Reflectors

Reflectors usually use a concave primary mirror to collect and focus incoming light onto a secondary mirror (flat in Newtonian designs, convex in other designs) that in turn reflects the image into an eyepiece for viewing.

Advantages

  • Lower cost per inch of aperture than offered by refractors and catadioptrics, since mirrors can be produced at less cost than lenses in refractors in medium to large apertures;
  • Reasonably compact and portable;
  • Excellent for faint, deep-sky objects, such as remote galaxies, nebulae and star clusters, because of their larger apertures;
  • Deliver very bright images with few optical aberrations.

Disadvantages

  • Generally not suited to terrestrial observation,
  • Slight light loss due to obstruction from the secondary mirror.

Types of Reflectors

Newtonian

Schematic of a Newtonian Reflector

The Newtonian telescope was invented by Sir Isaac Newton in 1672. Newton realized that better optical performance could be achieved by using curved mirrors to reflect light instead of the relatively poor quality glass of the time used to refract and focus light. Because of its simplicity and ease of construction, the Newtonian telescope has become the standard optical system for amateur and many professional telescopes. The Newtonian telescope consists of a single parabolic mirror and an optical flat diagonal mirror to redirect the image to the outside of the optical tube. In large professional telescopes the diagonal is not used rather the astronomer and instruments ride directly at the focus. This arrangement is called the prime focus.  One advantge to the Newtonian design is the size and location of the secondary.  Since the flat diagonal secondary mirror is positioned close to the prime focus, the size of the secondary is smaller than the equivalent aperture/f-ratio in either the Cassegrain or Ritchie-Chretien, which means that it obstructs less of the primary.  Typical obstruction percentages for Cassegrain (classical or R-C) are 20-35% and can range into the mid 40% ranges, as opposed to 10-18% for Newtonian designs.

Cassegrain

cassegrain_optics.jpg

The Cassegrain telescope was invented by Guillaume Cassegrain in 1672. The Cassegrain design uses a folded optical path achieved by two mirrors; a perforated concave primary and a convex secondary. The Cassegrain has been very popular among amateurs and professionals alike. The three primary types of Cassegrains are the Classical, Dall-Kirkham, and Ritchie-Chretien. The layout of each type is identical, only the shape of the optical surfaces (or their figure) set them apart. The Classical and Ritchie-Chretien are both widely used among professional astronomers and the Dall-Kirkham is popular among amateur telescope makers.

Classical

The Classical Cassegrain consists of a concave parabolic primary mirror and a convex hyperbolic secondary mirror. The Classical Cassegrain is often used in conjunction with a Newtonian design. A Newtonian reflector can be converted to a Cassegrain by changing the secondary mirror form the optical flat of the Newtonian with the hyperbolic secondary of the Classical Cassegrain. Even though the classical Cassegrain has a difficult to manufacture secondary it suffers from less coma than does the Dall-Kirkham. Moderate coma and the ease of converting an existing Newtonian to a Classical Cassegrain make it a logical choice for a second focus on a telescope.

Dall-Kirkham

The Dall-Kirkham is popular among amateur telescope makers. Invented independently by the English amateur telescope-maker Horace Dall (1901-1986) and the American Alan Kirkham, it is particularly suited to planetary observation where good resolution is more important than wide field of view. The Dall-Kirkham consists of an elliptical primary mirror and a spherical secondary mirror. The spherical secondary is much easier to fabricate than the steep hyperbolic secondary of the Classical Cassegrain. The elliptical primary is also fairly easy to fabricate. Despite the fact that the Dall-Kirkham is easier to fabricate it suffers form more severe coma in shorter focal lengths. This combined with the inability to convert the optical path to a Newtonian make the Dall-Kirkham less desirable than a Classical Cassegrain.

Ritchie-Chretien

The Ritchie-Chretien is very popular among professional astronomers because of its optical properties. The Ritche-Creitien telescope consists of a hyperbolic primary and secondary. Both of these optical surfaces are difficult to fabricate and test, which accounts for it not being a popular design in the amateur telescope-making community. Special equipment is needed to manufacture and test the mirrors. Each mirror has to be tested with an interferometer. Factory manufactured Ritchie-Chretien telescopes will cost more per aperture-inch than other reflector designs, simply because of the complexity of making the mirrors. Here is the price list for one manufacturer, which ranges from $20,000 on the low end, to nearly $800,000 on the high! Even though the Ritche-Creitien optics are harder to make the Ritche-Creitien has advantages over both the Classical Cassegrain and Dall-Kirkham. The Ritche-Creitien does not suffer from coma, thus it can be made in shorter focal ratios and is well suited to photography. However, the image at the edges of the field is enlarged. Some famous examples of the use of the Ritchie-Chretien design are the Keck telescopes in Hawaii, and the Hubble Space Telescope.

Note: Meade Telescopes has a tube that they are advertising as an ‘Advanced Ritchie-Creitien‘ design. It is actually a catadioptric design, complete with corrector plate.

2 thoughts on “Reflectors

  1. you mention that it is easy to convert a Newtonian to a Cassegrain…but would you not need to make a hole in the Newtonian primary mirror?

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