What is Catadioptric telescope? Important Advantages and different types

What is Catadioptric telescope? Important Advantages and different types

CATADIOPTRIC TELESCOPE

Contents

  • What are Catadioptric telescopes?
  • Catadioptric Telescope History
  • Parts of Catadioptric telescope
  • What are Catadioptric dialytes?
  • How does a catadioptric telescope work?
  • Why do catadioptric telescopes use both concave and convex mirrors?
  • The advantages of catadioptric Telescopic designs.
  • The disadvantages of catadioptric Telescopic designs.
  • What are the different types of Catadioptric Telescopic designs?
  • What are Photographic catadioptric lenses or short tube catadioptric equatorial reflector telescope?
  • Catadioptric Telescopic designs vs Reflector vs Refractors
  • Catadioptric Telescopic designs uses

What are catadioptric telescopes?

Catadioptric Telescopic designs are a variant of optical telescopes that form an image by combining specially designed mirrors and lenses. This telescopic configuration is intended to obtain a greater degree of error rectification overall compared to all-mirror or all-lens telescopes. The catadioptric design provides a wider aberration-free field of view. These designs incorporate “correctors” that are lens or curved mirror elements in the image-forming optical system for correcting the reflective or refractive aberrations caused by its counterpart.

Catadioptric Telescope History

Several designs of early optical systems incorporated the catadioptric setup. Augustin-Jean Fresnel is credited for designing several catadioptric lighthouse reflectors in the late 1820s. A. Mangin, a French engineer designed the Mangin mirror in the year 1876. The Mangin mirror is a concave glass reflector that has a silver surface present on the glass’s rear side. Many catadioptric telescopes are built incorporating the Mangin mirror.

Parts of Catadioptric telescope

Lenses: Every catadioptric telescopic design incorporates one or more lenses. The lenses are generally placed at the objective and receives the incoming light. Typically, biconcave lenses are used in such designs. However, the use of plano-convex or concavo-convex lenses is also seen in certain catadioptric designs. The use of lens-mirror arrangement reduces the aberrations and produces a better quality image.

Mirrors: Every catadioptric telescopic design incorporates one or more mirrors. The mirrors can be placed as an objective setup along with lenses or as secondary reflectors depending on the telescopic design. Typically, two spherical mirrors are used in such designs, one as primary mirror and the other as secondary. In this form of arrangement, the lenses are used for reducing the divergence of the light rays. The use of lens-mirror arrangement reduces the aberrations and produces a better quality image.

Telescope tube: The telescope tube serves as the body of the telescope. The telescopic tube is designed corresponding to the lens-mirror arrangement used. Typically, a telescopic tube comes with a radius of around 10 cm or 4 inches. The telescopic tube has a knob built below the visual back. This knob helps to adjust the focal length setting of the system.

Corrector plate: Some catadioptric telescopic designs use corrector plates to improve image quality. These corrector plates, as the name suggests is either a single thin lens or a group of thin lenses that are strategically placed in front of a catadioptric telescopic design for correcting the aberrations caused by the telescopic system. These corrector plates are sometimes also referred as correcting lenses. We can see the application of such corrector plates in the Schmidt-Cassegrain catadioptric reflector.

What are Catadioptric dialytes?

Catadioptric dialytes are considered to be one of the initial varieties of catadioptric telescope. This design comprises a silver-backed negative lens (comparable to a Mangin mirror) and a single-element refracting telescope objective. The first Catadioptric dialyte was proposed by W.F. Hamilton in 1814 and was known as the Hamiltonian telescope. Another design proposed by German optician Ludwig Schupmann in the 19th century known as the Schupmann medial telescope had the catadioptric mirror placed after the primary refractor’s focus. This design also added a third correcting or focusing lens to the telescopic configuration.

How does a catadioptric telescope work?

Catadioptric telescopes are found in several variants and each variant has its own specialized system of working. However, all these variants follow the same primary lens-mirror principle. These telescopes use both reflection and refraction of light to form images.

The incident light rays, entering the tube falls on the primary objective. This objective can be a single lens, a lens system, or a lens-mirror system (common). In the lens-mirror objective system, the lens converges the diverged beam of light entering the telescope and makes it fall on the mirror. This primary mirror then reflects the light to the second spherical mirror that is placed at either side of the telescopic opening. The reflected light received by the secondary mirror is then reflected again to form the final image on the focal plane.

Why do catadioptric telescopes use both concave and convex mirrors?

Some catadioptric telescope designs use both concave and convex mirrors to reduce the spherical aberrations caused by marginal rays. These mirrors can be placed as corrector plates or as a part of the objective depending on the requirements. Obtaining a proper degree of divergence and convergence is necessary for accurate image formation. These lenses are strategically placed to suit the need of the particular telescope.

THE ADVANTAGES OF CATADIOPTRIC TELESCOPES

  • Catadioptric Telescopic designs operate with a folded-path optical system, unlike refractors and Newtonian reflecting telescopes that use a linear optical path. This allows the telescope to be shorter than the size implied by its focal length.
  • These telescopes and telescopic mounts are designed to be compact and weigh a lot less compared to telescopes of other designs having the same configuration.
  • Catadioptric Telescopic designs have increased portability and are easy to transport. This is because they have a reduced mechanical size and weight.
  • These telescopes use mirrors that have purely spherical figures and refractive elements that can be conveniently (generally termed as corrector lenses). Using such elements reduces the overall manufacturing costs of the telescope. 
  • The Catadioptric telescope’s secondary mirror provides additional power while reflecting the cone of light coming out of the primary mirror. 
  • A large aperture Catadioptric Telescopic design provides a better inherent angular resolution and better light gathering ability than a small aperture telescope. 

