With their light-gathering power, telescopes are undoubtedly the most important investigative tools in the world of astronomy. They magnify and resolve the view of space, letting us explore the distant celestial objects that we could never see with our naked eyes.
Telescopes collect, settle, and focus light from the night sky and magnify it to produce images that give greater detail about distant objects. Telescopes also provide a means of collecting and analyzing radiation from deep sky objects, even those in the far reaches of the universe.
However, they don’t all work the same way.
From reflecting telescopes to refractors and catadioptric scopes, these devices take on a diverse array of designs and performances in various sizes.
Some are fairly affordable and great for getting started in astronomy while others are high-end and expensive, developed for professionals, featuring the latest and most ingenious features to explore distant galaxies.
To help clarify the various types of telescopes, below we’ve shared a quick overview and a complete article thereafter, discussing each of these investigative tools in greater detail.
Some Telescope Terminology
There are various terms used to describe the features and properties of a telescope. Knowing these specifications and understanding what they mean will help you interpret information present on a telescope.
A telescope’s aperture is probably the most important pattern specification. Aperture represents the scope of light a telescope is able to gather. Aperture is determined by the size of the primary lens or mirror in the telescope.
Telescopes with greater aperture will capture more light, producing a brighter and cleaner image that’s capable of resolving objects and finer celestial details compared to a telescope of a smaller aperture.
Surprisingly, this feature has no direct relationship with the telescope’s magnification. A large aperture is desirable, but on the flip side, telescopes with great apertures are usually very bulky and heavy, and potentially very expensive.
Focal length simply refers to the distance between the primary lens/mirrors and the point at which the image of the object is in focus. Longer telescopes tend to have loner focal lengths but that’s not always the case, as some models can manipulate the path of light.
Focal length in telescopes is used to determine by what spectrum a certain telescope magnifies. A telescope’s magnification can be obtained by dividing the focal length of the telescope by the focal length of the eyepiece employed.
For instance, having a 25mm eyepiece in a telescope with a focal length of 1200mm would provide magnification of up to 48 times. Although it can be tempting to magnify as high as possible, high magnifications are not always possible, or even desirable.
High magnifications are often limited by atmospheric conditions during viewing as well as the telescope’s aperture (the ability to resolve images). Using telescopes at high magnifications also leads to a narrow viewing field or being able to observe less of the sky.
Focal Ratio (F-Ratio/F-Number)
This is simply the number that presents the relationship between the aperture and focal point of a telescope. The focal ratio can be determined by dividing the focal length by the aperture.
In most telescopes, focal ratios usually range from f/4 to f/144. Models with a lower focal ratio (f/4 – f/6) are often associated with lower magnification.
With their aperture, these telescopes provide wider scopes of view, which makes them ideal for astrophotography. They are, however, more susceptible to encountering certain forms of optical distortion.
Telescopes with greater focal ratios (f/10 – f/14) are known to deliver higher magnifications relative to their aperture size. They provide narrow scopes of views and are favored for activities that require high magnifications such as observing planets, the moon or double stars.
Numbers don’t determine performance per se, but they do suggest how the telescope will perform and what it is more or less capable of.
Different Types of Telescopes
Now, this is where telescopes’ choice becomes more exciting- and somewhat confusing. These devices are available in a few different designs and every construction has its own pros and cons. Generally speaking, most telescopes will fall into one of these three main categories:
- Refracting Telescopes (Refractors)
- Reflecting Telescopes (Reflectors)
- Catadioptric Telescopes (Compound Scopes)
Each of these telescope types is available in many variations and hybrid designs, some of which have been around since the 1600s.
Put simply, refractor telescopes are designed with lenses to bring light into focus, while reflector telescopes are built with mirrors that reflect light into focus. Catadioptric scopes meanwhile are designed to use a combination of both mirrors and lenses.
Note that you might come across other terms for telescopes types such as “Schmidt-Cassegrain” or “Dobsonian” but most of them are actually variants of the basic three.
For instance, a “Schmidt-Cassegrain” is a type of catadioptric while a “Dobsonian” is a type of reflector telescope.
Consumer and Professional Class Telescopes
Telescopes can vary in price from as low as $200 to upwards of ten thousand. Normally, only scientific institutions and top professionals will dish out more than $10,000 on a telescope. But this doesn’t mean that the average person doesn’t stand a chance to explore the heavens.
