Telescope types

Telescope types

In astronomy, a distinction is made between refractors (lens telescopes) and reflectors (reflecting telescopes), and there are sub-variants of both.

Refractors:

A simple and inexpensive form of a refractor is an achromat. In this case, the light is focused via two lenses.

Schematic structure of a refractor [https://www.bresser.de]

Illustration of a refractor [Tamasflex, CC BY-SA 3.0, via Wikimedia Commons]

Since light is split spectrally by glass surfaces (different refraction of the wavelengths), the different wavelengths do not meet in one focal point (chromatic aberration).

Imaging error with an achromat [DrBob (Transfered by nbarth), CC BY-SA 3.0, via Wikimedia Commons]

These color errors can be corrected by one or two additional lenses. Color-correcting refractors are called apochromats.

Focusing with an apochromat [Egmason, CC BY-SA 3.0, via Wikimedia Commons]

Instead of a large refractor, a small camera telephoto lens can be used. This is a refractor with partially short focal lengths that can be used to capture large sections of the sky.

Reflectors:

Reflectors focus the light via curved mirror surfaces. The best-known representative is the Newtonian telescope.

Schematic structure of a Newtonian telescope[Szőcs Tamás Tamasflex, CC BY-SA 3.0, via Wikimedia Commons]

Illustration of a Newtonian telescope [Szőcs Tamás Tamasflex, CC BY-SA 3.0, via Wikimedia Commons]

Newtonian telescopes always have coma error (rays that are not parallel to the optical axis are distorted at the edges of the image due to the parabolic shape of the primary mirror and have a tail). Therefore, Newtonian telescopes require coma correction lenses in the focuser.

A special form are catadioptric systems, where a Schmidt plate with integrated correction secondary mirror is used for the image field corrector (e.g. Schmidt-Cassegrain telescopes). Such systems achieve large focal lengths despite their compactness.

Schematic structure of a Schmidt-Cassegrain-Telescope [Szőcs Tamás Tamasflex, CC BY-SA 3.0, via Wikimedia Commons]

Illustration of a Schmidt-Cassegrain-Telescope [Kapege.de, CC BY-SA 2.5, via Wikimedia Commons]

With both systems, it is necessary to adjust the deflection or secondary mirrors. Especially with the Newtonian telescope, the two mirrors have to be adjusted very precisely to each other with a collimator.

Both the small secondary mirror in the Newton telescope and the secondary mirror in the Schmidt plate of the Schmidt-Cassegrain-Telescope are located in the beam path and lead to a shadowing, the so-called obstruction. This changes the amount of light collected and thus the image brightness and contrast.

Summary overview table:

Device	Features	Advantages	Disadvantages
Refractor Image refractor: Tamasflex, CC BY-SA 3.0, via Wikimedia Commons	Closed tube Typical apertures: 50 - 150 mm Typical focal lengths: 400 - 1200 mm Telescope length is approximately equal to the focal length Especially suitable for photography of large objects	No impurities due to closed tube no additional parts in the beam path → no obstruction → high contrast no adjustment necessary relatively easy to set up	Very expensive for large apertures due to large glass lenses → high weight Due to closed system longer cooling times to reduce tube-seeing If not color corrected, color errors occur
Newtonian-telescope Image Newtonian: Szőcs Tamás Tamasflex, CC BY-SA 3.0, via Wikimedia Commons	Open tube Typical apertures: 130 - 350 mm Typical focal lengths: 500 - 1600 mm Telescope length corresponds approximately to the focal length Particularly suitable for photography of galaxies and nebulae, partly also for planets and planetary nebulae	Fast cooling times due to open tube Due to simple design low-priced producible Large aperture Since there are no lenses, color errors are almost eliminated In contrast to the Schmidt-Cassegrain-Telescope, higher contrast because the secondary mirror is smaller	Risk of impurities on the mirrors due to open tube Adjustment necessary Large models can become unwieldy Coma correction necessary In contrast to a refractor, lower contrast due to the secondary mirror
Schmidt-Cassegrain-telescope Image Schmidt-Cassegrain: Kapege.de, CC BY-SA 2.5, via Wikimedia Commons	Closed tube Typical apertures: 130 - 350 mm Typical focal lengths: 1250 - 4000 mm Telescope length corresponds to about a quarter of the focal length Especially suitable for photography of small galaxies, planets and planetary nebulae	No impurities due to closed tube Large aperture New systems have only very small aberrations Short tube length with very long focal length	The large aperture number results in a slow optic Longer cooling times due to closed system to reduce tube-seeing In contrast to the other two systems lower contrast due to larger obstruction More expensive than Newtonian telescopes

The image orientation of the telescopes

Compared to the naked eye, images viewed through a telescope may appear upside down or mirrored depending on the used accessories.

View with the naked eye in the direction of the North

Image orientation of optical systems and accessories in the North direction

Used tube	Image orientation without accessories	Image orientation with zenith mirror/ zenith prism (opening directed upwards)	Image orientation with Amici prism / erecting lense
Refractor	upside down and laterally reversed	laterally reversed	with Amici prism upright and laterally correct
Newtonian	upside down and laterally reversed (depending on the viewing position)	Not used with the Newtonian telescope, since the light path would lengthen to such an extent that the focus point would no longer be reached.	with erecting lense upright and laterally correct
Schmidt-Cassegrain	upside down and laterally reversed	laterally reversed	with Amici prism upright and laterally correct

Image Refractor: Tamasflex, CC BY-SA 3.0, via Wikimedia Commons
Image Newtonian: Szőcs Tamás Tamasflex, CC BY-SA 3.0, via Wikimedia Commons
Image Schmidt-Cassegrain: Kapege.de, CC BY-SA 2.5, via Wikimedia Commons
Image zenith prism: https://www.astroshop.de
Image Amici-prism: Fred the Oyster, CC BY-SA 4.0, via Wikimedia Commons
Image erecting lense: https://www.astroshop.de

A comparison of the field of view widths

To get an impression of what commercially available telescopes can normally display for a section of the sky, an illustrative image is shown below.

The following assumptions apply:

• The same camera chip is always used (APS-C format, 22.2x14.8 mm)
• No accessories like reducers or barlow lenses are used
• The following telescopes are compared:
  • Telephoto lens with 280 mm focal length
  • Refractor with 480 mm focal length
  • Newtonian telescope with 1000 mm focal length
  • Schmidt-Cassegrain-Telescope (SC) with 2000 mm focal length