Choosing a telescope – a beginner’s guide

Every amateur astronomer wants to own a telescope – and the bigger the better.  But for a starting instrument don’t overlook binoculars, which are often cheaper and better value than small telescopes, and will show you many interesting objects.

[caption id="attachment_1077" align="alignright" width="200"]Infinity 76 The Sky-Watcher Infinity 76 reflector costs under £50[/caption]

Other basics for beginning skywatching include a planisphere (a circular star chart with a mask that rotates to show the stars on view at any particular time) and a simple star atlas, though these days many people will be using a computer mapping program such as the free Stellarium or an app on their phone or tablet. Armed with these and a pair of binoculars, you can learn the basic sky sights with little outlay. And the money will not be wasted – the binoculars will always be useful, even if you eventually graduate to a large telescope.

But, let’s face it, many people really want to see details on the planets, closeups of the Moon and galaxies galore. For that you’ll need a telescope. The good news is that if you choose wisely, you can now get an excellent telescope for a reasonable sum. The cheapest that give a good image cost under £50, though these have limitations. You can get one that will perform well for around £200, and a computer-controlled one that will find objects for you for under £300.

Telescope basics

There are two main types of telescope: refractors (the spyglass type) which use lenses to collect and focus light; and reflectors, which collect light with a mirror. There are also telescopes combining the two principles, known as catadioptrics, which combine the two principles to create a more compact instrument. To choose the right telescope, you need to know the pros and cons of each type.

[caption id="attachment_1080" align="alignnone" width="650"]L-R: Celestron Astromaster 70 mm refractor on altazimuth mount; Sky-Watcher 130 mm reflector on equatorial mount; Meade ETX90 catadioptric on altazimuth Go To mount L-R: Celestron Astromaster 70 mm refractor on altazimuth mount (£105); Sky-Watcher 130 mm reflector on undriven equatorial mount (£179); Meade ETX90 catadioptric on altazimuth Go To mount (£469)[/caption]

The first thing to know is that telescopes are judged not by their magnification or length but their aperture — the diameter of the main lens or mirror. When astronomers refer to a ‘small’ telescope they mean one with a small aperture. The aperture governs how much light the telescope collects – and the more light it collects, the more you can see. So for the best views it’s best to get the largest aperture telescope you can afford. But there are other things to consider, such as portability.

The smallest refractors, of 50 to 70 mm (2 to 3 inches) aperture, will show the Moon’s craters, Saturn’s rings, Jupiter’s cloud belts and its four main satellites, some attractive double stars, and the brighter nebulae and galaxies. The views will not be bright, but with care and in good, dark skies there’s a lot you can see. So they are good starter instruments, though if you think you might want to see just that bit more, it’s better to start with something larger.

[caption id="attachment_1082" align="alignright" width="320"]Spot the difference: on the left, a 50 mm refractor with achromatic lens giving a good image, and at right, a 50 mm refractor with aperture stop behind the lens reducing the apertures and giving a poor image Spot the difference: on the left, a 50 mm refractor with achromatic lens giving a good image, and at right, a 50 mm refractor with aperture stop behind the lens reducing the apertures and giving a poor image[/caption]

A word of warning. There are some small refractors which are not what they seem, because although they have, say, a 50 mm lens at the front end there is a disc with a hole behind it which cuts the aperture down to around 25 mm, because the lens is just a simple lens rather than the more expensive achromatic variety. So the view is very dim, and of very little use. From the outside, they look impressive, but in practice they are so bad that most people give up on them straight away. You may still find these being sold in toy shops or on the Internet, and even in some prestigious department stores, not necessarily very cheaply.

So if you get the chance, look down the tube through the main lens. Is there a stop – a disc with a small hole in it – a short way down the tube? (This should not be confused with light baffles, which are a series of rings of decreasing diameter positioned at intervals down the tube.) Views through the telescope at a low magnification of about 20 or 30 should be sharp and free from significant false colour (blue or orange fringes around bright objects). But never look at the Sun when testing a telescope – a TV aerial or chimney pot seen against a bright sky is good enough.