There’s a whole range of telescopes available for anyone interested in exploring the sky. Whether you’re a professional or beginner, there’re many options purposely designed for the average person to the most seasoned hobbyist.
Plus, they are basically equipped with the necessary tools and features to help you find, track, and observe different celestial objects.
Refracting telescopes, commonly known as refractors are the type of telescopes developed with specially designed lenses that refract light, sending it along a focal path (light path) within the telescope tube.
These lenses/objectives act as the main focusing unit where light is refracted as it passes through the tube to deliver a clear image with minimal distortion.
These telescopes are usually long relative to their size because the light has flow in a straight path through the telescope tube to the eyepiece. Refractors with larger lenses basically feature a longer optical tube in order to get the image into focus.
Larger refractors are usually expensive mainly because large lenses are intricate and costly to manufacture at high quality.
That said, though, most refractors are typically smaller compared to other types of telescopes, which makes them one of the most portable telescopes available on the market.
It’s good to note that modern refracting telescopes are now available in two styles: achromatic and apochromatic. Both styles are developed with varied capabilities to reduce chromatic aberration or rather color distortion/dispersion- which is a common downside to most lenses.
Chromatic aberration occurs when the lenses are either incapable of focusing all the wavelengths to the same focal plane and/or when the wavelengths are focused at different parts of the focal plane, leading to a mismatch in color at the focal point.
That means you may encounter visible color fringing/dispersion, especially at high magnification, though it mostly depends on the type of lenses utilized for the optics.
There are various ways to minimize chromatic aberration. For instance, you can use multiple compensating lenses to offset distortion or alternatively get along the objective focal length to counteract the effect. Below is a quick overview of the two types of refractor telescopes:
Apochromatic refractors or apochromats have lenses/objectives built with special, extra-low dispersion materials.
This particular design is capable of bringing three wavelengths, usually green, blue, and red into focus in the same plane. Moreover, the residual color variance can be minimized to a magnitude less than that of an achromatic lens.
Given the special materials involved in the fabrication, apochromatic refractors are generally more expensive compared to other models with comparable aperture. In fact, these models used to be extremely expensive but prices have gone down significantly in recent years.
Thankfully, current technology plays an essential role in integrating the pros of a long-tube and a short-tube refractor at a cost. Apochromats or APOs come with lenses developed with extra-low dispersal (ED) glasses alongside other materials to offset false-color considerably.
This allows the manufacturers to produce refractors that are effectively color-free with short focal ratios. It also helps to ease the problem of overlong tubes, while enabling these scopes to provide generous wide-field sights at low magnifications and gorgeous high-power images.
While they’re still excellent, cheaper models are often marked as ED refractors and not APOs. Prices tend to vary a lot but the difference is relatively subjective.
An ED refractor can be a great choice for someone looking for a serious, highly versatile instrument or astronomers who can tolerate the limited scope of image and resolution often associated with a small aperture.
Achromatic refractors or achromats have lenses that are built with conventional flint and crown glass. Distorted color is basically invisible when the focal ratio is three times the aperture at a minimum.
For instance, for optimal planetary views, a 3-inch achromat should be f/9 with a focal length of 3 x 9 = 27 inches, which is an overall manageable tube size.
However, to work equally well with a high-power 6-inch achromat, it would need to be at least f/8, with a tube 9 feet or 6 x 18 inches long.
Overall, refractor telescopes are worth consideration, and here’s why:
Despite chromatic aberration, refractor telescopes are pretty much reliable. With proper alignment, the optical system is said to be more resistant to misalignment compared to that of a reflector telescope.
The glass surface inside the tube is also covered to minimize dust and other lens disturbances, which as well reduced the need to clean the lens.
High-quality refractors are known to provide crisp, high-contrast images with excellent magnification, making them a great choice for astrophotography and planetary/lunar viewing.
Plus, they are generally smaller and more portable than other types of telescopes. That means there’s no need to have a large equatorial telescope mount as you would for some of the larger models out there.
Unlike refractors, reflector telescopes come with a spherical mirror that bends inward to reflect light at various angles within the optical tube. This light travels down a tube into a secondary mirror near the top of the tube, focusing light into the eyepiece.
