Different Types of Telescopes

Looking to buy a new telescope? Read our guide to the different types of telescopes to see what telescope would suit you, whether it's your first telescope or your next upgrade.','different-types-of-telescopes','Which Telescope is Right for Me?'

You are sure you want to buy a telescope (if you’re not, why not have a quick read here for the differences between some optical instruments?), and you’re possibly ready to dip your toes in, or jump right into, the deep and wonderful waters of astronomy, but which telescope is the right one for you? There are few decisions in life that are capable of seriously changing your life, we believe purchasing a telescope is one of them: a bad purchase can plague your experiences with astronomy forever or a good one can introduce you to a rewarding, lifelong hobby.

This article will help you sort out the important bits of information about telescope and help make an informed decision whether you are looking for a telescope for a child or beginner or a telescope for a specialised task such as planetary viewing or astrophotography.

Some Telescope Terminology

The following are some of the specifications that are present on all telescopes. Knowing these terms and what they mean will help you to interpret information present on a telescope.

Aperture

A telescope’s aperture is arguably the most important specification (and it surprisingly has no direct relationship with a telescope’s magnification!). Aperture measures the amount of light that a telescope is able to gather; the larger the aperture the more light that enters the telescope. This is determined by the size of the primary lens or the mirror in your telescope (more on that later). A larger aperture allows the telescope to capture more light, resulting in a brighter and clearer image that is capable of resolving dimmer objects and finer details compared to a telescope of smaller aperture. So a large aperture is desirable but extra-large aperture telescopes can be very bulky and heavy, not to mention potentially very expensive.

Focal Length

Focal length is the distance between the primary lens or mirror and the point at which the image of the object is in focus. While longer focal lengths generally result in longer telescopes, it isn’t always the case as some types of telescopes can manipulate the path of light. A telescope’s focal length is important in that it determines by what factor your telescope magnifies. The magnification of a telescope can be calculated by dividing the focal length of the telescope by the focal length of the eyepiece being used: 

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For example, using a 25mm eyepiece in a telescope with a focal length of 1200mm would give a magnification of 48 times. 

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While it is tempting to magnify as much as possible, a high magnification is not always desirable or possible. High magnifications are limited by atmospheric conditions during viewing and the telescope’s ability to resolve images, i.e. its aperture. Using a telescope with magnifications also result in a narrow field of view, or being able to see less of the sky.

Focal Ratio (F-Ratio or F-Number)

Focal ratio is a number that denotes the relationship between the aperture and focal length of a telescope. It can be obtained by dividing the focal length by the aperture. The focal ratios of telescopes usually range from f/4 to f/14. Telescopes with low focal ratios (f/4 to f/6) are lower in magnification for their aperture, provide wider fields of view, are favoured for astrophotography, and are more susceptible to showing certain forms of optical distortions. Telescopes with high focal ratios (f/10 to f/14) provide higher magnification for their aperture size, with narrow fields of view, and are ideal for viewing that requires high magnification such as looking at planets, the moon, or double stars. The number does not determine performance per se, but does suggest how the telescope will behave and what it is more or less suited for.

Types of Telescopes

This is where telescope choice becomes very exciting (and confusing!). Telescopes do come in a few different designs and each construction has its own strengths and weaknesses. The types of telescopes you can find in the saxon range are:

  • Refractor Telescopes
  • Reflector Telescopes
  • Dobsonian Telescopes
  • Maksutov-Cassegrain Telescopes

Refractor Telescope

The refractor telescope is likely what comes to mind when one thinks of a telescope. Just as its name suggests, a refractor telescope works by refracting, or bending, light in order to form an image.

Many entry level refractor telescopes come on a simple to operate mount, known as an Alt-Azimuth mount, and make for great first telescopes. They also require little to no maintenance, making them ideal for those new to the field or who want something convenient and easy to set up. Refractors provide excellent colour contrast and thus are often used for looking at the moon and planets, especially refractors with a relatively high focal ratio (f/8 to f/10). Refractors with a low focal ratio (f/5) provide superb low magnification, wide field views of the night sky.

Refractor telescopes are, however, not without their drawbacks. They are relatively more expensive per inch of aperture compared to other telescope constructions. This is due to their utilisation of lens prisms which are more expensive to produce. The use of prisms in these telescopes is also behind the optical distortions in these telescopes, known as chromatic aberration or colour fringing which shows itself as a purplish ring of light around bright objects. This happens because the different wavelengths in light do not all focus at the same point due to the different rates of refraction (light bending) when light passes through the prisms.

High end premium refractor telescopes overcome this by using special types of glass with certain characteristics which enhance the telescope’s ability to focus the wavelengths at one point. This makes them highly suitable for astrophotography due to their colour correction and excellent contrast.

Reflector Telescope

In contrast to the refractor telescope is the reflector telescope, invented by Sir Isaac Newton. Reflector telescopes work by utilising mirrors to reflect light that the telescope gathers to form an image at the eyepiece.

