Types of Telescope

Different Types of Telescope

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The solar system is a wonderful, exciting and vibrant space that’s filled with eye-catching sceneries such as the planets, the comets, black holes, asteroids, and quasars just to name a few. When it comes to astronomy, the telescope has been the backbone in the study of the stars and the entire solar system for ions of years. It provides a way by which new discoveries are made as well as provide a way to refine scientists’ thinking about old concepts regarding the universe. So, to guarantee efficiency, astronomers use different types of telescopes to study the universe.

As you all know, the discovery of the telescope dates back to the 17th century thanks to the efforts of Galileo Galilei, Thomas Harriot, and Isaac Newton. Since then, technology has taken over by offering astronomers a variety of telescopes that can be used to view the universe from various angles.

Now, if you’re a beginner in the dynamic world of astronomy, choosing a telescope can be quite difficult. That’s because there are different types of telescopes available, some obsolete and others functional though rare. So, with that said, this guide will focus primarily on the different types of telescopes, how they work, a few examples of telescopes that fall on each category.

 

How far do Telescopes See?

Now, if you’re using a telescope to view the galaxy, you’ll definitely see objects that are at much further distances as compared to viewing them with naked eyes. Viewing the galaxy with your naked eyes means that you’ll only view objects that appear as tiny spots.

You see, the further an object is to your eyes, the lesser the light you’ll see. And the lesser the light that gets to your retina, the more difficult it is for the eye to process the image.

We all know that light travels fast, at a speed of 186,000 miles per second just to be specific. The solar system on its side is large and quite vast. This takes light thousands of years to travel from one side to the other.

Speaking of the stars, most of these stars we usually see at night take up to 20, 30 or even a few hundreds of years for their light to reach our eyes. Since this distance is so long, scientists usually use terms such as light-years to measure them.

So, to answer how long telescopes see, I will use an example of the Hubble Space Telescope. This telescope can see several billions of light-years. In fact, the furthest it has seen so far is about 10 to 15 billion light-years away.

Do Bigger Always Mean Better?

This is another key question most people ask regarding telescopes. Now, depending on your viewing conditions, there are times when bigger becomes better. When I speak of bigger, what I mean is the size of the objective lens. Now, when the aperture is large, then it means there’s an increase in the light which results in brighter images.

So, if you’re looking to reviewing images that are buried far away in the galaxy, then a telescope with a large aperture, combined with a short focal length, will generally make these distant objects appear large, bright and clear.

Telescope Basics

Now, before we get to our main topic, we would like to define some of the common terms and specifications that are present in the world of optical tubes. By understanding these terminologies, it will be easier for you to interpret most of the things you’ll see or hear regarding telescopes.

  • Aperture:

This is arguably one of the most important features of a telescope. It is the diameter of the primary mirror or the objective lens. The larger the aperture is, the more the light the OTA (Optical Tube Assembly) gathers.

Since more light is being captured in the OTA, the results are brighter, clearer and easily magnified images. This feature makes telescopes with large apertures the best for viewing distant objects in the galaxy as compared to those with smaller apertures.

  • Focal Length:

Another term you’re likely to hear in the world of telescopes is the focal length. In simple terms, this is the distance between the objective lens and the point where the image being viewed is in focus. The focal length is a very important factor in any telescope as it dictates how well a telescope can magnify images.

  • Magnification:

This is another term you’ll expect to hear. All telescopes are capable of magnifying images from the galaxy to make it easier for you to view them. Since it’s related to the focal length and the point of focus of an object, magnification can, therefore, be calculated by dividing the focal length by the eyepiece.

For example, if your telescope has a 25mm eyepiece and a focal length of 1200mm,

Then you’ll have 1200 ÷ 25 = 48

This means your telescope has a magnification of 48 times.

  • Focal Ratio:

Finally, we have the focal ratio. This is basically the relationship between the aperture and the focal length and is calculated by dividing the focal length by the aperture. In most cases, the focal ratio of a telescope will range anywhere from f/4 to f/15.

Now, a telescope with a low focal ratio, let’s say f/4 to f/6, has a low magnification power, a wider field of view and is susceptible to producing images with optical distortions.

On the other hand, a telescope with a high focal ratio of f/10 and above has high magnification, narrow field of view and present uniformly good images. This makes such telescopes the best for viewing the solar system such as the stars, the moon, and the planets.

 

Different Types of Telescope: Know the Basics

 

Having discussed a few important optical principles regarding telescopes and their performance, this guide will now explore the different types of telescopes that are available. Before we start, you will be forgiven if you thought that telescopes are available in infinite varieties.

