Editorial Feature

Optical Telescope - How Does it Work?

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An optical telescope is an instrument that can make distant objects appear much closer. To make this possible, an optical telescope uses the following two instruments:

  • Objective lens or primary mirror
  • Eyepiece lens

The ability for a telescope to collect light is related to the objective lens’ diameter. This is also commonly referred to as the aperture. The main purpose of the aperture is to allow light to enter the telescope. In general, a larger aperture will result in a brighter image, as more light has been ‘collected’. 

Between the objective lens/primary mirror and the eyepiece lens, the light is focused because the lenses are curved. The magnification of a telescope is performed by the eyepiece lens and depends on the combination of lenses used. The eyepiece lens captures the bright, focused light from the objective lens, and magnifies it or spreads it out over the retina of the observer’s eye. As a result, the image appears larger to the observer.

By combining the objective lens/primary mirror with the eyepiece, a telescope is born.

Types of Optical Telescope

There are three main types of optical telescope:

•    Refractor telescope, which uses glass lenses
•    Reflector telescope, which uses mirrors
•    Catadioptric telescope, which uses the combination of lenses and mirrors

Refractor Telescope

Refractor or refracting telescopes use a convex glass lens to refract or bend light, bringing it into focus. This convex glass lens is commonly referred to as the objective lens. Convex lenses are designed specifically to be thicker at the center and thinner at the edges. This design allows the light to be bent into a single point of focus. This focus point is where the image is created.

Unfortunately, two main problems are associated with the refracting telescope. Firstly, the power of the convex lens is governed by the size of the convex lens. This also influences the physical size of the telescope, so they can end up being very large. Furthermore, because the light is refracted or bent, the image generated is not always clear.

Reflector Telescope

Instead of using convex lenses to refract light, reflector or reflecting telescopes use curved mirrors to gather and focus light. Large concave mirrors are used to gather and reflect the light to generate an image. Concave mirrors are designed so that the thinnest part of the lens is at the center; the exact opposite to the convex lens. The eyepiece lens then spreads or magnifies the image formed. When viewing dark objects, the reflecting telescope is extremely useful.

Catadioptric Telescope

Catadioptric telescopes can be considered a hybrid option. They use the combination of both a convex lens and mirrors to permit very fast focal ratios. The most famous catadioptric telescope designs are:

  • Schmidt–Cassegrain design
  • Maksutov–Cassegrain design

Schmidt–Cassegrain Design

The first catadioptric telescope is known as the Schmidt telescope. At the back of the Schmidt telescope is a primary mirror, and at the front of the telescope is a glass corrector plate. The purpose of the glass corrector plate is to eliminate spherical aberration.

The Schmidt–Cassegrain design is the most popular telescope type to date. The telescope employs a secondary mirror, which bounces light through to an eyepiece through a hole in the primary mirror.

The Schmidt–Cassegrain telescope offers many advantages. For example, the telescope is very portable because of the folded light path, this allows the optical tube to be very short. The Schmidt–Cassegrain telescope is suitable for planetary observations, as well as deep-sky observation due to its optics.

Maksutov–Cassegrain design

The second most famous type of catadioptric telescope Is the Maksutov–Cassegrain. The Design of the Maksutov telescope is similar to the Schmidt–Cassegrain telescope, the only difference is that the Maksutov design uses a meniscus corrector plate or lens rather than a glass corrector plate.

The Maksutov–Cassegrain telescope also performs superbly at planetary imaging. It is used extensively in military, aerospace, and industrial applications.

This article was updated on the 28th January, 2020

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