Choosing a telescope

Robin Scagell will help through some of the pit falls

Pity the beginner these days! There are just so many telescopes available, all offering the wonders of the heavens at prices that are lower in real terms than they have ever been. How can you choose between them all?

Before you start poring over the ads, there is one thing to think about: do you really need a telescope in the first place? A lot of people say that binoculars are a good way to start, and even with the wide range of telescopes available today, it's still true, if not more so. Many people buy a telescope with high hopes of seeing amazing sights, like those in the books, but only use it on a few occasions because they find that the reality does not match up with their expectations. Apart from the Moon and Saturn, many objects can be a disappointment. It can be hard to see any detail on the planets; deep-sky objects are faint, fuzzy and quite possibly invisible against the light pollution. But don't let us put you off -- there are also many people who accept these limitations and enjoy making the most of their conditions.

However, if you are not sure, binoculars can be an excellent and low-cost way to start observing and get a taste of what's visible. They will allow you to see fainter stars than you can with the naked eye, which in effect means that you can cut through some of the light pollution. Even from the city, you can begin to see some deep-sky objects and find your way around the sky. 10 x 50s are a good start (10 magnification, 50 mm aperture) for city observers, but higher powers can be useful once you have got the hang of using them.

However, many people will not be happy until they have a telescope as well. So this is a brief guide to get you up to speed and help you start making a choice. The things you have to consider are:

Type of telescope

Type of mounting

Where you live (town or country)

What your interests are

How portable the telescope must be

Your budget

All the above are linked in one way or another, so there isn't a fixed choice for each aspect.

All telescopes have some specifications in common. First there is the aperture, or the diameter of the lens or mirror. The larger the aperture, the more light you get. As you might expect, in general the larger the better, but quite often you can get good results from quite a small telescope, depending on your interests and circumstances.

The other specification you have to consider is the focal ratio, or f-number, which is a measure of how short the tube is. This varies between about f/4 for a very compact and short tube, up to about f/15 for a long tube. There is a trade-off. A telescope with a low f-number gives a brighter image for its size, but it is harder to get high magnifications out of it. By and large, a low f-number is not good news if you want to look at detail on planets, for example. At the other end of the scale, while a telescope with a large f-number is great for observing planets, it's not suitable for wide-field views of such things as nebulae and comets. Some telescopes have short tubes but large f-numbers, as explained below.

You might imagine that the magnification is an important feature, but it's not as critical as you might think. You vary the magnification by using different eyepieces. Often, turbulence in our atmosphere (what we call the seeing) restricts the amount of magnification to 100 or so, and many deep-sky objects need aperture rather than magnification. Typical magnifications (or powers, as astronomers often say) vary from a low of about 30 to 50, up to a high of maybe 200 or 250, with higher powers for occasional use.

Types of telescope
There are three basic types: refractor, reflector and what are called catadioptric telescopes. Refractors focus the light using lenses, reflectors use mirrors, and catadioptrics use a combination of the two, with a mirror to focus the light and a corrector plate to reduce the length of the tube.

Refracting telescopes are the traditional type of telescope, with a lens at the top end of the tube. There are the ordinary refractors and there are the posh apochromatic types, known as APO or ED or Fluorite. The difference is that ordinary refractors (called achromatic, not to be confused with apochromatic) give a certain amount of false colour round objects, typically a blue rim to a planet. In cheap telescopes this is very bad, but in the better ones it is usually quite acceptable as long as you don't try to magnify the image too much. In apochromats, the false colour is almost unnoticeable, but you pay three or more times as much.

Refractors start at about 60 mm aperture (the diameter of the main lens) and go up to about 150 or 175 mm aperture, but above that size they get very expensive. Their advantages are that they are easy to use and to store, require little maintenance and, if well made, give sharp, bright and wide fields of view for their size. The contrast of the image is also very good. The downsides are that business of the false colour, and the fact that they get very pricey in the larger sizes. In addition, because the eyepiece through which you view the image is at the bottom of the tube, you often need either a tall tripod, or you crouch down on the ground, or you use a star diagonal which gives a more comfortable viewing position but turns the image back to front. This is particularly confusing when you are viewing the Moon, say, or trying to find something using a star map.

