Limiting magnitude
Also called: NELM, naked-eye limiting magnitude.
The faintest magnitude you can see — with the naked eye (NELM) or through a given telescope.
Your naked-eye limiting magnitude measures the sky: about 4 or less in a city, 5–6 in the suburbs, 7 or more at a truly dark site. A telescope's limit adds roughly 5 × log₁₀(aperture ÷ 7 mm) magnitudes to that — so the darker your sky, the deeper every telescope goes.
At the eyepiece
Estimate your NELM by counting stars inside a known triangle of stars, or simply note the faintest star you can hold in the Little Dipper.
In AstroPilot
Gear pages give each telescope's limiting magnitude under a dark sky.