Reflecting telescopes are said to be the best value for money because they offer the most aperture. For instance, a six-inch diameter refractor can cost as much as ten times the cost of a six-inch Newtonian reflector.
In fact, the most common telescope under this category is the Newtonian reflector,which as you may have guessed was invented by Sir Isaac Newton.
Considering that large mirrors are easier to manufacture than large lenses, it’s easy to expect reflectors telescopes to be less expensive than refractors at large apertures. Most reflecting telescopes are best for viewing planets such as Mars, Jupiter, and Saturn.
However, the fact that light doesn’t have to travel in a straight line makes reflectors to be generally shorter than refractors of the same aperture.
The presence of an inward-bent spherical primary mirror rather than a glass lens also reduces chromatic aberration as wavelengths reflect off the mirrors to a single point.
On the flip side, though, the fact that light doesn’t have to travel in a straight line makes reflectors to be generally shorter than refractors of the same aperture.
These types of telescopes can also fall out of alignment quite easily. And with an exposed wide-open tube, the optics will require regular cleaning.
Other than that, reflectors can be of great value with many conveniences.
They can get quite large, and are commercially available in a variety of sizes. After all, buying the largest reflector you can afford is a great budget approach to achieve a high-aperture scope. You just need to make sure you can store and transport it safely.
Among the different types of telescopes, we have the Catadioptric. Simply put, catadioptric or compound telescopes are a hybrid based on both refracting and reflecting telescopes. The name was actually derived from the two names; refractors (dioptrics) and reflectors (catoptrics).
They combine the optical benefits of both mirrors and lenses into a compact package, making them smaller and more portable than either reflectors or refractors of the same aperture.
The inside of the tube features a corrective lens that magnifies the incoming light internally. In this case, the light is reflected from a primary mirror to a secondary mirror and then reflected back to the primary mirror and the eyepiece.
There are many variations in this category but some of the most common include the Maksutov-Cassegrain and Schmidt-Cassegrain telescopes.
These two options have a front corrector plate- which are lens, a rear primary mirror with a hole in the center, and a secondary reflector which is normally attached to the correct plate.
Since it is designed with mirrors just like a reflector, a catadioptric telescope will require occasional optical collimation. This procedure needs to be conducted far less frequently to avoid compromising the integrity of the compound scope.
Due to their compact size and portability, catadioptrics telescopes are likely to be more convenient than reflectors of the same class, making them a considerable investment for average users and advanced professionals alike.
That said, they also have their downside. Most Catadioptric models have a focal ratio of f/10, which means they might not be capable of producing noticeably wide low-power fields of view.
Some models, however, allow for a bit of focal adjustment to manage the adequate focal ratio to f/6 or so which can provide noticeable refinement.
As for the cost, catadioptric telescopes fall halfway between the refractor and reflector. Like Newtonians, common catadioptric telescopes are designed with a secondary mirror in the light path. This just impedes the results for high-magnification lunar and planetary viewing.
Catadioptrics telescopes also take longer to cool down than any other type, which might compromise their ability to produce high-power images.
Catadioptric telescopes, therefore, may not really be the best option, especially for quick casual views of the sky, unless it’s okay with you to leave your telescope outside for a while to pre-cool.
Even so, with proper construction, a Maksutov or Schmidt-Cassegrain telescope will provide satisfactory images of a wide variety of celestial objects.
Well, this is arguably the only category that looks least like a telescope.
But too, it’s an easy recommendation for anyone looking for their first telescope or someone who wants something with a large aperture, and ease of use at a budget price.
Dobsonian telescopes are basically reflector telescopes, usually quite large, which sits on an easy-to-operate, sturdy, telescope mount.
The stated telescope mount is typically inexpensive and was developed and popularized by Jon Dobson. This brings about certain advantages and disadvantages.
On the bright side, there’s simplicity. Dobsonian telescopes are very intuitive to operate as the mount only pans right and left (azimuth axis) and up and down (altitude axis), compared to an equatorial mount that is meant to compensate for the rotation of the earth.
Another major advantage is the ease of use. A Dobsonian telescope can be installed in a matter of minutes with a little effort. In fact, you will just need to lift the telescope tube onto the mount and secure it in place.