If the telescope fails either of these tests, do not buy it or return it for a full refund.

Incidentally, many refractors show some false colour unless you go for the much more expensive variety with what are called ED or apochromatic lenses. But reflectors and catadioptrics are usually free from it.

[caption id="attachment_1090" align="alignright" width="320"]Reflector vs refractor. At left, a 130 mm reflector, showing the main mirror at the bottom of the tube and the small diagonal mirror which brings the image to then side of the tube. At right, an 80 mm refractor. Reflector vs refractor. At left, a 130 mm reflector, showing the main mirror at the bottom of the tube and the obstruction caused by the secondary mirror that brings the image to the side of the tube. At right, an 80 mm refractor.[/caption]

Reflecting telescopes are generally made in larger sizes than refractors. A typical small reflector is 100 – 130 mm aperture, whereas a refractor of this aperture would cost considerably more. There is some light loss compared with a refractor because of the optical design – there is a small mirror in the optical path needed to bring the light to the side, where you view the image, and this blocks a little light. With a reflector you view at right angles to the object you are observing, but this can also make the telescope easier to view through.

With catadioptric telescopes you view in the same direction as the object, as with refractors. They generally cost more than a refractor or reflector of the same aperture, as you are paying for a complex optical design to make the instrument more compact.

Magnification

Another problem at the cheaper end of the market is ‘empty magnification’. The magnification of a telescope depends on the eyepiece used, and they usually come with a selection of eyepieces that offer low, medium and high powers. Do not get carried away by advertisements for small telescopes that claim magnifications of many hundreds of times. Too high a magnification will show less rather than more, since an over-magnified image will be faint and indistinct.

A good rule of thumb is a magnification of twice the aperture in millimetres (or 50 times for each inch of aperture. And if the telescope’s aperture is stopped down, the maximum usable magnification is correspondingly reduced. So be wary of small telescopes offering top magnifications much more than 100.

[caption id="attachment_1092" align="alignright" width="224"]Jupiter photographed through 130 mm Sky-Watcher reflector. This amount of detail can be seen with a magnification of 150. Jupiter photographed through 130 mm Sky-Watcher reflector. This amount of detail can be seen with a magnification of 150.[/caption]

Even experienced astronomers with large telescopes rarely use magnifications much more than about 250 because our own shimmering atmosphere often causes what astronomers call; ‘bad seeing’, when the image is fuzzy and indistinct. And low magnifications are every bit as useful as high ones. When finding an object, every astronomer starts off by using a magnification of only around 30 or 40, then increases the power if circumstances warrant it.

What are focal length and the f-number?

This technicality is actually quite important. The focal length is the distance between the lens and the eyepiece, which for many telescopes is a guide to its overall length. So a telescope with a focal length of 900 mm would be just under a metre long. The f-number is the focal length divided by the aperture and tells you whether the telescope is short and fat or long and thin. Put simply, a telescope with a small f-number (such as f/5) is short and fat and gives bright images of low magnification, while one with a large f-number (such as f/12) is long and thin and will give dimmer images at higher magnification, assuming that the aperture is the same in both cases.

Catadioptric telescopes are different, as their optical systems provide a long focal length and high f-number in a short tube, which makes them more portable for their size than would otherwise be the case.

Mountings

Having got this far, you are now faced with an increasing range of different mountings. Which you choose depends on how far you want to take your interest. Until you start to look through a telescope, you might not realise how important a good mounting can be. The worst ones are very spindly and wobbly, and make it hard to follow an object as it moves slowly across the sky. An ordinary photographic tripod, for example, is pretty useless when you are magnifying things even 50 times. On the smaller mounts there are ‘slow motions’ which are flexible cables to allow you to move the telescope slowly, but these don’t always work particularly well.

