Society for Popular Astronomy

Choosing a telescope

If you’re interested in astronomy, it stands to reason that you need a telescope, and the bigger the better. But for most of us, there’s a lot of research to do before you can make the plunge. There is a bewildering range of instruments available, but the good news is that there are some excellent telescopes out there, at very reasonable prices.

And if you’re just starting out, you might well have a good beginner’s instrument already – a pair of binoculars. Even small binoculars will show you a wide range of objects in the sky much better than you can see them with the naked eye,  and even in city skies they can help you find whole constellations that are otherwise hidden in the light pollution. We’ve a separate article on binoculars and using them, so check that first.

But although beginners are often advised to start with binoculars – and even advanced amateurs wouldn’t be without them – you are probably going to want a telescope as well. This is fair enough. Even super-duper binoculars have their limitations, and when it comes to viewing the planets and the vast majority of nebulae, star clusters and galaxies, you can’t do without a telescope.

What can you see?

Let’s get one thing out of the way to start with. Forget those glorious photos of colourful nebulae, sparkling star clusters and gorgeous galaxies. There’s no telescope ever made that will show the same amount of colour and detail in faint objects that photographs can reveal. Our eyes are just not sensitive enough to faint light, and although telescopes can make objects much more easily visible, even large ones can’t pour more light into your eye than the laws of physics allow. Ye cannae break the laws of physics, captain, as someone once said.

So what’s the point? Seeing those objects for yourself, with your own humble telescope in your back garden, gives a real thrill. The Andromeda Galaxy may appear to be a fuzzy oval of light rather than a dramatic spiral, but you are seeing with your own eyes light that has been on its way for some 2½ million years before it reaches you. And there’s also the fun of finding the objects for yourself that up to now have just been photos in books.

And for bright objects, such as the Moon and major planets, the view through a telescope is much more real than anything you can see in a photo. The Moon becomes a true world, full of mountains and features that you can explore, with brilliant highlights and deep shadows. Jupiter’s belts and moons, ever changing position, and especially Saturn’s famous rings, can be breathtaking.

How large?

300 mm Dobsonian and 80 mm refractor

It’s easy enough to say ‘the larger the better’, and the fact is that a larger telescope will show you more than a small one. We define a telescope’s size by its aperture – that is, the size of its main mirror or lens – rather than by its length or magnification. Telescopes start at about 60 mm aperture (2½ inches), and even these will show you a wide range of objects, but if you can afford something larger you will enjoy better views. Around 100 mm is a good size, and there are many reasonably priced 130 mm telescopes on the market these days. 

Once you get to sizes larger than about 250 mm you are into seriously large telescopes, though you can get off-the-shelf instruments considerably larger than that.

With an 80 mm telescope the Cassini Division in Saturn’s rings is just visible, but with a 130 mm it is clear

As well as gathering more light and giving a brighter image, the larger the telescope, the more detail it can show. This is known as resolving power. The standard way of testing this is to look at double or binary stars – stars which consist of two individual stars close together, usually in orbit around each other (though usually taking hundreds of years for a single orbit). Stars which appear single in a small telescope are resolved into doubles in a larger one. And with a 60 mm telescope no amount of magnification will show the gap in Saturn’s rings called the Cassini Division, for example, whereas with a 100 mm telescope it is usually obvious. The Earth’s unsteady atmosphere also limits the visibility of fine detail, so you need to get steady conditions to make the most of larger apertures and their improved resolving power.

Bear in mind that virtually all astronomical telescopes give an inverted image – that is, upside down. This is the simplest way to produce an image, and to bring it the right way up requires extra lenses, which not only absorb a little light but can add slight distortion to the image. Astronomers get used to this but it does make it harder to find objects in the sky as the view is always upside down compared with the map. You can get what are called erecting prisms to bring the view the right way up, but astronomers rarely use them. With many telescopes you also get a mirror image – so the view is back to front as well as upside down! Talk about making things difficult! 

Telescope types

There are three general categories: refractors, reflectors and catadioptrics. Refractors are the traditional type of telescope, with a lens at the top end of the tube, and you view at the bottom end. Reflectors have mirrors at the bottom end of the tube, and you view at the top end, looking sideways into the telescope. Catadioptrics combine the two and are basically reflectors but with a corrector plate at the top end to reduce the length of the telescope, so you get a more compact instrument. Each type has its pros and cons, and works best in a particular environment, but all will give good views of the full range of astronomical objects, as long as they are from a reputable manufacturer.

Here are the pros and cons in a nutshell, but whole books could be written about each type.

Refracting telescopes (refractors)

The Celestron Astromaster 70AZ is a basic refracting telescope suitable for a beginner

These range in size from the very smallest telescopes at about 30 mm aperture, used for finder telescopes and indeed binoculars, up to about 180 mm for very expensive observatory models. Their advantages are that they are low-maintenance and robust, and can give excellent contrast images. They are particularly suited to planetary observing because of the inherent good contrast, and are good for use in towns where light pollution is a bit of a problem. They can also have good definition across a wide field of view, which is good for  photographing nebulae and other extended objects.

Drawbacks are that they get very expensive once you go above about 120 mm aperture, and they are prone to dewing up as the lens is exposed to clear sky. In the larger sizes they can be a bit unwieldy, as they need long tripods to avoid observing positions too low to the ground. And one big problem is that they are inherently likely to suffer from false colour, known as chromatic aberration – that is, coloured fringes that are obvious around the edges of bright objects.

Comparison of images of the lunar crater Pythagoras from three small telescopes of similar focal length. L-R: 80 mm Sky-Watcher ED refractor; unbranded 80 mm achromatic refractor; Bresser Skylux 70 mm achromatic refractor. You can see the chromatic aberration (false colour) clearly in the images from the two cheaper instruments, but it is absent from the ED refractor.

