How to Choose a Telescope

Being captivated by the starry night sky almost inevitably leads to the next step: buying a telescope. With myriad options available, it can be overwhelming to know which telescope if best for you.

There are a couple of things to consider to help make your decision easier. This introductory guide will give you everything you need on how to choose a telescope that will give you endless many nights of joy.

The best place to start is by deciding what type of telescope you would prefer. When it boils down to it, there are two types to choose from: reflectors and refractors; or a hybrid telescope which combines properties of both.

Reflecting / Newtonian Telescopes

Newtonian telescope2

The simple design of Newtonian telescopes always makes them a winner with beginners. These telescopes use primary and secondary mirrors to gather and focus light. Because mirrors are relatively cheap to manufacture, Newtonians do not cost as much as refractors would. This is true even for really quality makes, and especially true when you buy a telescope of a larger aperture.

As you will see later, aperture is an important feature to consider when choosing a telescope. Basically, the bigger you go the more light your telescope can gather. Newtonians are a great choice if you want a big aperture of good quality at a fraction of the cost of similar refractors.

Another advantage of reflecting telescopes is that mirrors gather the different wavelengths of light and focus them all equally. Lenses don’t do this – they suffer from chromatic aberration.

Because mirrors are cheaper to manufacture and lighter too, they are the ideal optics for very large aperture telescopes. These telescopes, in turn, are then mounted on the simple but sturdy alt-azimuth type mount, which can be beneficial to beginners because of how straightforward they are to use.

Reflecting telescopes aren’t perfect though. Although they avoid chromatic aberration, they can produce coma. Comas are when the image you are viewing through your telescope look comet like toward the edges.

Newtonians also require a fair bit of maintenance. Your optics will need to be cleaned regularly, and your mirrors may need recoating every couple of years.

Because the optics are exposed, the telescope is prone to air currents. If you do not allow your telescope proper time to cool, your images can be blurry.

Newtonians may not also produce as sharp an image as some refractors because of the secondary optic and its supporting structures. They act as an obstruction to light, causing some of the light to be lost before it is focused.

Lastly, consider that you will need to learn how to collimate your telescope’s lenses. Bumps and shock from handling your telescope can cause the mirrors to go out of alignment, which will lead to a very out of focus image. It can seem daunting at first, but once learnt it will be a skill you will always have. There are some wonderful and easy to follow video tutorials online.

Refractors


Telescope-schematic

Refractors use lenses in place of mirrors. They are well worth considering as a beginner telescope for several reasons.

Firstly, very little maintenance if any is required. A refractor’s lens is sealed from the outside environment, and so isn’t susceptible to the elements like a reflector would be. The refractor’s build is hardier than that of Newtonians, and so they aren’t as sensitive to bumps and handling as reflectors. If you handle your refractor with care, it can be an investment for life.

There is no secondary optic blocking the path of light in refracting telescopes, so they can offer better contrast in images.

Another consideration which is often overlooked is that the refractor’s design is perfect for people of a smaller build, like children, and those unable to stand for extended periods of time. This is because the eyepieces are located at the end of the tube, rather than off to the side and closer the top as is in most Newtonian makes.

An added bonus is that you will never need to collimate your telescope!

On the down side, refractors are more expensive because manufacturing lenses over mirrors costs more. A larger aperture refractor wold cost you a whole lot more than a reflector of the same size. The weight (and cost) of lenses also makes it nearly impractical to build refracting telescopes as large as the Newtonians can – and do – get.

You also can’t avoid chromatic aberration with lenses, where the lens bends the wavelengths of light unevenly, causing a slight rainbow effect.

Catadioptric

Finally, it may be worth looking into catadioptric – or hybrid – telescopes, which use both lens and mirrors.  Schmidt-Cassegrain telescopes are a good example of catadioptrics. They can be rather worthwhile investments but are quite expensive too.

Does Size Matter?

Earlier I mentioned aperture, and that generally the bigger the aperture size the better. Aperture is the main opening or optic of your telescope, and the larger it is, the more light can enter. This produces better detailed and quality views if the optics are good.

However, I would not recommend going out and buying the largest aperture telescope you can afford merely on a whim! There are other key considerations to take into account. Will you be carrying and setting up your telescope alone? Do you need to travel for clear, dark skies? What kind of storage space do you have for your telescope? Is the telescope compact enough for your car? These questions are very important to think over. Large apertures are great, but if you aren’t able to lift, transport, set-up or store the telescope because of its large size, then it won’t be much of an investment anyway.

Focal Length

How much you can see of the sky at any given moment, also known as field or angle of view, is determined by your telescope’s focal length which is measured in millimetres. A very simplified definition of focal length is the distance light travels between the telescope’s primary optic to the point where the light comes together to focus.

It is incorrect to automatically assume that the length of a telescope’s tube is the sole determiner of focal length, because there are in fact telescopes that have a compact tube but longer focal length due to their design.

Telescopes with a longer focal length will produce a narrower field of view but good magnification, whereas shorter focal lengths offer a wider field of view but with limits on magnification compared to longer focal lengths. Both long and short focal lengths have their advantages and disadvantages. This article shows the pros and cons of each in greater detail.

Eyepieces

If you can, choose a telescope that comes packaged with good quality eyepieces, namely Plössl. The unfortunate reality is that many telescopes, even excellent brand telescopes, come packaged with poor quality eyepieces. Bad eyepieces can ruin a telescope. These low grade eyepieces are usually marked H or SR.

Eyepieces are important because they are what magnify your images and give detail to what you are seeing. They also play a part in determining your angle of view. You can learn more about eyepieces here. For now, know that the most important aspect is definitely the quality of the make.

Mounting Matters

We took a brief look at alt-azimuth mounts earlier on, but there are also other options to consider.

The advantage of the alt-azimuth design is that it is easy and straightforward to use, and a sturdy mount that can hold heavier telescopes quite well. The downside is that they are problematic when it comes to accurately tracking the skies, and they are near impossible to use for long exposure astrophotography.

Equatorial mounts do have the advantage of tracking the sky with excellent accuracy, and so they are a must if you want to get into astrophotography later down the line. The cons include them being a little pricier, less stable for larger telescopes, and a little tricky for some novices to get the hang of at first.

Choose What You’ll Use

At the end of the day it really comes down to choosing a telescope with quality optics that you will actually end up using. Remember the questions asked earlier? Make a checklist of these. If you know that you live in a light polluted place and often travel for more pristine skies, a 20–inch Dobsonian mounted telescope isn’t going to help you!

It is so important that you choose a telescope that you are comfortable transporting, setting up and using. You want a telescope that can give you years of viewing pleasure, not one that frustrates you or can’t serve its purpose according to your needs.

Go ahead and shortlist a handful of your favourite telescopes. Next time you attend a star party you will surely find good people willing to let you trial run the telescopes that you are most interested in. This is a wonderful way to make a final decision.

Clear and steady skies!

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