Most reflectors are of the basic Newtonian type, with the mirror at the bottom end of the tube and the eyepiece at the side of the tube near the top. This means that you look sideways on to the object you are viewing, but it can make for much comfortable viewing positions, at least in the smaller instruments. There is no false colour with reflectors, which is a big advantage, but they don't have such good wide fields of view as refractors. The image is not as bright, size for size, and the mirrors will lose their reflectivity over the years so they may need to be recoated and the optics realigned if not looked after very carefully. But if you want a large telescope, particularly for observing faint deep-sky objects, a reflector is by far the cheapest option. A reflecting telescope of about 130 to 150 mm aperture is a pretty standard all-round instrument for a lot of people, but you can get them as small as 75 mm and as large as 450 mm (or more, if you are totally obsessed). About f/6 is a standard all-purpose focal length, but the larger ones tend to be f/5 to keep the tube length down.

There are two main types of catadioptric telescope, the Schmidt-Cassegrain and the Maksutov. The Schmidt-Cass has what looks like a flat corrector plate at the top end of the tube, while the Mak has a corrector which curves inwards like a dinner plate. The idea of both these types is that they give a lot of focal length in a short tube, so they are more manageable than standard reflectors or refractors for their aperture. Typically they are f/10 for a Schmidt-Cass and f/14 for a Mak. The portable Meade ETX 90 and ETX 125 telescopes are Maks, while the Meade LX 90, LX 200 and the Celestron C8, C9.25, C11 and C14 are Schmidt-Casses. Cats offer some portability compared with the equivalent reflector or refractor because of their short tubes, but at higher cost.

So having looked at the telescopes available, next come the mountings, which are every bit as important as the type of telescope.

Types of mounting
The simplest mounting is an altazimuth mount, which means that you can move the telescope from side to side and up and down on separate axes. This is fine for speed of use, but the trouble is that objects in the sky rarely move horizontally like ships on the horizon, but a mixture of up a bit and along a bit. It's not until you start to view a planet at a magnification of 100, say, that you realise just how quickly objects move through the sky. After a matter of seconds you have to recentre the object and then wait for the telescope to stop quivering if it is on a lightweight mounting before you can view it again for another few seconds.

Manual altazimuth mounts are found on the smallest of telescopes, plus many larger reflectors which have what are called Dobsonian mounts. There is a lot to be said for the altazimuth mount if you are a beginner – no complicated setting up, in particular. The cheapest are a nightmare, but the types used on the mid-sized refractors are usually quite acceptable, and come with smooth slow-motion knobs on flexible cables which allow you to track objects without too much trouble. Dobsonians have low-friction bearings which should allow you to move the tube smoothly. A Dobsonian gives you the largest aperture at the lowest cost.

But motorised mounts are much more convenient for much serious observation, particularly of the planets or if you want to do photography (a whole can of worms which we won't go into here). Altazimuth mounts can be motorised if they have an inbuilt computer, which these days always includes what's called a GO TO facility. That is, having set it up on the sky, it then knows where everything is and will (in theory) find any object for you. This sounds great, and when it works properly it is, but be warned – some GO TO systems, even from one well-known manufacturer in particular, are notoriously difficult to use despite the claims that you can be up and running within minutes. Running, yes, but for the aspirin bottle.

The other way of doing it is to use an equatorial mount. This has the usual two axes but one has to be pointed at the Pole Star. Then a single motor will compensate for the Earth's rotation. Sounds simple, but many beginners find the whole thing very daunting. If you think you can cope with this, and are prepared for a bit of a learning curve, then you will be happy with an equatorial mount, which is also really the only sort you can use if you want to do long-exposure photography, by the way. Most advanced amateur astronomers use equatorial mounts. It's a bit like learning to ride a bike. Once you've got the hang of it, it's fine.

