Aperture

The diameter of the main mirror, which decides both how much light and how much detail.

Light-gathering scales with the area, so a mirror twice as wide collects four times as much and reaches far fainter objects. Resolution scales with the diameter directly. That single number is why telescopes are described by it and why every generation builds bigger: the 39-metre ELT will collect about a hundred times the light of a 4-metre. Above about 8 metres a single piece of glass becomes impractical, which is why everything larger is segmented. What aperture cannot buy is contrast against a bright sky. A larger mirror gathers more of the object and more of the airglow behind it in the same proportion, so on an extended target under a poor sky the view improves far less than the numbers suggest. This is why an hour's drive is often worth more than the next size up, and why nobody who sells telescopes leads with it.