Home / Guides / Telescope aperture: the single most important number
Telescope aperture: the single most important number
Aperture is the diameter of a telescope's main lens or mirror, and it is the single most important specification. It decides how much light the telescope gathers and how fine a detail it can resolve.
Aperture gathers light
The wider the aperture, the more light the telescope collects, and light grasp rises with the square of the aperture. A 200 mm telescope gathers four times as much light as a 100 mm one, not twice, because it has four times the area. That extra light is what lets you see fainter galaxies, nebulae and stars.
This is why experienced observers chase aperture for deep-sky work: there is no substitute for raw light-gathering when the target is faint.
Aperture sets resolving power
Aperture also sets the finest detail a telescope can show, its resolving power. A larger aperture can split closer double stars and reveal finer structure on the Moon and planets, when the atmosphere is steady enough to allow it.
A rough guide to the highest useful magnification is about twice the aperture in millimetres, so a 150 mm telescope tops out near 300x on a night of good seeing. Pushing past that shows a bigger but blurrier image, not more detail.
The trade-off with portability
More aperture means a bigger, heavier telescope that costs more and takes longer to set up and cool down. The best telescope is the one you will actually carry outside and use, so a modest aperture used often beats a large one left in the cupboard.
For many observers a 130 mm to 200 mm telescope is the sweet spot: enough aperture for rewarding deep-sky views, still portable enough to use on a weeknight.
Match an eyepiece to your telescope
Open the Fit My Telescope toolSee the exit pupil your aperture and eyepieces giveFind telescope on eBayaffiliateRelated
Sources: Telescope Optics, Rutten and van Venrooij; Sky & Telescope optics primers ยท Optical telescope (Wikipedia, CC BY-SA)