LearnInstruments & observingNaked eye
Light pollution and the Bortle scale
A 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.
Why it matters
Sky brightness is a noise source. It sets how long a single exposure can usefully be before the background swamps the target.
In practice
It is also the argument for narrowband: a 7 nm filter rejects almost the entire spectrum that streetlights emit while passing the line you actually want.
Rests on
2 concepts in the full chain back to first principles.
Leads to
Nothing here builds on it yet.