What is Cassegrain Telescope ? | It’s Working with reflectors | Important variants



  • What is a Cassegrain telescope?
  • What are the variants of the Cassegrain telescope?
  • Ritchey–Chrétien telescope
  • Dall–Kirkham telescope
  • Off-axis configuration
  • Catadioptric Cassegrains 

What is a Cassegrain telescope?

A Cassegrain telescope (proposed by Laurent Cassegrain in 1672) uses a primary parabolic mirror and a secondary hyperbolic mirror to reflect the light to the primary through a hole. The secondary mirror produces a diverging and folding effect creating a telescope having a short tube length with a long focal length.

How does a Cassegrain telescope work?

cassegrain telescope
light rays from a Cassegrain telescope.
Image source: Szőcs Tamás TamasflexCassegrain.enCC BY-SA 3.0

The Cassegrain design places the focal point behind the primary mirror, forming a convenient location. The primary and secondary reflectors share a common focus on this design. The secondary convex mirror produces a telephoto effect that creates a much longer focal length in a comparatively short mechanical system. The concave parabolic primary mirror reflects all received light rays parallel to its symmetry axis to the focus. The convex hyperbolic secondary mirror (that has two foci) directs all light rays falling on one of its focus towards the other. The second focus of the secondary hyperbolic mirror occurs at the same point where the image formation takes place (usually just outside the eyepiece where the mage is observed).

What are the variants of the Cassegrain telescope?

The classic Cassegrain design operates with a parabolic reflector as the primary mirror and a hyperbolic reflector as the secondary. This results in a compact design due to the folding of optical rays. Most of the smaller telescopes and camera lenses mount the secondary on an optically flat and transparent glass plate that closes the telescope tube. This helps in removing the “star-shaped” diffraction effects produced by the straight-vaned support spider. The closed tube remains clean, and the primary stays secure. However, this happens at the price of some light-gathering power loss.

Symmetrical Cassegrain telescopes have both the primary and secondary mirrors aligned about the optical axis. The primary mirror generally has a hole in the center that allows the reflected light to reach an eyepiece, an image sensor, or a camera. Some radio telescopes are designed to place the final focus in front of the primary mirror. 

Ritchey–Chrétien telescope

The Ritchey–Chrétien telescope (built by George Willis Ritchey and Henri Chrétien in the 1910s) is a specialized Cassegrain reflector. This design incorporates two hyperbolic mirrors in place of a parabolic primary mirror. The Ritchey–Chrétien telescope is devoid of any kind of comatic and spherical aberration. It has a nearly flat focal plane; this happens with the appropriate aligning of the primary and secondary mirror curvature. It is considered to be well suited for wide-field and photographic observations. The Ritchey–Chrétien telescope design is now considered to be one of the most famous professional reflector telescopic designs in the world.

Dall–Kirkham telescope

The Dall–Kirkham telescope (invented by Horace Dall in 1928 and proposed by Allan Kirkham in 1930) is another specialized Cassegrain telescope design type. This design operates with a concave elliptical primary mirror and a convex spherical secondary mirror. The Dall–Kirkham telescopic design is a more comfortable building than the traditional Cassegrain or Ritchey–Chrétien design. However, this design does not solve the problem of the off-axis coma. The Dall–Kirkham telescope comes with a smaller field curvature than a general Cassegrain design. Because of the small field curvature of the telescope, it becomes less evident at longer focal ratios. For this reason, Dall–Kirkham telescopes are rarely found to be faster than f/15.

Off-axis configuration:

Schiefspiegler telescope

Schiefspiegler telescope (invented by Anton Kutter, also called the “Kutter telescope”) is another variant of the Cassegrain reflector. This design incorporates tilted mirrors in order to prevent the secondary mirror from casting a shadow on the primary mirror. This design may eliminate the diffraction pattern, but this further forms several other aberrations, which needs to be corrected. Asymmetrical Cassegrain telescopes have tilted mirror(s) to prevent obscuration of the primary mirror or to prevent the requirement for a hole in the center of the primary mirror (or both).

Yolo reflector

The Yolo reflector (invented by Arthur Leonard) is another off-axis variant of the Cassegrain reflector. The unobstructed design operates with a parabolic or spherical or primary mirror and a mechanically warped spherical secondary mirror that fixes the off-axis induced astigmatism. With a proper setup, the Yolo can deliver unhindered observations of planetary objects and non-wide field targets. There is no lack of contrast or no compromising in image quality caused by spherical aberration. 

Catadioptric Cassegrains 

Catadioptric Cassegrain telescopes comprised of two mirrors along with a spherical primary mirror for reducing cost. Refractive corrector element(s) are often combined in the design to rectify the resulting aberrations.

Schmidt-Cassegrain Telescope

The Schmidt-Cassegrain reflector design was built based on the wide-field Schmidt camera. However, the Cassegrain arrangement provides a relatively narrower field of view. Amateur astronomers widely use these telescopes. 

GriffenjbsSchmidt-Cassegrain-Telescope, marked as public domain, more details on Wikimedia Commons

Maksutov-Cassegrain Telescope

The Maksutov-Cassegrain is a Maksutov telescope variant. It is baptized after the famous Russian astronomer and optician Dmitri Dmitrievich Maksutov. This design has an optically transparent corrector lens, a section of a hollow sphere in the beginning.

GriffenjbsMaksutov-Cassegrain-Telescope, marked as public domain, more details on Wikimedia Commons

Argunov-Cassegrain Telescope

The Argunov-Cassegrain telescope is another modified type of the Cassegrain telescope. In this telescopic design, all the optical instruments are spherical. The secondary mirror of the classical Cassegrain telescope is substituted by the sub-aperture corrector in this telescopic design that uses three air-spaced lens elements. The Mangin mirror element is held at the opposite end from the primary mirror and it serves as a second mirror that comes with a surface reflective coating applied to its sky-facing side.

Klevtsov-Cassegrain telescope

The Klevtsov-Cassegrain is similar to the Argunov-Cassegrain telescope. It is built of a sub-aperture corrector, a small meniscus lens, and a Mangin mirror that acts as a secondary mirror.

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About Sanchari Chakraborty

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