Types of Telescopes

September 22, 2019
12:23 am

Reflector telescopes, refractors, catadioptrics… What do these words even mean? Moreover, does it even matter what kind of a telescope you are using?

It turns out that it does matter. Each telescope type has its own specifics that make it the best for a particular purpose. With that being said, let’s overview the purpose of each telescope type, as well as the difference between types of telescopes.

Refractors

Refractor telescopes boast a simple design that requires very little maintenance and is very reliable.

These telescopes are particularly suitable for observing closer objects lying within the Solar System. However, the specs of a refractor telescope, as well as the used accessories are going to have a significant impact on what you will be able to see.

Cheaper refractor telescopes offer decent visibility for observing the Moon, as well as for terrestrial observations. However, pick a good telescope and equip it with the right eyepiece, and you are going to see much farther into the Solar System.

The optical system in refractor telescopes is essentially a straight line, which means that there are no obstructions or secondary mirrors that would make the telescope complex and more difficult to maintain, like it is with other telescope types. Aside from that, the simple optics of refractors allows for little to no aberrations.

With that being said, refractor telescopes tend to be more expensive per mm of aperture than other types. Aside from that, their lens systems weigh more than in reflectors or catadioptrics. Lastly, since refractors tend to have smaller apertures than other telescope types, they are worse at seeing dim deep-space objects.

Overall, here are the pros and cons of refractor telescopes:

Here are some really great examples of refractor telescopes

MAXPLATER 70mm telescope

Features & specs:

Inexpensive
Suitable for shorter-distance Moon and terrestrial observations
Lightweight design
400mm focal length & 70mm aperture
16x and 66x eyepieces

Sky-Watcher ProED Doublet APO Refractor Telescope

Features & specs:

Great for long-distance observations
Rugged design
900mm focal length & 100mm aperture
45x and 180x eyepieces

Orion 9005 AstroView 120ST Refractor Telescope

Features & specs:

Excellent for observing distant objects
Height-adjustable tripod
Smooth and stable German equatorial mount
600mm focal length & 120mm aperture
24x and 60x eyepieces

Reflectors

Comprised of mirrors, reflector telescopes (also known as Newtonian telescopes) offer greater value per mm of aperture since mirrors are cheaper to manufacture than lenses. However, the build of reflector telescopes is more complex.

To focus the light into the eyepiece, reflectors bounce it from a primary mirror into a secondary mirror placed near the front of the tube at a 45-degree angle to the primary mirror. The secondary mirror then sends the light to a side-mounted eyepiece.

The secondary mirror causes a slight obstruction to the light entering the telescope tube. Due to this, some light diffraction and loss occur in reflectors. Aside from that, traditional reflectors’ tubes are open to exterior elements, which means that reflector telescopes require occasional cleaning.

A variation of reflectors called Schmidt-Newtonian has a corrector plate at the front of the tube, whose purpose is to shield the tube’s interior from the elements and also reduce spherical aberration.

With that being said, reflectors can have much larger focal lengths and apertures than refractors, which allows for better deep-sky performance. Reflectors quite easily see galaxies and nebulas.

There are even the so-called Dobsonian telescopes, which are reflectors with extra-large focal lengths and apertures. These can get very heavy and bulky, but they offer unparalleled performance for deep-space observations among any telescope type.

In conclusion, the pros and cons of reflectors are the following:

Here are some really brilliant examples of reflector telescopes:

Orion 10019 SkyQuest XT10i IntelliScope Dobsonian Telescope

Features & specs:

Computerized tracking with a database of over 14,000 celestial objects
Stable telescope base
1200mm focal length & 254mm aperture
48x and 120x eyepieces

Celestron PowerSeeker 127EQ Telescope

Features & specs:

Inexpensive
Light and compact for a reflector
Adjustable tripod height
Sturdy and smooth German equatorial mounting system
1000mm focal length & 127mm aperture
50x and 250x eyepieces plus a 3x Barlow lens

Orion 09007 SpaceProbe 130ST Reflector Telescope

Features & specs:

Stable and smooth German equatorial mount
Quite lightweight
Adjustable tripod height
650mm focal length & 130mm aperture
26x and 65x eyepieces

Celestron NexStar 130SLT Computerized Telescope

Features & specs:

Motorized alt-azimuth mount system
Database of over 4,000 celestial objects
Adjustable tripod height
650mm focal length & 130mm aperture
26x and 72x eyepieces

Catadioptrics

Lastly, there are catadioptric telescopes. These telescopes combine some of the pros of refractor and reflector telescopes, but they do have a very different design.

