Two things: how much light arrives, and how fine a detail can be separated. Both scale with the diameter of the opening, and neither can be recovered later by any eyepiece, filter or accessory.
Aperture is the first number on a telescope and the one most often crowded out by the second.
Light gathering
The opening collects light over its area, so a small increase in diameter is a larger increase in light. That is what decides whether a faint object is visible at all, and no accessory adds to it.
Resolving power
The same opening sets a limit on how close two points can be and still be seen as two. A larger aperture separates a tighter double star and shows finer structure on a planet, and this limit is a property of the optics rather than of the magnification used.
What aperture does not decide
Field of view, which comes from the focal length and the eyepiece. Image brightness at a given magnification, which involves the exit pupil. And whether a telescope is pleasant to own, which is decided by the mount and by how easily it comes out of the house.
The ceiling above it
The atmosphere. On most nights, turbulence limits the finest detail visible well before the optics do, which is why a larger aperture shows its advantage most clearly on faint objects and least clearly on planets.
If this page and the manufacturer’s own documentation disagree, follow the documentation. Every aperture, focal length, payload rating, field, eye relief, transmission figure and thread size on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And the distinction this page keeps: a magnification is not a property of a telescope. It is the focal length of the tube divided by the focal length of whichever eyepiece is fitted, so it changes every time the eyepiece does, and a figure printed on packaging is a statement about a supplied eyepiece. This page publishes both focal lengths and the aperture, and reproduces no packaging magnification as a specification.
The questions that keep arriving
Does doubling the aperture double what I see?
It more than doubles the light collected, because light gathering follows the area rather than the diameter. It also improves the finest detail that can be separated, which follows the diameter directly.
Is a larger telescope always better?
Only if it gets used. A larger aperture is heavier, takes longer to cool and is more affected by unsteady air, and the largest instrument that actually gets carried outside beats the larger one that does not.
Last reviewed 17 September 2026