Although you’ve seen so many models of telescopes from ads, all of these are divided into three main categories which are refractors, reflectors, and catadioptrics. Courtesy of the advancement in technology, we’ll also cover wave-length sensitive telescopes which are the X-Ray, Gamma-ray and Ultraviolet-ray telescopes.

  • 1. Refractive Telescopes

This type of telescope is exactly what comes to your mind whenever you think of a telescope. It’s a typical example of the Galileo Galilei’s telescope discovered in the early 1600s. Just as its name suggests, this telescope works by refracting or bending light from the front lens (the objective lens) to the eyepiece.

Refractive telescopes have longer bodies and larger objective lenses. Note that, the larger the objective lens is, the longer the OTA. The longer the optical tube is, the more efficient the telescope is when it comes to bringing an image into focus.

Since their design means nothing is obstructing the optical path of light, these telescopes are the most sort out by astronomy students and other planetary observers looking to take high-contrast images under high magnification.

Advantages of Refractor Telescopes

  • One advantage of refractor telescopes is that their lenses are fixed meaning they won’t get misaligned when focusing. This is quite different from reflective mirrors.
  • Since the optics are sturdier, handling these telescopes during transportation is quite easy, as they can’t be affected by bumps and shocks.
  • Refractive telescopes don’t have a secondary optic or mirror-like reflectors do. This means nothing is obstructing the path of light thus making the resulting images crisp and high in quality.

Disadvantages of Refractor Telescopes

  • Due to their large sizes, there’s always a tradeoff between performance and portability. Their large sizes make them hard to transport meaning you have to find a permanent location to mount them.
  • Another drawback is the high-quality lenses they use. Since it’s quite expensive for glassmakers to manufacture them, it generally means that these refractor telescopes can get rather expensive.
  • Since they use prisms for magnification purposes, these telescopes can suffer possible color fringing where the edges of an image appear to have rainbow colors. This condition is known as chromatic aberration where rings of colors appear around the edges of an image. Since the issue was highly evident in low-cost refractors, modern high-standard refractors, also known as triplets, have been designed to eliminate this issue.

Examples of Refractor Telescopes

  • Achromatic Telescopes: The achromatic telescope was introduced as a way to reduce chromatic aberration. This modern refractor telescope combines two lenses, a Flint glass, and a Crown glass, to achieve different light dispersion and eliminate chromatic aberration.
  • Apochromatic Telescopes: Unlike our previous telescope, this one uses a special lens that disperses three wavelengths to eliminate chromatic aberration. Although the lenses used are similar in design with the achromatic telescope, they contain a liquid in between them that offers added dispersion.

 

  • 2. Reflector Telescopes

Also known as the Newtonian telescope, the reflector telescope was discovered in the mid-1600s by Isaac Newton. Unlike Galileo’s refractor telescope, this one utilizes mirrors instead of lenses to reflect light and gather the required images on the eyepiece.

Now, with this telescope, light enters from one end and hits a concave mirror at the back. This light is then focused on a second mirror angled at 45 degrees before it’s reflected to the eyepiece straight above.

Originally, polished metal was used instead of the mirrors but due to advancement in technology, glass with an aluminized layer is commonly used as it’s able to create a more reflective surface.

Advantages of Reflective Telescopes

  • The first advantage of reflective telescopes is the fact that they use mirrors instead of lenses. This allows them to eliminate chromatic aberration—a major problem with lenses.
  • Another huge advantage of reflective telescopes is the low cost of production. Since mirrors are quite inexpensive to manufacture, this makes the overall cost of these telescopes low.
  • Since these telescopes are not as complex as the refractor telescopes, the mirrors can be housed in a small tube making them compact and less bulky. This is a huge advantage if you’re looking to transport the telescope from one location to another to have a clear orientation of the sky.
  • Finally, mirrors can reflect all wavelengths of light equally. Since they’re inexpensive to manufacture, a large mirror will, therefore, gather more light leading to high-quality crisp images.

Disadvantages of Reflector Telescopes

  • Although they solve the issue of chromatic aberration, reflector telescopes suffer from various optical distortions such as spherical aberration and coma. These are conditions whereby the image being focused appears comet-like on the edges.
  • Although it’s easy to transport these telescopes, slight knocks or impacts can easily misalign the mirrors. They also require a high level of maintenance as dust and debris settling on the mirrors can affect the quality of images being processed. However, to ensure that they work perfectly well, the mirrors are aligned through a process called collimation.
  • Lastly, the main drawback with reflector telescopes is that the image produced is usually inverted. Although this is not an issue when you’re viewing the solar system, it becomes a real problem when you’re viewing nature.