THE DISADVANTAGES OF CATADIOPTRIC TELESCOPES

1. With an increase in aperture, the Catadioptric Telescopic designs tend to become heavier rather quickly.

2. These telescopes have the requirement to be optically aligned more frequently compared to refractors. The methods of aligning these telescopes is also a bit complex. 

3. The mobile parts of a catadioptric telescopic design are generally more complex or complicated in design than those present in reflector or refractor telescopes.

4. Catadioptric telescopes face an intrinsic optical performance inadequacy due to the central obscuration of their aperture produced by their own secondary mirror. However, this particular issue is also experienced in Newtonian and classical Cassegrain reflecting telescopes.

What are the different variants of Catadioptric Telescopic designs?

Argunov-Cassegrain Catadioptric Telescopic design

The Argunov-Cassegrain telescope is a variant of the Catadioptric Cassegrain telescope in which the majority of the optical instruments used are spherical. P.P Argunov introduced this design in 1972. These lens elements are placed at the focus of the larger objective. The Mangin mirror element is placed furthest from the primary mirror and works as a second surface mirror with a surface reflective coating applied to its sky-facing side.

Maksutov-Cassegrain Catadioptric Telescopic design

catadioptric telescope
Image source: GriffenjbsMaksutov-Cassegrain-Telescope, marked as public domain, more details on Wikimedia Commons

The Maksutov-Cassegrain is a Maksutov telescope variant that is named after the Russian astronomer and optician Dmitri Dmitrievich Maksutov. This variant comprises an optically transparent corrector lens along with a section of a hollow sphere. This design requires a spherical primary mirror and a spherical secondary mirror, which is usually a section of mirrored section corrector lenses. The Maksutov telescope can be air-sealed and collimated, so these telescopes are considered to be low maintenance.

Klevtsov-Cassegrain Catadioptric Reflector

The Klevtsov-Cassegrain has a similar design and mechanism to the Argunov-Cassegrain telescope. This telescopic design was first envisioned by G. I. Popov and was practically implemented by Yuri A. Klevtsov. This instrument is fitted with a sub-aperture corrector, a small meniscus lens, and a Mangin mirror as its “secondary mirror”. The corrector and the Mangin mirror are held with a spider vane in an open telescopic tube.

Schmidt-Cassegrain Catadioptric Reflector

What is Catadioptric telescope? Important Advantages and different types
Image source: GriffenjbsSchmidt-Cassegrain-Telescope, marked as public domain, more details on Wikimedia Commons

The Schmidt-Cassegrain reflector was influenced by the wide-field Schmidt camera in 1940 by James Gilbert Baker. The Cassegrain setup gives us a comparatively narrower field of view. The Schmidt corrector plate is known for being one of the first optical element. It works by the generation of a vacuum on one end of the plate and accurately adjusting for rectifying the spherical aberration generated by the spherical primary mirror. Amateur astronomers widely use these telescopes. 

What is Photographic catadioptric lenses or short tube catadioptric equatorial reflector telescope?

Several variants of catadioptric lenses (CATs) are incorporated into camera lenses. These lenses are also known as mirror lenses and reflex lenses. The design in which these lenses are incorporated reduces the length of the entire optical setup by folding the optical path and by the telephoto effect of the convex secondary mirror. This effect is used for multiplying the focal length of the optical system by 3 to 4 times. These lens systems are therefore much more compact and have a tube length ranging from 200mm to 1000mm. The photographic catadioptric lens system also reduces or eliminates the chromatic and off-axis aberrations. The images formed by such an optical system can fill the large focal plane of a camera.

Catadioptric telescope vs Reflector vs Refractors

A Reflector telescope incorporates the use of only mirrors in its design to gather and focus light for forming images. The Newtonian reflector is the most common example of a reflecting telescope that is built with a primary and a secondary mirror. Reflectors or reflective telescopes often have to deal with the issues of off-axis aberrations. The primary mirror gathers the light and directs it towards the secondary mirror that further reflects the light towards the eyepiece.

A refractor or a refracting telescope incorporates the use of only lenses in its design to gather and focus light for forming images. These telescopes often have very long tubes and therefore, suffer from chromatic aberrations. One lens is used to gather light and form an inverted image falling on the focal point of the magnifying eyepiece lens.

A Catadioptric telescope, as mentioned above forms an image by combining specially built mirrors and lenses. The chromatic and off-axis aberration is eliminated in the case of these telescopes.

Catadioptric telescope uses:

Catadioptric telescopes have a shorter tube length, a better balancing structure, lighter weight, and adjustable eyepieces. All these factors contribute to making these telescopic variants convenient for astronomical observation at different locations by amateur astronomers. These telescopes are also modified and combined with digital technology for astrophotography. These telescopes can also be used as general function telescopes. A large aperture telescope provides a better inherent angular resolution and better light gathering ability than a small aperture telescope.

To know more about telescopes https://lambdageeks.com/reflecting-telescope/ & https://lambdageeks.com/steps-to-use-a-telescope-parts-of-a-telescope/

About Sanchari Chakraborty

What is Catadioptric telescope? Important Advantages and different typesI 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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