculator reports variance contributions for the full aperture and exposure stack, their root-sum-square noise and the exposure duration at which the chosen sky variance exceeds read variance by the selected factor.
Calibration limitations
A magnitude zero point must be expressed in electrons per second for the same filter, atmosphere, gain convention and aperture calibration. Moonlight, airglow, light pollution, extinction, vignetting and gradients can change during an imaging sequence. Use measured image medians when available and treat this calculator as an exposure-planning model rather than a substitute for detector characterization.
Sky Background Electron Rate Calculator questions
Why is sky brightness entered in mag per square arcsecond?
It describes surface brightness independently of pixel size. Pixel scale then converts the surface rate into electrons collected by one detector pixel.
What is a sky-limited exposure?
It is an exposure long enough that sky shot-noise variance dominates read-noise variance by a chosen factor. It does not mean the sky is the only noise source.
Does binning change the result?
Hardware binning changes effective pixel scale and readout behavior. Enter the binned scale and the read noise appropriate to the camera mode.