Making your choice: where you live
Let's face it, if you live in the middle of an urban area and can only see about five stars in the sky, it's a waste of time trying to see faint galaxies and other deep-sky objects. But that doesn't mean to say that it's a waste of time having a telescope at all. You just have to concentrate on the bright objects – the Moon, the planets, maybe some planetary nebulae, and of course the Sun (taking proper precautions). In this case, it makes sense to get a telescope which is most suited to planetary work at fairly high magnifications, and that usually means one with a largish f-number. You'll also be quite likely to have problems with lights shining onto your scope, and good contrast is critical. All this tends to point in favour of either a refractor or a catadioptric telescope, though an all-purpose reflector would still give good views and shouldn't be written off as a bad idea if the idea of one appeals to you. But a large Dobsonian or other reflector of f/4 or f/5 is not really suitable for this location. Alternatively, you may decide that you want a telescope which you can take out in the car to a darker site, in which case portability (or transportability, which is a different thing) will be an issue.

If you are in a good, dark site, however, you could observe any type of object and other factors, such as your interests, portability and budget will affect your choice.

Making your choice: what your interests are
Probably as a beginner you haven't had a chance to decide whether you are more keen on deep-sky, planetary, lunar or solar work, or maybe photography. If you say 'yes' to the whole lot, then fine. You need a general-purpose instrument rather that something specific. But bear in mind that saying 'yes' to photography, particularly of the deep sky, can involve you in far more stringent requirements.

In a nutshell, for planetary and lunar observing a fairly long focal length is a good idea. At least f/10 is probably a good idea. An aperture of at least 100 mm is needed, and preferably quite a bit larger. For solar observing, however, you can use a smaller instrument as light grasp is not a problem.

When it comes to deep-sky observing, aperture really is important. Actually, in perfectly dark skies you could see a great deal with a very small telescope, but in average UK skies a bit of aperture is worth having. This is where Dobsonians come into their own. The shorter focal ratios here are no drawback, as the give a brighter and wider field of view.

For photography of any type of object you do need a good and sturdy mounting. Planetary and lunar photography, which is now often carried out using webcams, is much less demanding than deep-sky imaging. For the latter, a very sturdy equatorial mount with accurate motors is really important.

Portability
For many people, portability is the Holy Grail of telescopes. They may want something they can take on a plane on holiday, where they will get dark skies, or take in the boot of the car to darker skies than they get at home. Or they may have little room to keep a telescope at home, so they need something compact. The sad truth is that a very portable telescope is often a disappointment. The mounting can be fiddly and wobbly, and the aperture too small to show anything well.

At the same time, bear in mind the impracticality of a telescope that is too large for you. If you can't readily move the whole instrument around, you will have to spend some time putting the mount on the tripod, losing screws and washers on the grass, then lifting the tube onto the mount and holding it there while you tighten the clamp. By the time you have got everything ready it has clouded over. And the ideal home for a telescope is not behind the sofa next to the radiator, where it gets so warm that it will take hours to cool down in the night air, and will give lousy images while doing so, but out in a garage or shed where it is close to the night-time temperature when you come to use it. In these cases, security may be a problem.

So think carefully about how small a telescope you really need, and how large a telescope you would find practical. Somewhere in there is the one you need.

What can you afford?
For some people, particularly young stargazers (or their parents), this really is the deciding factor. You know you should go for the best telescope you can afford, but how long to wait for something worthwhile? There's no easy answer to this, but here are a few hard facts:

* Don't buy from catalogues or newspaper small ads -- these suppliers are not people who know telescopes. Camera shops are not always the best place, either, though some are good and have people who know what they are talking about. If the supplier is not able to give personal advice on your choice, be sure you know what you are doing.

* Be very careful about offers on the Internet other than from sources that advertise in the astronomy magazines. There are bargains out there, but there is also a lot of rubbish, and you don't want to be caught out.

* Don't assume that you will be able to trade in your telescope for a better one after a few months. There is a fair mark-up on telescopes, and secondhand prices are not particularly high.

* This means that a secondhand telescope can be a good buy, but try to get a look at and through it, preferably with an expert with you, before you part with your money.

And finally...
Don't be so worried about whether the telescope is ideally suited to your circumstances that you spend too long deciding. Most telescopes of reasonable quality will perform well on a wide range of objects. But also, be aware that the photos you see in books and magazines represent an ideal that is rarely met in practice. You just don't see vivid colours in nebulae, or fine detail on most planets, and certainly not on starter scopes. Even so, your first view of the Moon through your own telescope is a great thrill, and tracking down those faint fuzzy objects is great fun. With practice, it's amazing how your observing skills improve.

Find out about books by Robin Scagell at www.stargazing.org.uk