Reflecting Telescope (Invented in 1663): Definition, Working, Variations

Reflecting Telescope

What is a reflecting telescope?

reflecting telescope is developed based on the principle of light reflection by a mirror or a combination of curved mirrors to generate an image. These telescopes come in different design variations and also includes additional optical elements at times for enhancing the image quality or mechanically improve the position of the image. Since reflecting telescopes/reflectors involve mirrors, they are termed as “catoptric” telescopes. These telescopes are commonly used for astronomical purposes. Prominent telescopes like the Hubble Space Telescope and some amateur telescopes are based on this microscopic design. Additionally, telescopes that operate with wavelengths of light other than the visible range (such as X-RAY telescopes) also use the principle of reflecting telescopes. 

Who invented the reflecting telescope?

The use of parabolic mirrors in such telescopes has reduced spherical aberration leading to several telescopic designs following the reflection principle. One of the most important telescopic designs was the Gregorian telescope proposed by James Gregory in 1663 and was built by experimental scientist Robert Hooke in 1673. 

Sir Isaac Newton is considered to be the creator of the first reflecting telescope in 1668. This design is referred to as the Newtonian telescope. The Newtonian telescope uses a spherical-ground metal primary mirror and a small diagonal-mirror.

In the late 20th century, the field of adaptive optics and lucky imaging has witnessed a development helping to overcome the difficulties of seeing. Now, reflecting telescopes have become omnipresent on space telescopes and several other types of spacecraft imaging devices.

How does a reflecting telescope work?

Path of light in a reflecting telescope.

The reflector telescope has a curved primary mirror as its fundamental optical element. This mirror is used for creating an image at the focal plane. The distance between this mirror and the focal plane is termed the focal length. A digital sensor or film can be kept on the focal plane for recording the image produced. At times, a secondary mirror is added to redirect/forward the focused light to a film, digital sensor, or an eyepiece for visually observing the optical characteristics.

In a majority of modern telescopes, the primary mirror is made of a solid glass cylinder with its front surface ground to a parabolic or spherical shape. A highly reflective front surface mirror is created by vacuum to deposit a thin layer of aluminum onto the mirror.

Different methods make primary telescopes. One such method involves rotating molten glass in order to make it is surface a paraboloid. This is continued till the glass cools down and solidifies. The mirror developed is paraboloidal in terms of shape approximately and requires minimal polishing and grinding for achieving the accurate figure.

Why are reflecting telescopes used for astronomical research?

At present, almost all large astronomical telescopes used for research are reflectors/reflecting telescopes. There are a variety of reasons why the reflectors are preferred for astronomical research:

· The glass elements/lenses used in refracting and catadioptric telescopes absorb specific wavelengths of light or a certain amount of incoming light. Reflectors do not absorb any such wavelength, and hence, they work on a broader spectrum of light.

· For a lens to work correctly, it should be devoid of any form of aberration, imperfection and inhomogeneities. The entire structure should be accurate. But in the case of mirrors. Only the reflecting surface requires to be perfectly polished.

· Lenses are made up of different materials with different refractive indices. Different wavelengths of light travel at different speeds and angles in different mediums. This results in giving rise to chromatic aberration. In order to correct these aberrations, one needs to incorporate a combination of two or more aperture-sized lenses. This increases the monetary investment of the system and also makes it significantly bulkier. The images formed by mirrors do not suffer from chromatic aberration. Moreover, mirrors prove out to be comparatively cost-efficient and are compact in size.

· Manufacturing and setting up lenses with large apertures can create problems. Lenses can be attached only with their edge. The central part of the lens slumps because of gravity. This leads to the distortion of the image formed. Using mirrors eradicates the possibilities of such problems. Mirrors can be held with back support and hence, can have large apertures without affecting image formation. The largest lens aperture currently stands at 1 m, whereas the largest mirror aperture stands at 10 m. 

What are the different designs of reflecting telescope?

