aperture
When astronomers describe a telescope, the first number they quote is not its length or its magnification but its aperture — the diameter of its main lens or mirror. A '10-metre telescope' means its primary mirror is 10 metres wide. Aperture is the single most important figure of merit, because it sets both how much light the telescope collects and how fine the detail it can resolve.
Aperture matters because the front opening acts like a bucket catching rain: the wider the opening, the more photons fall in each second. Since the area of a circle grows as the square of its diameter, doubling the aperture quadruples the light collected. An 8 m mirror has roughly a million times the area of a fully dark-adapted human eye (about 7 mm), so it can register stars a million times fainter. Aperture also sets the sharpest possible detail: a bigger opening makes a tighter diffraction pattern, so larger telescopes are inherently sharper.
This is why the history of astronomy is a race for ever-larger apertures, from Galileo's few centimetres to today's tens of metres. The catch is cost and engineering: aperture grows linearly but mirror mass and price balloon, which is exactly why telescopes switched from lenses to mirrors and then to segmented mirrors. Aperture is not magnification — a small telescope can magnify a lot, but it cannot make a faint galaxy bright or a close double star separate.
Going from a 4 m to an 8 m telescope is not twice as good but four times: it collects four times the light, so it can see objects two magnitudes fainter in the same exposure time.
Light grows as aperture squared, not aperture itself.
Aperture, not the eyepiece, is the limit on what you can see. No eyepiece swap can let a 60 mm telescope reveal a galaxy that needs a metre of aperture to register.