Optics 101

Optical Telescope - How Does it Work?

Types of Optical Telescope
Refractor Telescope
Reflector Telescope
Catadioptric Telescope
Schmidt-Cassegrain Design
Maksutov-Cassegrain Design


An optical telescope is an instrument which can make distant objects appears much closer. To make this possible, an optical telescope uses the following two pieces of instruments:

  • Objective lens or primary mirror, and
  • Eyepiece lens

The ability for a telescope to collect light is related to the lens’ diameter or mirror used. This is also commonly referred to as the aperture. The main purpose of the aperture is to collect light. In general, the larger the aperture will result in a brighter final image, as there is an increase in the amount of light for the telescope to collect.

The magnification of a telescope depends on the combination of lenses used. The magnification of the telescope is performed by the eyepiece lens. The eyepiece lens captures the bright light from the focus of the primary mirror or from the objective lens and magnifies it or spreads it out over the etina of the observer’s eye. As a result, the image appears big to the observer.

By combining the objective lens or 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 lens
  • Reflector telescope, which uses mirrors
  • Catadioptric telescope, which uses the combination of lens and mirrors

Refractor Telescope

Refractor or refracting telescope uses a convex glass lens to refract or bend light and bringing the light into focus. This convex glass lens is commonly referred to as the objective lens. Convex lens 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 refracting telescope. First, the power of the convex lens is governed by the size of the convex lens. This will also influences the physical size of the telescope. Furthermore, due to the light is refracted or bent, the image generated is not always clear.

Reflector Telescope

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

Catadioptric Telescope

Catadioptric telescope can be considered as a hybrid telescope. It uses the combination of both 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 Schmidt-Cassegrain 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. The telescope is very portable due to the folded light path. This allows the optical tube to be very short. The Schmidt-Cassegrain telescope are suitable for planetary observations as well as deep sky observation due to its optics.

Maksutov-Cassegrain design

The second type of catadioptric telescope. 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 also uses extensively in the military, aerospace and industrial applications.

Source: AZoOptics

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