MagnitudeThe brightness scale of objects, running backwards: the smaller the number, the brighter the object. Five magnitudes make a factor of 100, one magnitude a factor of about 2.5. Vega is 0, Sirius −1.5, Venus up to −4.6; the eye reaches 6 under a dark sky.
Light-gathering powerHow much light an instrument collects compared with the eye; it grows as the square of the aperture. Against a 7 mm pupil, a 200 mm telescope gathers (200 ÷ 7)² ≈ 800 times more light. That is what brings galaxies invisible to the naked eye into view.
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.
TransparencyHow clear the air is: free of haze, moisture, dust and thin cloud. Good transparency shows faint stars and contrasty nebulae. It does not always go with good seeing: hazy, still nights are often excellent for the planets.