Bortle scaleA 1 to 9 scale describing the quality of a night sky, proposed by John Bortle in 2001. In class 1 the sky is so dark that the Milky Way looks textured and full of detail; class 9 is a city centre where only a few stars show. Typical countryside in France or the UK is class 4 or 5.
SQMThe Sky Quality Meter, a small device that measures the brightness of the sky background in magnitudes per square arcsecond. The higher the number, the darker the sky: about 21.7 to 22 under a perfect sky, 19 in the suburbs, 17 or less in a city. It allows sites to be compared objectively.
Nebula filterA filter that passes only the few colours emitted by nebulae, notably ionised oxygen (OIII, around 500 nm) and hydrogen. It darkens the sky background and brings out emission and planetary nebulae, such as the Veil Nebula. It does not help with galaxies or star clusters.
Limiting magnitudeThe brightness of the faintest stars you can see. With the naked eye it ranges from 3 or 4 in a city to 6.5 or more under a very dark sky. A telescope of aperture D (in mm) pushes it further by about 5 log₁₀(D ÷ 7) magnitudes: a 200 mm telescope reaches roughly magnitude 14.