Different Types of Telescopes

Astronomy is a science that benefits greatly from advances in technology. As a hobby, the same is true, but the starting point should always be to keep things simple… But it can start getting complicated quickly when you are looking to buy your first telescope! At first it can seem that there are so many types of telescopes to choose from and factors to consider.

Essentially, as a beginner you only need to decide between three basic types of telescopes. These are the reflecting telescope, the refracting telescope and a hybrid of the two called a compound or catadioptric telescope.

Reflecting Telescopes / Newtonian Telescopes

Reflecting telescopes, also known as Newtonians, are possibly the type of telescope most novice enthusiasts go for. This simple design uses mirrors to gather and focus light.

Advantages

There are several advantages to using a reflecting telescope over a refracting telescope, especially as a beginner.

  • One of the main advantages of reflecting telescopes is in the simple fact that they are constructed with mirrors instead of lenses. This is because mirrors avoid chromatic aberration, a problem common to lenses. Lenses – found in refracting telescopes – bend light differently depending on its wavelength (think of a prism!).

Mirrors have the potential to produce better images because they reflect all wavelengths of light equally.

  • The simpler design of these telescopes makes them generally cheaper to construct and therefore kinder on your pocket. An expensive telescope isn’t necessarily better, especially when considering your specific needs. You need a telescope that will be easy to use and do the basics well. This is how you keep your passion for observing alive, rather than becoming easily frustrated by something complicated that you don’t understand.
  • Their design and how cost effective they are to make also means that it’s easier to make bigger reflectors. Aperture – the size of your main mirror or lens – helps determine the light gathering power of your telescope. Generally, the larger the surface area, the more light can be gathered.

Aperture is an important factor in choosing a telescope, but be careful! A large telescope is cumbersome to move, set up, and pack away. What good is that if you need to travel for clear skies? Rather get a telescope that fits in your car and is ready to use in a matter of minutes.

Disadvantages

Reflectors are great but they certainly aren’t perfect. The main issues they suffer are comas, collimation, and air currents – all of which affect the clarity of images.

  • Coma is a defect that causes a focused image to look almost comet-like (hence the term coma) around the edge of the field of view.
  • Picture this: it is a cold day out. You blow your warm breath against a cold window or mirror and it clouds up. The same thing can happen to your mirrors in your telescope! If there is a significant difference in temperature of the mirrors and the air outside, you could end up with fuzzy images.
  • Reflecting telescopes are usually built with two mirrors – a primary and secondary. Light is gathered at the primary mirror (the large one) and is then reflected to the secondary (smaller) mirror. The light is then reflected to the eyepiece which magnifies the image.

The thing is, this secondary mirror and its supporting structures, is essentially also obstructing the light and can reduce contrast and cause diffraction spikes (those cross-shaped lines that radiate from stars and other bright sources of light) respectively.

  • Another issue results from the primary and secondary mirrors – collimation, or keeping the mirrors aligned. They can get out of alignment from the transporting the telescope or even just handling the telescope.

The mirrors also need to be cleaned regularly and recoated every few years.

In other words, reflecting telescopes require a good level of maintenance. Knowing how to collimate your reflecting telescope is important, and with a good telescope, it is not difficult to do at all.

Refractors

Refracting telescopes use lenses in place of mirrors.

Advantages

  • Refractor telescopes can be great for novices because they require little maintenance when compared to reflectors. Of the maintenance niggles you can avoid with refracting telescopes, not having to collimate your telescope is probably the most advantageous.

A refracting telescope’s lenses are fixed, therefore won’t become misaligned like a reflector’s mirrors would.

  • The design of refractors tend to be hardier, meaning bumps and shocks from handling and transporting your telescope won’t affect the optics as it would with reflecting telescopes. They stay in better condition for longer, and so they often work out to be much more of an investment.
  • Refractors don’t have a secondary optic (the secondary mirror in reflectors) obscuring the path of the light to an eyepiece, so they generally produce images with far better contrast than reflecting telescopes.
  • Images can also be sharper due to the fact that a refractor’s tube is closed off from the outside. This means that air currents and the temperature from outside won’t affect the lens.
  • They can actually be easier to use for viewing (the whole point of a telescope!) for children, people of smaller stature, or even people that may have issues standing for extended periods of time. This is because the eyepiece is located at the end of the tube of the telescope, unlike in reflectors where the eyepiece is located on the top of the tube. So, you can even sit while you look through a refracting telescope if you would like.

Disadvantages

  • Refracting telescopes can be expensive pieces of equipment, especially for someone who is a part time hobbyist or a novice.
  • Refracting telescopes, no matter how good they are, suffer from chromatic aberration – the different wavelengths/ colours of light don’t all focus at the same point, producing a rainbow of colours that can affect the quality of an image.
  • It is tricky making refracting telescopes with a very large apertures because of the weight of large lenses, among other factors.
  • Long objective focal lengths are common in older refractor designs and some newer ones too, so they can be difficult to transport.

Catadioptric

Finally, consider compound or catadioptric telescopes when looking at different types of telescopes to choose from. These can be considered hybrids of the reflecting and refracting telescopes that combine the best of both worlds. They have a shorter length overall and are easily portable. They also do well in correcting the aberrations the separate telescopes suffer from.

On the other hand, they are more expensive than reflecting telescopes, and still have some of the issues the individual telescopes face. Examples include having to collimate the telescope (not as often as a reflecting telescope though), and obstructions from secondary mirrors.

Overall, I would recommend a small to medium sized reflector to the first time telescope owner and novice stargazer, but it will depend entirely on your individual needs and preferences, as well as your budget and level of experience.

If you get the chance, attend a star party! Some kind people will probably be willing to let you test out their telescopes, and that way you can get an idea of the usability of each kind.

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