A list of the most important telescope specifications (Terence Dickinson)

Telescope Glossary

Can’t tell a Maksutov from a Newtonian? In the dark about the difference between resolution and focal ratio?  This  glossary of optical terminology will help.

The following terms represent the most important optical specifications of any telescope:

Note: Magnification is one trait that can be ignored. The general magnification limit for a telescope is 50x the aperture in inches or 2x the aperture in millimetres. For example, the maximum usable power for a 60mm telescope is only 120x. Claims that such a telescope can magnify 400x are misleading, intended solely to lure the unsuspecting buyer.

Aperture and Light-Gathering Power

Telescopes are rated by their aperture. A 4-inch instrument has a main lens or mirror 4 inches in diameter. The larger the lens or mirror, the more light it collects, providing brighter and sharper images. An 8-inch telescope has four times the surface area, and therefore light-gathering power, of a 4-inch, making its images four times brighter.


In theory, an 8-inch telescope can resolve twice as much detail as can a 4-inch instrument. The resolving power of a telescope can be estimated with a simple formula: Resolving power (in arc seconds) = 4.56 divided aperture of telescope (inches); or 116 divided aperture of telescope (mm). This is the empirical rule devised by William Dawes in the 19th century. When manufacturers list a resolving power, they are merely stating the Dawes limit for the aperture of telescope, not a measure performance value for that specific model.

Focal Length

The length of the light path from the main mirror or lens to the focal point (the location of the eyepiece) is the focal length. With Maksutov- and Schmidt-Cassegrains, the optical path is folded back on itself, making the tube shorter than the focal length.

Focal Ratio

The focal ratio is the focal length divided by the aperture. For example, a 100mm telescope with a focal length of 800mm has a focal ratio of f/8. For photography, faster f/4 to f/6 systems yield shorter exposure times (therefore, these are known as fast focal ratios). But when used visually, image brightness depends solely upon the aperture. Focal ratio has nothing to do with it.


A promise of diffraction-limited optics means aberrations in the optics are small enough that image quality is affected primarily by the wave nature of light and not by errors in the optics. This is equivalent to stating that the optics provide a final error at the eyepiece of only one-quarter of a wavelength of light (the wavefront error), meeting the so-called Rayleigh criterion, a minimum standard for amateur telescopes. Anything worse, and planets will look soft, if not blurry. Contrary to some ad claims, diffraction-limited does not mean the optics cannot be improved upon. Premium telescopes can do better, with wavefront errors of 1/6 to 1/8 wave. Under good conditions, tests have proved that date difference is noticeable, but the performance edge over 1/4 wave optics comes at a high cost.

Central Obstruction

While the secondary mirror in a reflector blocks some light, the loss is not significant. The noticeable effect is the smearing of image contrast caused by the added diffraction of light from the obstruction. This effect is proportional to the diameter of the secondary mirror. As such, central obstruction should be stated as a percentage of the diameter of the aperture. An 8-inch scope with a 2.75-inch-diameter secondary mirror has a central obstruction of 34 percent. To make numbers even smaller, some companies state obstruction as a percentage of area (12 percent in this example). In general, a central obstruction of 20 percent of lower by diameter produces a negligible effect.

Types of Telescopes

Achromatic Refractor

Uses a doublet lens with elements made of crown and flint glass. In f/10 to f/15 focal ratios, chromatic aberration is negligible.

Apochromatic Refractor

To eliminate false colour, some apos use triplet lenses with elements of Super ED glass. Others use flourite doublets or small corrector lenses near the focuser.

Newtonian Reflector

Invented by Isaac Newton in 1668, this classic design uses a concave primary mirror (preferably with a parabolic curve) with a flat secondary mirror.


An aspherical corrector plate compensates for aberrations in the f/2 spherical mirror. A convex secondary folds the light path down the stubby tube.


Based on a design invented by Dmitri Maksutov in 1941, the Mak-Cass uses a steeply curved corrector lens. The all-spherical surfaces are easy to mass-produce.


This hybrid design, usually f/4 or f/5, combines a Schmidt corrector with Newtonian optics to reduce the off-axis coma inherent in fast Newtonians.


Usually made in f/6 focal lengths, this design boasts a view free of aberrations across a wide field of view at low power and refractorlike images at high power.

(Excerpted from The Backyard Astronomer’s Guide)