How Telescopes Work

The purpose of a telescope is not to magnify, as commonly thought, but to collect light. The larger the telescope's main light-collecting element, whether lens or mirror, the more light is collected. Importantly, it is the total amount of light collected that ultimately determines the level of detail—in a distant landscape or in the rings of Saturn—visible through the telescope. Although magnification, or power, is useful, it has no inherent effect whatever in determining the level of detail visible through a telescope.

Types of Telescopes

All telescopes fall into one of three optical classes. The relative advantages of each of these telescope designs will be made clear below.

Refracting   In the refracting telescope

(a) light is collected by a 2-element objective lens and brought to a focus at F. By contrast the reflecting telescope
Reflecting   (b) uses a concave mirror for this purpose. The mirror-lens, or catadioptric, telescope
(c) employs a combination of both mirrors and lenses, resulting in a shorter, more portable optical tube assembly. All telescopes use an eyepiece (located behind the focal point, F) to magnify the image formed by the primary optical system.


Refracting Telescopes use a large objective lens as their primary light-collecting element. Meade refractors, in all models and apertures, include achromatic (2-element) objective lenses, in order to reduce or virtually eliminate the false color (chromatic aberration) that results in the telescopic image when light passes through a lens. Example: Meade NG70-SM, 90AZ-ADR, DS2080 AT-TC, ETX80 AT-TC.

Reflecting Telescopes use a concave primary mirror to collect light and form an image. In the Newtonian type of reflector, light is reflected by a small, flat secondary mirror to the side of the main tube for observation of the image. Example: 114EQ-ASTR, DS2114.

Mirror-Lens (Catadioptric) Telescopes employ both mirrors and lenses, resulting in optical configurations that achieve remarkable image quality and resolution, while housing the optics in extremely short, highly portable optical tubes. Example: Meade ETX90 AT-TC and ETX125 AT-TC .


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