Optical design of a telescope

  • Maja 
Optical design of a telescope

The optical design of a telescope is a feature that defines what sort of observing the telescope is suitable for. Depending on the design, some telescopes are built for casual lunar and planetary viewing while others are meant for “deep-space” observations.

In general, there are three main types of optical telescopes:

Refractors

A refractor is a type of telescope with a long tube and a combination of glass lens at its objective (objective lens) and a second glass at its eyepiece (eyepiece lens).
The light enters a refractor telescope at the front where it is refracted (bent) as it passes through the lens. Refractors are much more rugged than other types of telescopes – the lens is less likely to come out of alignment. Because of a long tube, the eyepiece is at the lower end of the telescope, meaning that a high tripod is required for comfortable viewing.

A significant advantage of refractor telescopes is its closed tube.

Optical design of a telescope

A refracting telescope (Available from: https://www.britannica.com/science/refracting-telescope [accessed 28 Nov. 19])

A closed tube is good for two reasons:

  • Firstly, the glass surface is air sealed, which means that it is protected against dust and dirt – because of that, the lens doesn’t need much cleaning.
  • Secondly, the image is steadier and sharper than on other types of telescopes because the effects of changing temperatures are eliminated.

The bad side of refractors is an effect called chromatic aberration. It occurs because the lens is unable to focus all colors at a single point. The effect is seen as a rainbow of colors around the image. To reduce it, a compensating lens is added. The older models of refractors use a very long objective focal length to minimize the effect.

Refractors produce an image very high in contrast. This makes them great for lunar and planetary observations. There is not enough light-gathering power for observing “deep-sky objects” such as nebulas and galaxies.

Reflecting telescopes

This type of telescope uses a combination of curved mirrors to collect and focus light. The main reason for its invention was to eliminate chromatic aberration. Parabolic mirrors focus the light rays at the same point and do not suffer from chromatic nor spherical aberration.

A reflector features an open tube, which means that dust and other particles can accumulate in the tube. Because of that, frequent cleaning is needed. Besides cleaning, this type of telescope requires occasional maintenance. Mirrors can sometimes get out of alignment, especially if the telescope is moved frequently.

Optical design of a telescope

A light path in a reflecting telescope (Available from: https://www.britannica.com/science/optical-telescope/Reflecting-telescopes. [accessed 28 Nov. 19 ])

The most popular types of reflecting telescopes are:

  • Newtonian reflectors – The most common and simple type of a reflecting telescope. Because of its somewhat low cost and simple design, it is great for amateurs and professionals also. It features a side-mounted eyepiece located near the top of the tube. A Newtonian reflector uses a parabolic shape of the mirror, which means that if the focal ratio is less than f/3, it suffers from the defect called a coma.
Optical design of a telescope

A light path in a Newtonian reflector (Available from: https://www.qsstudy.com/physics/reflecting-telescope [accessed 28 Nov. 19 ] )

  • Cassegrain reflector – This is a combination of a concave mirror (primary mirror) and a convex mirror (secondary mirror). The primary parabolic mirror features a hole in the center where the light enters the eyepiece. A small secondary hyperbolic mirror is mounted on the front plate. Like a Newtonian reflector, a Cassegrain is also free from spherical aberration. 

Catadioptric telescopes

The Catadioptric telescope is a mixture of a refractor and a reflector telescope. It uses a combination of mirrors and lens to form an image. With the right position of mirrors and lenses, the light is “optically folded,” meaning that the effective focal length of the telescope increases while the mass reduces.

The primary mirror is located at the back of the telescope while the glass corrector is in the front. A glass corrector is used to remove spherical aberration. The secondary mirror bounces the light into the eyepiece.
The optical tube is quite small in size, which means the telescope is very light and easy to mount, but it does need occasional optical collimation.

The most popular designs are:

  • Schmidt-Cassegrain – it uses a combination of a spherical primary mirror and a secondary mirror. In general, this is a Cassegrain reflector telescope with a special a Schmidt corrector plate (lens) to correct for aberrations.
Optical design of a telescope

A Schmidt-Cassegrain design (Available fromhttps://www.britannica.com/science/Schmidt-telescope [accessed 28 Nov. 19 ] )

  • Maksutov-Cassegrain – this type is very similar to the Schmidt-Cassegrain telescope. The difference between them is in the corrector lens and secondary mirror. Maksutov-Cassegrain uses a thicker corrector lens and a different secondary mirror that is located on the inside of the Maksutov corrector lens. The difference between Schmidt and Maksutov-Cassegrain is also in the aperture – Maksutov usually has a smaller aperture than Schmidt-Cassegrain. 

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