The simplest type of mounting is the altazimuth design, which allows you to move the scope up and down and from side to side. Then there is the equatorial mount, which needs to be set up more carefully with the polar axis pointing to the north celestial pole, near Polaris. An equatorial mount is more complicated but has the advantage that objects can be kept within the field of view as the Earth rotates by turning the telescope around the poleward-pointing axis only.

[caption id="attachment_1085" align="alignright" width="184"]The Sky-Watcher Skyliner 200P Dobsonian The Sky-Watcher Skyliner 200P Dobsonian. £315[/caption]

Some people say there are grounds for preventing beginners from buying equatorial mounts! They can be more trouble than they are worth, though once you know what you’re doing they do make life easier. Those with motors definitely help, as they keep objects in the field of view for a long time with no need to touch the telescope. But a firm altazimuth mount is often easier to use than a spindly equatorial.

One example is the Dobsonian mount, used for reflectors of all sizes, which can allow you to move the telescope easily just by pushing it. You can also get motorised auto-tracking mounts which, like equatorials, do need to be aligned on the sky before they will work, so you really need to know the names of the brighter stars at least. And then there are Go To mounts, available as both altazimuth and equatorials, which will find any object in their databases once they are aligned on the sky. Some, such as Celestron’s Sky-Align system, don’t require you to know any of the names of the stars or even where north is.

So what’s best for me?

In terms of buying a telescope, and very broadly speaking, if you live in a town where there is a lot of light pollution, you’d be better off with a larger f-number which will enable you to push up the magnification and get detailed views of the planets which are not affected by city lights. It will also give good views of many deep-sky objects such as nebulae, star clusters and galaxies, so don’t think you are cutting yourself off from viewing these.

Out in the country, a shorter f-number will give you bright, wide-field views of nebulae and galaxies, but the telescope may struggle a bit to give very high magnification views of planets unless it is of rather good quality. But again, there’s a good overlap so don’t rule them out for town viewing if the package suits you otherwise.

Reflector, refractor or catadioptric? If you just fancy having a small telescope for occasional use, a refractor is probably best as it’s virtually maintenance-free and is pretty straightforward to use. An altazimuth mount is probably all you need. But if you want to see fainter objects than a small refractor can show, a larger refractor or a reflector might suit you better. You get more aperture for your money with a reflector, but the mirrors do tend to lose their reflectivity over time so they need more looking after.

[caption id="attachment_1088" align="alignright" width="222"]NexStar 130 The Celestron NexStar130 SLT is a 130 mm reflector with Sky Align Go To. £365.[/caption]

Go To telescopes sound like the answer to a beginner’s prayer, but bear in mind that if you use one, you won’t learn much about the sky. Just pressing a few buttons and seeing a few faint objects appear somewhere in the field of view is not always very satisfying.

If you want to indulge in photography, bear in mind that you really do need to get more deeply involved. A good equatorial mount is pretty essential, and the costs keep on going up!

Bear in mind that most telescopes designed for astronomy give an upside-down view so if you want one for daytime viewing as well you should really get a refracting telescope with a 45 degree star diagonal that gives an upright image.

Where to buy

If your first port of call for any equipment is a catalogue shop, because they offer the best value for money, think again. Quite often, the instruments they sell are available from specialist suppliers more cheaply, and you will get the benefit of advice as well. To find the specialists, get hold of one of the monthly astronomy magazines such as Astronomy Now or Sky at Night, which carry ads from most of the major suppliers. If they have a showroom, try to visit them and you will get advice from people who actually use the instruments.

Or you could join your local astronomical society. Don’t worry that their members are probably all experts who would look down their noses at you – the majority are probably just like you. But do your homework as well – they probably won’t have time to talk you through every stage of the process, but if you have specific questions they will be happy to help. There’s a list of most societies on the Federation of Astronomical Societies website.

Original text by Steve Tidey of the Association for Astronomy Education, revised by Robin Scagell, 2014.

© National Astronomy Week and AAE. This information may be copied freely for non-commercial purposes.