Telescope types

Telescope types
  1. Refractor
  2. Reflector
  3. Catadioptric (combined type)

Refractor

The Refractor is a telescope that uses lenses to refract, (bend), the light that it collects. This refraction causes parallel light rays that converge at a focal point at the opposite end, where they can be magnified by an eyepiece. The large lens at the front is called the objective lens. The objective lens usually comprises of two or more individual lenses that are bonded and or arranged together to make up what is called the objective lens cell. The glass material used can also vary which will help in the overall performance of the objective lens. There two types of refractors - Achromatic and apochromatic.

Advantages

  • Little or no maintenance and is easy to use.
  • Reliable due to the simplicity of design.
  • Excellent for lunar, planetary or binary star viewing.
  • Good for terrestrial viewing.
  • High contrast images due to its clear aperture (no central obstruction).
  • Good colour correction in achromatic designs and excellent in apochromatic, flourite and ED designs.
  • Sealed optical tube reduces image degrading air currents and protects the optics.
  • Objective lens is usually permanently mounted and aligned.

Disadvantages

  • Usually more expensive per inch of aperture than Newtonians or Catadioptrics.
  • Heavier, longer and bulkier than equivalent aperture Newtonians or Catadioptrics.
  • The cost and bulk factors usually limit the maximum size to smaller apertures.
  • Less suited for observation of deep sky objects such as distant galaxies and nebulae because of practical aperture limitations.
  • Some colour aberration in achromatic designs (doublet).
  • Poor reputation due to low quality imported toy telescopes; a reputation unjustified when dealing with a quality refractor from a reputable manufacturer.

The Achromatic refracting lens was invented in 1733 by an English barrister named Chester Moore Hall although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal legths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, "crown" and "flint glass", to limit the effects of chromatic and spherical aberration. Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. All achromatic refractors provide the blue or purple halo around high contrast objects (bright stars, planets and Moon). This is due to the lack of the correction of the blue wavelenght because of the two used lenses.

Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (secondary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. Such telescopes are sold in the high-end amateur telescope market. Apochromatic refractors are available with objectives of up to 553 mm in diameter, but most are between 80 and 152 mm.

Technical considerations

Refractors have been criticized for their relatively high-degree of residual chromatic and spherical aberration. This affects shorter focal lengths more than longer ones. A 4" f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 4" f/16 will have little color fringing.

In very large apertures, there is also a problem of lens sagging, a result of gravity deforming glass. There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished by using reflecting telescopes, that can be made in far larger apertures.

20 inch Achromatic refractor in California

68cm Achromatic refractor in Vienna

Modern 120mm achromatic refractor with computerised mount
Modern ED apochromatic refractor 120mm optical tube

Reflector

The Newtonian Reflector is a telescope which uses a spherical or concave parabolic primary mirror to collect, reflect and focus the light onto a flat secondary mirror (diagonal). This secondary mirror in turn reflects the light out of an opening in the side of the tube and into an eyepiece for focus and magnification.

Advantages

  • Lowest cost per inch of aperture compared to refractors and Catadioptrics since mirrors can be produced at less cost than lenses in medium to large apertures.
  • Reasonably compact and portable up to focal lengths of 1000mm.
  • Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters due to the generally fast focal ratios (f/4 to f/8).
  • Reasonably good for lunar and planetary work.
  • Good for deep sky astrophotography (but not as convenient and more difficult to use than Catadioptrics).
  • Low in optical aberrations and deliver very bright images.

Disadvantages

  • Requires regular alignment (collimation) of optics in order to perform at its best. Badly aligned optics can make the image quality suffer quite dramatically.
  • Primary mirror may require re-coating (usually after years of service).
  • The open nature of the tube assembly could mean more complicated cleaning of mirror surfaces when compared to telescopes of other designs.
  • Generally not suited for terrestrial applications.
  • Slight light loss due to secondary (diagonal) obstruction when compared with refractors.

Very popular among amateur astronomers are so called Dobsonian reflectors. They are the same Newtonian reflectors (from the optical piont of view), but are built to be used in manual and simple Alt-Az mount instead of Equatorial (EQ). The manual and simple Alt-Az mount cut's the price so even large Dobsonian reflectors are popular among amateur astronomers.

Nowadays Orion telescopes and binoculars company Dobsonian reflector systems (also known as Orion Intelliscope dobsonian reflectors) can be equiped with the optional Intelliscope computerised controller. It will not point your Alt-Az dobsonian reflector to the desired object nor it will track the object, but it will help you to point your tube where the desired object is.

Other variations of Dobsonian reflectors (Meade, for instance) belong to "truss" type. These reflectors are very very portable, so you can fit your 16" monster into small car. This could be achieved by the unique tube and thair parts construction... Base and top tube are connected with the light Aluminium alloy truses, so the telescope has to be "built" every time you are planing to use it.

Few special modifications of Newtonian reflectors are available nowadays. These are modified Schmidt-Newtonians and Maksutov-Newtonians. These modifications were offered for the optical systems to minimize their abberrations and produce better images for both visual and photographic applications. Schmidt-Newtonian telescope systems use the same primary, and secondary mirror contructions, but also has the corrector plate in the fron of the tube. The corrections should be made because Schmidt-Newtonian's have a spherical primary mirror (cheaper to made), so the light source coming to the secondary mirror should be the same, as it comes from the parabolic mirror. The corrector plate configures the light to the spherical primary, so the seconadry gets the light from the spherical primary as from the parabolic primary. Schmidt-Newtonian optical systems have the flatter field (less coma o the edges of the Eyepiece and film or CCD) and small focal ratio (even f/4 systems provide much better edge correction than ordinary Newtonians). Therefore the Schmidt-Newtonian optical systems has no vanes holding the seconadry mirror, so no diffraction spikes are visible during visual and photographic situations.

One disadvantage for such optical systems is dew... In order to enjoy the use of Schmidt-Newtonian optical system during the night observations, you must buy the optional dew cap, because the front corrector lens are placed on the top of the tube.

Other disadvantage of the Schmidt-Newtonian is a little more light loss than from the ordinary Newtonian reflectors... This is because the corrector plate absorbs and reflect the very small amount of incoming light. So be sure to choose the Schmidt-Newtonian optical system with quality coating on the corrector plate and mirrors. Nowadays manufacturers such as Meade company (USA) offer the special enhanced coatings (they call it UHTC coatings) on their production mirrors and corrector plates.


First ever Isaac Newton reflector

Modern Newtonian reflector on computerised Equatorial mount
Modern Dobsonian reflector with computerised object locator

Modern "Truss" type dobsonian reflector

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