Reflector telescopes are the most cost-effective telescopes available, as mirrors are much easier and more inexpensive to produce. As such, large aperture reflector telescopes can be manufactured for relatively far less than a refractor telescope of equal aperture. For those seeking to get the most out of their astronomy experience on a budget, a large aperture reflector telescope is the way to go. Modestly large, low f-ratio (f/5) reflectors also make excellent wide-field astrophotography telescopes.

While refractors do not share the problem of chromatic aberration with refractors, they can exhibit their own optical distortions known as spherical aberration and coma. Reflectors also require some degree of maintenance as dust and debris can settle on the mirrors inside the telescope and the mirrors can fall out of alignment due to movement or impact. While regular cleaning is not necessary, reflector telescopes should be checked and have their mirrors aligned (through a process called collimation) before use to ensure that the telescope performs well. There are a number of accessories that can allow a reflector telescope to be used for terrestrial viewing, but due to the construction of a reflector telescope it does not lend itself to be used for regular use in this manner.

Dobsonian Telescopes

Perhaps the category that looks least like a telescope, but the one that we would recommend to anyone looking for their first telescope or seeking something with a large aperture, ease of use, and at an affordable price.

The Dobsonian telescope is actually just a reflector telescope, usually fairly large, which sits on top of a simple to operate, sturdy, inexpensive mount that was designed and popularised by John Dobson. This brings with it certain advantages and disadvantages.

The first advantage is the simplicity of it. A Dobsonian telescope is very intuitive to use as the mount only pans left and right (azimuth axis) and up and down (altitude axis), in comparison to an Equatorial mount which is designed to compensate for the rotation of the Earth. The second advantage is the ease of setup. Dobsonian telescopes can be set up in a matter of minutes with little effort by simply lifting the telescope tube onto the mount and securing it in place. A third advantage is the lower cost of a Dobsonian telescope, which can be attributed to the fact that a heavy duty mount capable of bearing the telescope tube’s weight is not required.

The Dobsonian telescope has the same shortcomings as a reflector telescope as they share the same telescope tube design and optical elements. That they are not mounted equatorially also means that they do not track objects as easily as other telescopes, making them unsuitable for anything beyond the simplest forms of astrophotography.

If you are starting out in astronomy or want a telescope that is truly “bang for your buck”, the Dobsonian telescope is what you should be looking at. It doesn’t look fancy and does not provide much room for upgrades like other telescope designs but, for pure visual astronomy observation, a Dobsonian telescope is more than worthy of consideration for both beginners and experienced astronomers.

Maksutov-Cassegrain Telescopes

A Maksutov-Cassegrain Telescope is a “catadioptric telescope”, this means that the telescope utilises both lenses and mirrors to form an image. This comprises of the spherical mirror at the back of the telescope that gathers light and a curved lens at the front of the telescope tube.

A common first impression that many have when seeing a Maksutov-Cassegrain telescope is how small and portable it is, and concluding that therefore it must be a low power instrument. While it is indeed a small and portable instrument, it is anything but low power. Due to the nature of their catadioptric design, the path of light is bent inside a Maksutov-Cassegrain telescope, resulting in a long focal length but a short physical tube. This does more than compress a powerful telescope into a small package, the high focal ratio of Maksutov-Cassegrain telescopes (between f/12 to f/14) virtually eliminates any optical aberration seen in either refractor telescopes or reflector telescopes. They are excellent for planetary and lunar viewing due to their long focal lengths which allow higher magnifications to be achieved compared to both reflectors and refractors. Unfortunately, Makstuov-Cassegrain telescopes are not “do it all scopes”. The high focal ratios of these telescopes result in a much narrower fields of view than other telescope designs. This limits them when looking at objects which take up larger sections of the sky such as nebulae and galaxies as it may be difficult or impossible to fit the entire object within the field of view. Maksutov-Cassegrain telescopes are also relatively smaller in aperture as large sizes would require large and heavy lenses. The more modest apertures on these telescopes also limit their performance in regard to faint and dim objects.

In summary, the Maksutov-Cassegrain is comparatively more specialised in what its design allows. Its small size, low weight, and need for little maintenance, a Maksutov-Cassegrain telescope may be the perfect telescope for those who favour convenience and are looking for a grab and go telescope, especially for lunar and planetary viewing.

Closing Thoughts

So, which telescope is the right one for you? Well, that depends. Unfortunately, there is not a single answer to that question each time. Each telescope design has its own strengths and weaknesses, and will be more suitable for certain things and not others.

Other considerations such as what area of astronomy you would like to major in, distance from a location with dark skies may make portability more or less important, subjective personal factors such as mobility and strength may also limit choices, and so on.

Whichever one you choose, any saxon telescope can open up the universe to you. But there is one more thing we need to consider: Telescope Mounts. But that’s another article.