In terms of cost, Dobsonian telescopes are fairly inexpensive, which can be attributed to the fact that they don’t require heavy duty mounts to bear all of the instrument’s weight.
That said, however, Dobsonian telescopes also have their shortcomings. They are still reflector telescopes, meaning, they share the same scope tube design and optical elements.
The fact that they’re not mounted equatorially also means they do not track objects as easily as other models, making them unsuitable for anything beyond the modest forms of astrophotography.
To someone new to the world of astronomy, or seeking a telescope that’s budget and great enough for pure visual celestial viewing, a Dobsonian telescope is more than a worthy consideration, for both newbie and seasoned astronomers.
Just keep in mind it doesn’t look fancy and does not provide much room for improvements like other telescopes’ designs.
As the name suggests, Radio telescopes are designed to use radio waves to produce images of distant celestial objects. Note that every object in the sky gives off radio waves. These waves are resolved by huge antennas that have large dishes attached.
In other words, radio telescopes are those huge dishes you see with three spokes in the middle. Radio telescopes are normally not used by amateur astronomers, though.
X-Ray and Gamma-Ray Telescope
Like radio telescopes, X-ray and Gamma-ray telescopes are designed to use the rays of the suns, stars, and supernovas to produce their images. This is how astronomers get to see the events that take place in deep space such as supernovas and black holes.
Gamma-ray telescopes, in particular, detect the burst of gamma rays that assist confirm events that occur in space that are rather hard/impossible to detect with visual confirmation.
The fact that these telescopes are used at high elevations means that images will hardly suffer distortion due to pollution.
Choosing the Right Scope for You
So what telescope suits you the best? Well, normally, you would want to have the largest telescope that you can easily afford and transport. However, you might have some specific needs that could make you reconsider purchasing a telescope based only on size.
For beginners and amateur astronomers looking to buy their first telescope, it’s important to have a clear understanding of what you want to use it for.
Is it for visual and photography or planetary deep sky viewing? How much are you willing to spend and what level of complexity are you planning to take on?
Answering these questions will help familiarize yourself with what’s on the market and increase your odds of getting a telescope that best matches your needs and preferences.
With any astronomical telescope, the primary goal of the optical tube assembly is to collect light, which is something they do pretty much well.
While people often concentrate too much on the magnification of the telescope, it isn’t actually more important than the desired focal length and aperture.
Newtonians telescopes generally fall between the F4 and F8 range. They are a little more complex on the correction ability of the eyepiece but there’s a device called a coma corrector that can assist with this.
Or, you can simply choose better, more expensive eyepieces to have a clear image across the scope. For most people, though, a little edge distortion is tolerable.
If you’re planning for deep-sky observing or wide-scope astrophotography of distant celestial objects, you might want to consider selecting a large reflector.
Meanwhile, if you want a high degree of flexibility in what you observe, a compound telescope like the Schmidt-Cassegrain would be an easy recommendation as it will give you many options for how to target your viewing and imaging experience.
While weighing their pros and cons, and keeping your available budget in mind, you will be wise to choose an instrument that will help you achieve the most enjoyment you can get out of the night sky.
As we have seen, there are several factors that go into deciding what telescope type suits you the best. Different telescope designs come with different levels of capabilities, strengths, and weaknesses, which means that some will be more suitable for certain things and not others.
Besides, telescope designs vary based on who is using the task they are made to perform. Amateur astronomers might pick up a telescope based on price and the kind of lenses utilized. The whole thing should be compact and easy to transport if you’re planning to travel with it.
The lens and mirror tend to have a dramatic impact on the celestial objects that the observer will be able to see. Therefore, this should be a consideration when making the purchase.
Other important things to consider include the area of astronomy you want to take on the instrument set up.
While the distance from a location with dark skies can make portability more or less important, subjective personal preferences such as strength and mobility may also limit choices.
Otherwise, there’s a perfect telescope out there waiting for you. And as seasoned astronomers suggest, the best telescope is the one you love; the one you use the most. So the point here is to get an instrument that you will be eager to use every clear night.
One more thing: consider reaching out to your local astronomy club. They may have some night sky viewing sessions where you can try different scopes, chat with their owners, and grasp more details for a serious telescope.