Catadioptrics have a folded optical path. Entering through a thin aspheric correcting plate, light first reflects off a spherical mirror at the back of the tube. It goes to a smaller secondary mirror placed right behind the corrector plate and is then reflected to the optical tube in the back. Finally, the light goes through an opening in the rear of the tube towards the eyepiece.

Due to their folded optical path, catadioptrics have short tubes with long focal lengths. The build of these telescopes also allows for maintenance-free operation. However, the use of a secondary mirror implies some light loss and diffraction just like in reflectors.

Catadioptric telescopes tend to be more expensive than similarly-sized reflectors, but they offer more mm of aperture for the money than refractors. Plus, they are excellent for deep-sky viewing, but reflectors tend to be better when it comes to observing extremely dim objects.

To summarize, the pros and cons of catadioptrics are the following:

Here are some outstanding examples of catadioptric telescopes:

Celestron NexStar 8 SE Computerized Telescope

Features & specs:

Database of over 40,000 celestial objects
Fully automated GoTo mount
Comes with 6 filters and a 2x Barlow lens
Height-adjustable tripod
2032mm focal length & 203.2mm aperture
81x eyepiece

Celestron NexStar 6 SE Telescope

Features & specs:

Database of over 40,000 celestial objects
Fully automated GoTo mount
Height-adjustable tripod
1500mm focal length & 150mm aperture
60x eyepiece

Orion 10022 StarMax 90mm TableTop Maksutov-Cassegrain Telescope

Features & specs:

Inexpensive
Very compact and lightweight
May be mounted to any sturdy surface
1250mm focal length and 90mm aperture
50x and 125x eyepieces

About Refracting Telescope

A refractor telescope, also known as a refracting telescope, is a type of telescope that uses a lens to focus light. It was one of the first types of telescopes invented and is still used by astronomers today. The lens at the front of the telescope is called the objective lens, and it is responsible for gathering light and bending it so that it converges to a point of focus at the back of the telescope, where an eyepiece is located to magnify the image formed by the objective lens.

The Optical Design

The optical design of a refractor telescope is relatively simple. The objective lens is a convex lens, meaning that it is thicker in the middle than at the edges. When light passes through the lens, it is refracted, or bent, by an amount that depends on the angle at which it hits the lens and the properties of the glass. The refracted light converges at a point called the focus, which is located a certain distance behind the lens. The distance between the lens and the focus is called the focal length, and it is an important characteristic of the objective lens.

The eyepiece is a small lens that is placed near the focus of the objective lens. Its job is to magnify the image formed by the objective lens so that it can be viewed by the observer. The magnification of the telescope is determined by the ratio of the focal lengths of the objective lens and the eyepiece. For example, if the focal length of the objective lens is 1000mm and the focal length of the eyepiece is 10mm, the magnification of the telescope would be 100x (1000/10 = 100).

In addition to the objective lens and eyepiece, refractor telescopes typically have a few other components to help with focusing and alignment. A diagonal mirror is often used to redirect the light from the objective lens to a more comfortable viewing angle. A focuser is used to move the eyepiece closer or farther away from the objective lens to achieve a sharp focus. Finally, a mount is used to support the telescope and allow it to be pointed at different objects in the sky.

The Advantages of Refracting Telescope

One advantage of refractor telescopes is that they produce high-quality images with good contrast and minimal chromatic aberration. Chromatic aberration is a phenomenon where different colors of light are refracted differently by the lens, causing a rainbow-like effect around bright objects in the image. This can be a problem with some types of lenses, but it is less of an issue with refractor telescopes because they use a single lens to focus the light.

Another advantage of refractor telescopes is that they are relatively low-maintenance. Because the objective lens is sealed inside the telescope tube, it is protected from dust and other debris. This means that the lens does not need to be cleaned as often as the mirrors in a reflecting telescope. However, it is still important to keep the lens clean and free of fingerprints or other smudges, which can degrade the image quality.

Overall, a refractor telescope is a simple but powerful tool for observing the night sky. With a well-made objective lens and a high-quality eyepiece, it is possible to see many of the wonders of the universe, from the craters of the Moon to the rings of Saturn and beyond. Whether you are a seasoned astronomer or a curious beginner, a refractor telescope is a great way to explore the cosmos and deepen your appreciation for the beauty and complexity of our universe.