* The Gregorian telescope (proposed by James Gregory) uses a concave secondary mirror to reflect the primary mirror’s image through a narrow hole. This is done to produce an upright image that is advantageous for conducting terrestrial observations. There are a few small spotting telescopes that are constructed in this manner. Many large modern telescopes also use the Gregorian arrangement. For example, the Magellan telescopes, the Vatican Advanced Technology Telescope, the Giant Magellan Telescope, and the Large Binocular Telescope.

reflecting telescope
Path of light of a Georgian Reflecting telescope.
image source:KrishnavedalaGregorian telescopeCC BY-SA 4.0

* The Newtonian telescope is a reflecting telescopic design variation that was developed by Sir Isaac Newton in the year 1668. Such telescopes incorporate a concave primary mirror and a flat diagonal secondary mirror. The Newtonian telescope is famous due to its effective and simplistic design, which is appreciated by telescope makers. In this design, the eyepiece is located at the top end of the telescope tube. The placement of the eyepiece with short focal ratios provides a compact mounting system, ensures mobility, and brings down the expenditure. [To know more about Newtonian telescope visit]

Path of light of a Newtonian Reflecting telescope. Image source: Krishnavedala – Own work CC BY-SA 4.0

* The Cassegrain telescope that was developed by Laurent Cassegrain in the year 1672 incorporates a parabolic primary mirror and a hyperbolic secondary mirror for reflecting the incident light to the primary mirror through a small hole. The secondary mirror is primarily used for diverging and folding. This results in a telescope having a short tube length with a long focal length. [To learn more about Cassegrain telescope visit]

Path of light of a Cassegrain Reflecting telescope. Image source; Krishnavedala – Own work CC BY-SA 4.0

* The Ritchey–Chrétien telescope (developed by George Willis Ritchey and Henri Chrétien around 1910s) is a special Cassegrain reflector. This design has two hyperbolic mirrors instead of a parabolic primary mirror. The Ritchey–Chrétien telescope is free of coma and spherical aberration and provides a nearly flat focal plane. This telescope is apt for wide-field and photographic observations. The Ritchey–Chrétien telescope design happens to one of the most commonly used professional reflector telescopes.

* The Dall–Kirkham telescope is another special type of Cassegrain telescope design. The Dall–Kirkham telescopic design is comparatively easier to construct than a regular Cassegrain or Ritchey–Chrétien telescope. However, this design is unable to correct the issues of off-axis coma. Its small field curvature makes it less evident or accurate at longer focal ratios; hence, Dall–Kirkham telescopes are barely seen to be faster than f/15.

* The Herschelian reflector (proposed by William Herschel in 1789) is incorporated for building very large telescopes. The Herschelian design uses a tilted primary mirror. This ensures that the light is not blocked by head of the observer. However, this reflector design comes with certain geometrical aberrations. Irrespective of that, it is used for avoiding the use of a Newtonian secondary mirror. The secondary mirror is generally built up of speculum metal mirrors that gets tarnished fast and provides a reflectivity of only 60%.

Path of light of a Herschelian Reflecting telescope. Image source: User:Eudjinnius – Own work
Diagram of Herschel-Lomonosov telescope system. CC BY-SA 3.0

What are the errors produced by reflecting telescope?

Reflecting telescopes are prone to producing specific errors while forming images, just like any other optical system. The images formed has object distances up to infinity, and these images are viewed at different light wavelengths. These factors cause specific errors in image formation.

· Coma – Coma is a type of aberration that focuses the center of the image to a point, but the edges generally appear radially smudged (comet-like) or elongated.

Diagramatic representation of comatic aberration, anonymous, Lens-comaCC BY-SA 3.0

· Field curvature – At times, the images are not focused well all across the field. This happens due to the image plane’s curvature and is corrected by using a field flattening lens.

· Astigmatism – Astigmatism is a type of aberration that causes an azimuthal focal variation around the aperture. As a result of this, off-axis point source images appear elliptical. Astigmatism causes more error when the field of view is large and starts varying quadratically with field angle. In the case of a smaller/narrower field of view, astigmatism is not usually a problem.

Diagramatic representation of Astigmatism. I, Sebastian Kroch, AstigmatismCC BY-SA 3.0

· Distortion – Distortion is an aberration effect that disturbs the shape of the image. Image sharpness is not affected by distortion. This aberration is generally corrected with the help of image processing. 

· Spherical aberration: Spherical aberration is a defect that occurs when a spherical mirror/lens is unable to focus light from different distant objects at the same point. This defect is solved by using parabolic mirrors instead of the spherical ones. However, the parabolic mirror does not work well with image formation of light falling on the edge of its field of view and produces off-axis aberrations. 

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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