LearnInstruments & observingSome maths
Photon noise
The irreducible grain in any measurement of light, arising because photons arrive at random and counting N of them carries an uncertainty of √N.
Why it matters
It is the noise floor no amount of better electronics removes. Everything else — read noise, dark current, light pollution — is something you can fight; this one you can only outlast.
Leads to
- Signal, noise and stackingCombining many exposures so that the signal adds linearly while random noise adds as the square root, improving the ratio as √N.
- Light pollution and the Bortle scaleA nine-point ranking of night-sky brightness, from a pristine site where the Milky Way casts shadows to an inner city where a handful of stars survive.