The aperture of a telescope is one of its key attributes. Telescopes, like camera lenses, are designed to gather image-forming light and produce a critically focused image which can be viewed or recorded. Fast lenses, that is those with wide apertures that consequently allow the use of fast shutter speeds, gather more light and are therefore generally more useful in low-light conditions. The same reasoning is valid for telescopes because they are required to gather as much light as possible from faint and very remote objects. A large aperture is therefore a significant advantage. telescope apertures are normally measured in inches, so a six-inch aperture scope will have an objective diameter of 152.4mm.

 Optical quality is also very important. The overall performance of a lens or telescope is determined by the weakest part of its specification. A huge maximum aperture is of little use if the images are of poor quality. When selecting a telescope it is therefore necessary to balance the various key attributes of each product. An instrument with a slightly smaller aperture might be a better choice if the overall optical quality is obviously better. Unfortunately, cost also rises with increasing aperture and optical quality. Large, high-quality lenses are more expensive to manufacture and will always cost more to purchase. In the end, you get what you pay for. Larger apertures also mean increasing expense, size and weight.

The focal length of a telescope is the optical distance from the objective lens or mirror to the plane of focus. Some telescopes use a mirror to reverse the path of the light before the focal plane is reached, thereby reducing the physical dimensions of the instrument. The greater the focal length, the larger the image formed and the higher the magnification of the telescope. Greater magnification is desirable when viewing small and remote objects but, as with a pair of binoculars, a high-magnification instrument is more difficult to position, track and hold stable. A point is eventually reached where a higher magnification reduces the detail that can be perceived.

Focal ratio, or f/number, is the relationship between aperture and focal length. It is defined by:

FR = FL / A

where FR is the focal ratio, FL is the focal length and A is the maximum aperture.

For example, a simple refracting astronomical (inverting) telescope with a focal length of 1200mm and an objective diameter (aperture) of 127mm would have a focal ratio (f/number) of 1200 / 127 = approximately f/9.5. This is reasonably "slow", and might be appropriate for photographing relatively bright objects such as the Moon or the major planets because exposures can be kept acceptably short.