Examples of Reflector Telescopes

  • Newtonian Reflector: This is usually considered the mother of all reflector telescopes. Here, the light enters through the primary parabolic mirror which bounces the light to a secondary flat mirror placed at a 45-degree angle. The secondary mirror then sends the light straight to the eyepiece.
  • Cassegrain Reflector Telescope: This particular reflector telescope uses a combination of concave and convex mirrors to fold the light path and improve magnification. A hole is also drilled at the center of the primary parabolic mirror to reflect light to the eyepiece.
  • Three-Mirror Anastigmatic Telescope: With this reflector telescope, three mirrors are used to help correct spherical, coma and astigmatic aberrations. The idea here is that the first mirror will correct spherical aberrations while the other two mirrors correct the rest of the aberrations to widen the field of view and produce crisp high-contrast images.

 

  • 3. Catadioptric Telescopes

Third, on our list, we have the catadioptric telescope. Just as its name suggests, this type of telescope is a hybrid or rather an intermarriage between the refracting (dioptrics) and the reflecting (catoptrics) telescopes.

Invented in the 1930s, this telescope employed a combination of lenses and mirrors as a way to correct the various aberration problems each of the two telescopes suffered.

So, how do they operate? First and foremost, these telescopes comprise of a front corrector plate that has a secondary reflector attached. The corrector plate is usually a lens. Next, there’s a primary mirror located inside the tube. The mirror is concave and has a hole at the center.

During operation, incoming light getting in via the corrective lens is reflected by the concave primary mirror to the secondary mirror (the one attached at the center of the corrector plate). This light is then reflected back to the primary mirror and exits through the eyepiece (which is the hole at the center of the primary mirror).

Advantages of Catadioptric Telescopes

  • Since they’re designed to fold the light path inside the telescope’s tube, they have the advantage of being small and easily portable.
  • The combination of mirrors and lenses means that the various aberration issues are eliminated leading to much better images.
  • Another advantage of these telescopes is that they have the characteristics of both the reflective and refractive telescopes. This makes them versatile and perfect for viewing both the solar system and the nature around you.

Disadvantages of Catadioptric Telescopes

  • Although they’re easy to transport, the fact that they contain mirrors makes them quite susceptible to misalignment meaning you have to collimate the telescope regularly to align the primary and secondary mirrors.
  • Just like the Newtonian telescope, the catadioptric telescope has a secondary mirror attached at the center of the corrective lens. What this means is that light getting in from the objective lens will have its path obstructed by the secondary lens thus degrading the quality of the resulting image.

Examples of Catadioptric Telescopes

Nonetheless, some of these aberration issues, from both the reflective and refractive telescopes, have been solved by the development of different variations of the catadioptric telescopes. These include:

  • Schmidt-Cassegrain Telescopes (SCT): These telescopes are designed to use spherical mirrors and lenses to help correct spherical aberration. Most catadioptric telescopes are typical examples of SCT meaning they operate the same way as discussed above.
  • Maksutov-Cassegrain Telescopes (MCT): By just looking at it, this telescope appears similar to the SCT. However, the only notable difference is with the correcting plate that is convex rather than being spherical, like it’s the case with the SCT.

Also, instead of having a mirror at the back of the corrector plate, a silvered section behind the convex plate is used to reflect light back to the eyepiece opening on the primary mirror.

 

  • 4. X-Ray and Gamma-Ray Telescopes

 

Finally, in this guide, we’re going to discuss X-Ray and Gamma-Ray telescopes. Now, why do scientists have to use these expensive and high-standard telescopes? Well, the answer is simply because they need to study and learn more about the high-energy environments in the universe.

Now, to view an image, there must be light. This light is part of the electromagnetic spectrum. This spectrum is made up of electromagnetic radiation which travels in different wavelengths. Visible light, radio waves, microwaves and infrared light all have longer wavelengths while X-rays, gamma-rays, and ultraviolet-rays all have shorter wavelengths.

Note: The shorter the wavelength is, the more energy the radiation has.

Since the three latter rays have high-energy radiation, they’re usually harmful to life on earth and are hence repealed by the ozone layer from the atmosphere. It is for this reason that X-ray and Gamma-ray telescopes are mounted on the outer space (in space) to prevent the short wavelengths from being compromised by the ozone layer.

This allows them to get crisp images of the sun, the stars, the supernova, huge explosions, the pulsars, collapsed neutron stars, and the black holes without any interruptions.

 

Conclusion

As you can see, the universe has immense mesmerizing wonders that are barely noticeable with the naked eyes. However, following major discoveries and contributions from Galileo and Newton, scientists have managed to create improved variations of different types of telescopes that have come to handle how we view the universe.

So, with that said, whether it’s choosing the refractor, the reflector, or the catadioptric telescope, everything melts down to your own personal intentions. Also, note that each telescope has its own strengths and weaknesses.

You should, therefore, choose a telescope that falls within your budget and one that you can easily afford to maintain.

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