Telescope Basics

Buying a telescope is a daunting process. With many pieces costing hundreds or even thousands, it’s no wonder why people are always trying to get the best pieces for the best prices. I dedicate this guide to first-time buyers and those who are finding the selection difficult to choose from, like I had when I first started out.

Telescope Types

When choosing a telescope, one of the first decisions you will face is the type of telescope: a reflector, or a refractor. Each is structured differently, using different methods to magnify the night sky.

Reflectors

Reflectors are popular telescopes which rely on two mirrors to reflect light into the eyepiece. One of the most used types of reflectors is the Newtonian, where light reflects off of a primary mirror onto a secondary mirror, then into an eyepiece on the side of the telescope.

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A useful advantage about reflectors is that there is a greater range of telescope sizes at much cheaper prices than a refractor. Thanks to the simplicity of the mirror, it can produce fantastic images when correctly collimated. My first telescope was a Newtonian from National Geographic that I received for my 10th birthday, which was on the less expensive side of the price spectrum, and yet still produces crisp shots of the night sky.

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Photo of my first telescope- a 700mm newtonian with a 76mm aperture, so quite small.

Let there be no mistake that it does not have to cost a fortune for good quality views of the night sky, and reflectors are great for beginners who aren’t ready to spend too much. There’s also virtually no false colour, since a plane mirror reflects all light equally, which will be covered in the refractor section.

With all telescopes there will be some drawbacks. To get a high magnification, reflectors usually have to be very long compared to their diameter, so some people opt for a catadioptric telescope, the most popular being the Cassegrain design. They use three mirrors to reflect the image to the end of the scope, which reduces the need of a long telescope. Catadioptric telescopes are much more compact and shorter, and are usually much easier to carry around. They are a bit more expensive, roughly around twice the price of the newtonian reflector of equal diameter.

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A photo of a catadioptric – Notice how short they are, and the mount they have.

Although colour is rarely an issue with reflectors, light can be slightly blocked from the primary mirror. This produces a hazy middle as the light diffracts, but with well-fitted telescopes, this issue is hardly noticeable.

Refractors

The second type of telescope is the refractor, the most popular of the two. Being easy to use and manipulate, they are brilliant for advanced stargazing and astrophotography.

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A Sky-Watcher Refractor, with a diagonal and mount.

Refractors produce images using lenses instead of mirrors, so they refract light that enters the tube. They make use of a large lens, called the objective, at the end of the telescope where light is refracted into a smaller lens, the eyepiece. Refractors are unique because there is such a large range of lens types to choose from, which alter the quality of the view immensely.

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The problem with refractors is that different colours of light have different wavelengths, thus they are all refracted differently, so refracting telescopes have a problem called false colour (sometimes called chromatic aberration). The refracted light splits into a spectrum of colours. So when buying refractors the quality of the telescope will largely depend on how the telescope deals with false colour.

Some refractors are stopped down; a disk with a hole is inserted into the telescope to stop false colour. It reduces the amount of the lens actually used, and dims the view. These types of lenses are called non-achromatic lenses, which are usually made of just one material (i.e. Glass). Stopped down refactors are usually much cheaper than others, so are ideal for those on a tight budget, but the dimmed images are not ideal for people in urban areas with high light pollution.

Achromatic lenses use 2 elements or compounds which form a doublet objective, thus false colour is cancelled out. Good achromatics create beautiful pictures. For those who are able to spend a little more, an apochromatic objective gives virtually no false colour, if you are willing to pay a higher price. An example this Skywatcher Evostar costs £262, whilst this one costs £3,999, both of which are the same brand telescopes, sold on the same website, which shows just how varied these types of lenses are in price (even within brands!)

Telescope Sizes

Sizing of the telescope is also vital to look at. Remember: the larger the diameter of mirror/lens, the bigger the magnification, but the smaller the area of sky observed. A large diameter is great for deep sky objects, and can be adjusted for closer objects with the use of a reducer if needed. However large lenses are rare and quite expensive, so mirrors are probably the best option, at a lower cost and maintain good quality. For example my Newtonian has a 20 cm (roughly 8 inches) primary mirror and takes crisp shots of the moon, but objects like Orion’s nebula cannot be seen at all. On the other hand the Celestron at my local astronomical society has a 50cm lens and takes wonderful images of deep sky nebulas and rings, and by using a reducer, we can also “de-magnify” and look at close objects such as the planets and the moon.

So What works best for me?

Living in an urban area takes a large toll on the range of scopes to choose from, even with light pollution filters, it’s not easy. Most people opt for a telescope with a larger ratio of the lens/mirror to the length of the telescope. Often called the f/ratio, it means the scope will be long, but that usually results in a larger magnification. In cities with lots of street lights, shop displays and lots of lighting, light pollution filters improve the clarity of views and images fantastically, I have found. Personally I love the reflector and the crisp images I can get from them. No false colour means I can invest in a larger mirror and higher magnification.

In the next few articles, I will be discussing different telescope accessories, including the use of different filters, eyepieces such as diagonals and reducers, and later we’ll touch upon mounts and tripods for your telescope.

In the meantime, happy stargazing!

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