What aperture means
A telescope’s aperture is the diameter of its main light-collecting element, which is the objective lens in a refractor or the primary mirror in a reflector. It is usually quoted in millimetres (for example 80 mm) or inches (for example 8 inches). One inch is about 25.4 mm, so an 8-inch scope has roughly a 203 mm aperture.
Think of it as the size of the “window” the telescope uses to collect light. A larger window collects more.
Why aperture matters
Aperture does two main jobs.
- Light collection. The amount of light gathered depends on the area of the opening, not the diameter alone. Doubling the aperture gives about four times the collecting area. That is why faint galaxies, nebulae and star clusters look noticeably brighter in a larger scope. See how to calculate light gathering power for worked numbers.
- Fine detail. A larger aperture can, in principle, separate finer details, such as close double stars or subtle planetary markings. The theoretical limit improves in proportion to aperture, though in practice the atmosphere often limits what you see.
Aperture is not magnification
Magnification depends on the telescope’s focal length and the eyepiece you choose, not on aperture. A small scope can reach high magnification with a short-focal-length eyepiece, but the image will be dim and soft. Aperture sets how far you can usefully push magnification. A common rule of thumb is about 50x per inch (2x per mm) of aperture under good conditions, and many nights allow less. More on that in our guide to useful telescope magnification, and on the focal length side in what focal length tells you.
What is a good aperture?
There is no single right answer, since it depends on targets, budget and how much you are willing to carry. As general guidance:
- Around 70 mm. Often found in small, inexpensive refractors. Suitable for the Moon, bright planets, and bright star clusters. Quality varies a lot at this price, and these scopes can feel limiting for faint objects.
- 80 to 90 mm. A step up in brightness and detail. Reasonable for a portable “grab and go” scope, giving good Moon and planet views and the brighter deep-sky objects.
- 100 to 130 mm. A popular range for beginners who want more deep-sky capability while remaining manageable.
- 150 mm (6 inch) and above. Noticeably better for galaxies and nebulae. Large Dobsonians in the 8 to 10 inch range are often recommended for visual observers wanting good value per dollar of aperture.
So yes, 70, 80 and 90 mm can all be “good” if your expectations match: they are fine entry points, but you will see more with extra aperture.
Caveats on what the number means
- Obstruction. In reflectors and compound telescopes a secondary mirror blocks a small part of the opening, slightly reducing effective light and contrast compared with an unobstructed refractor of the same size.
- Optical quality. Well-made optics matter. A good smaller scope can outperform a poorly made larger one.
- Seeing. Turbulent air limits detail on many nights, so very large apertures do not always show proportionally more at high power.
The limits of a bigger scope
Larger apertures are heavier, bulkier, and take longer to cool to outdoor temperature. The best telescope is often the one you will actually carry out and use. Mount stability also matters, since a large tube on a wobbly mount is frustrating.
Related specs to look at
Aperture works together with other numbers. Focal length determines image scale, and the ratio of the two gives focal ratio, which affects field of view and photography. Read fast vs slow telescopes to see how those combine. If you are curious how the optics inside actually bend light, see concave vs convex telescope lenses.