Astronomy Telescope Designs

There are actually many different optical designs of Astronomical Telescopes, particularly for specialised areas of Astrophotography, but its fair to say that the majority of them would fall into the following 3 main categories: Refractors, Newtonian Reflectors & Schmidt Cassegrains.

While all of these telescope types perform quite well in a variety of different fields, whether that's planetary or deep space work, some of them have their own special qualities that make them particularly well suited to certain types of astronomy viewing and astrophotography. We have listed some of the main features of each design below. 

REFRACTER TELESCOPES:

Refractor Telescopes

Refractor telescopes would be familiar to most people as a typical telescope design. They use a front lens (or series of lenses) to collect and refract(bend) the incoming light through the front of the telescope, re-directing it to a focus point on the eyepiece or camera sensor.

Refractors can suffer from what is called Chromatic Aberration which causes a colour fringing on the edge of bright objects such as stars, planets or the moon - depending on the quality of the optics, you may notice it as a blue/purple glow on the edge of bright objects. This is caused by the light splitting into separate colour channels as it passes through the main objective lens and is refracted. - You may have seen a similar effect if you've ever left a glass on the window on a sunny day, sometimes you can see a rainbow effect on the wall or other surfaces nearby. This is because the light splits as its refracted through the glass (which acts as a lens).

Modern Astronomical Refractors address this issue by using multiple lenses at the front of the telescopes such as Doublets (2 Lenses) which can really help to reduce this effect considerably or Triplets (3 Lenses) which pretty much cancel it out all together.

Refractor Telescope (Pro's):

  1. The glass optics and the lack of a central obstruction means they offer a very clear and sharp contrasting image - more so than most other telescope types.
  2. They are quite robust and require little or no maintenance other than occasional cleaning of the lens.
Refractor Telescope (Cons):

  1. Large aperture versions are generally quite long and heavy so refractors are normally limited to approx. 6" Apertures. Higher quality designs eg. Triplets are also very expensive to manufacture.
  2. They can suffer from chromatic aberration, however as mentioned above this can be reduced or even cancelled out using a good quality Doublet or Triplet design.
Suitable for viewing:
  • Moon & Planets: YES
  • Deep sky objects (e.g Galaxies & Nebula): YES
Summary: Refractors make ideal telescopes for beginners as they are relatively maintenance free and intuitive to use. Small focal length Triplet design Refractors are extremely popular with Astrophotographers as their extremely sharp high contrast optics and widefield view make them an excellent choice for imaging larger galaxies and nebula.

NEWTONIAN REFLECTOR TELESCOPES:

Newtonian Reflector Telescope

Unlike Refractors, Newtonian Reflector Telescopes do not bend the light but redirect it using a series of mirrors - For this reason they are Achromatic by design (don't suffer from the same level of chromatic aberration as refractors). As the light passes through the front of the tube, the primary mirror at the end of the telescope collects and redirects it to the secondary mirror near the front end and then to the eyepiece or camera sensor - For this reason the viewing end is normally at the front of the telescope.

Newtonian Reflector Telescope (Pro's):

  1. They are less expensive to manufacture and so are more affordable in very large apertures. They are probably the most cost effective telescope designs available.
  2. The ratio between their wider apertures and longer focal lengths give them a very good f/ratio (making them a fast optical system - Excellent for astrophotography)
Newtonian Reflector Telescope (Cons):

  1. They are typically much larger and more bulky than Refractor & SCT designs.
  2. As the optics are exposed to the atmosphere and open air, they do require more regular cleaning and maintenance - such as collimation (re-aligning) of the mirrors.

Suitable for viewing:

  • Moon & Planets: YES
  • Deepsky Objects (e.g Galaxies & Nebula): YES

Summary: Newtonian Reflectors are an excellent telescope for beginners and experienced Astronomers/Astrophotographers alike. Their larger apertures mean that at higher magnifications, images will appear clearer and brighter thanks to better resolution. 

SCHMIDT-CASSEGRAINS (SCT) & MAKSUTOV-CASSEGRAINS (MC):

Schmidt Cassegrain

SCT & MC telescope designs are somewhat of a hybrid between refractor and newtonian reflector designs - bringing together some of the positive features of both. The Internal layout of the SCT & MC differ slightly between them but essentially they use a corrector plate or objective lens like a refractor but also have a primary and secondary mirror like a newtonian reflector.

SCT & MC Telescope (Pro's):

  1. These telescope designs generally offer longer focal lengths and larger apertures (in a more compact OTA) which means they are able to achieve higher magnification with better resolution and minimal loss of image quality.
  2. They are smaller and more compact than newtonian reflectors and provide sharp and high contrasting views. Maksutov designs are particularly well known for their unrivalled views of the moon and planets.

SCT & MC Telescope (Cons):

  1. They tend to be more expensive than newtonians (but are generally less expensive than some premium triplet refractors)
  2. They do require occasional collimation (re-aligning) of the mirrors.

Suitable for viewing:
(Schmidt Cassegrains)

  • Moon & Planets: YES
  • Deepsky Objects (e.g Galaxies & Nebula): YES

(Maksutov Cassegrains)

  • Moon & Planets: YES (Extremely well suited)
  • Deepsky Objects (e.g Galaxies & Nebula): YES (but better suited to smaller brighter targets due to their narrower field of view)

Summary: Schmidt Cassegrains and Maksutov designs are generally more robust and more compact than their equivalent Newtonian designs and as their internal mirrors are not directly exposed to the atmosphere, they rarely require cleaning and only occasional aligning. They are a good middle ground choice between high end Refractors and Newtonian Reflectors as they offer extremely sharp optics with wider apertures and longer focal lengths but in a more compact and robust instrument.


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