Apart from toy telescopes at the cheap end of the market, which should be avoided like the plague even for a child’s first telescope as they are not corrected at all for false colour, the main method of correcting false colour is to use an achromatic lens. This type can work well in the more expensive instruments, but there is always some residual false colour. Today, apochromatic lenses are much more affordable than in the past, and have much improved correction to the extent that in a good instrument the false colour isn’t noticeable. This is made possible by the use of extra dispersive (ED) glass or a type of fluorite glass. But even ED refractors vary a lot, so don’t rely on the letters ED alone if you are considering one.

Bear in mind also that as chromatic aberration increases as the focal length gets shorter, compact refractors (with a focal ratio or f-number lower than about 10) are more likely to suffer from false colour and are therefore less well suited to high magnifications. But large f/10 refractors tend to be unwieldy.

Reflecting telescopes (reflectors)

The Sky-Watcher Skyhawk 1145P is a basic Newtonian reflecting telescope. The eyepiece is at the top end of the tube

Although you can get entry-level reflectors of 76 mm aperture, these are of limited use because the secondary mirror that brings light to the side of the tube has to be quite large and obstructs some light, so their performance is inferior to that of a similar sized refractor. But in sizes of 100 mm and above, they become increasingly good value compared the refractors as the secondary is smaller compared with the aperture. A 150 mm reflector is affordable, whereas a 150 mm refractor is much more expensive and may not perform as well.

The chief advantages of reflectors are their lower cost, aperture for aperture, and their total lack of false colour. They are unlikely to dew up, as the mirror is well down inside the tube, which in the UK’s moist climate is a bonus.

Their drawbacks are that the mirror can easily become dusty and require cleaning (not a trivial task) or even recoating, particularly in a poor environment such as by the sea (expensive). They are less robust than refractors, and the optics can require realigning from time to time. The open tube can suffer from swirling air within the tube, which spoils the image, and is also susceptible to nearby lights shining into the tube. Because of the secondary mirror, there is some light absorption and shadowing, so the image is less bright than with a similar-sized refractor, and fine detail lacks contrast compared with a refractor. And the outer parts of the field of view are subject to distortions such as coma, which makes stars look like small comets, though this is not noticeable unless you use wide-field eyepieces. As with refractors, the smaller the f-number, the more noticeable the distortions. However, many reflectors these days are f/6 or shorter. 

Despite all this, reflectors are very popular in the larger sizes. A 200 mm or 250 mm reflector is fairly affordable, and provides sufficient aperture for a large number of fainter objects, such as deep-sky objects and indeed the moons of the planets, to be seen. 

The standard design for a reflecting telescope is a Newtonian, but there are variants which are occasionally encountered, mainly the Cassegrain.

Catadioptric telescopes

This Meade ETX90 is a 90 mm Maksutov. The corrector plate of a Mak has a dished shape

There are two main types of these – the Schmidt-Cassegrain (S-C) and the Maksutov (Mak). They typically have much longer focal lengths than their tube size suggests, so they provide the potential for high magnification in a compact tube size. Their compactness is a great bonus, as the eyepiece is usually at a convenient height, even though you are observing at the bottom end of the tube.

S-C telescopes are made in large numbers and are often  provided on a purpose-built Go To mount, though you can also get them as tube-only (known as OTA, for optical tube assembly) for use on a mounting of your choice. The standard sizes are 200 mm and 250 mm, though Celestron have 9.25 inch (235 mm) and 14-inch (355 mm) models that are very popular with planetary photographers.

Maks are more tricky to make and are generally available in smaller sizes, such as 90 mm and 127 mm. They have long focal lengths for their size and are noted as planetary instruments, but are less suited to giving wide-field views of extended objects such as star clusters.

A word about eyepieces

Astronomical telescopes all require interchangeable eyepieces so that you can change the magnification, or in many cases substitute a camera for photography. Often, the telescope comes with one or two eyepieces only, and you buy more for higher magnifications or wider fields of view.

The magnification you get depends on the focal length of the telescope and the focal length of the eyepiece. You divide the one by the other to find the actual magnification. A typical standard eyepiece may have a focal length of 25 mm. On a telescope of 900 mm focal length this will give a magnification of 900/25 = 36. The same eyepiece on a 1200 mm focal length telescope will give a magnification of 1200/25 = 48. And you can work out for yourself that a 10 mm eyepiece on the latter telescope will have a magnification of… that’s right, 120.

A typical eyepiece set: standard 25 mm Plössl (left); 2x Barlow (centre); 9 mm Plössl (right)

The crucial thing here is that the magnification is higher the longer the focal length of the telescope, but the shorter the focal length of the eyepiece. So if you want a low magnification, choose the eyepiece with the biggest number on it. You might think you’d always want to use high magnifications, but in practice even expert astronomers start observing with lower magnifications, which give wider field of view, so that they can find the object they want and get it centred, then switch to higher magnifications if necessary.

Oh, and in the jargon, magnifications are often referred to as powers. So, low power and high power.

The other thing you need to know to start with is what Barlow lenses are for. These are not actually eyepieces, but they are adapters which typically double the power of any eyepiece they are used with. So a 2X (two times) Barlow will double the magnification of any eyepiece it is used with. If you have three eyepieces, of 25 mm, 10 mm and 7 mm focal length, getting one Barlow will give you six possible magnifications. If you are choosing eyepieces, bear this in mind so you can get a wide range of powers with the minimum outlay.

Telescope mountings

If you have kept up so far, you may be getting some ideas about what you want to go for, but there’s a further decision to be made – what type of mounting to choose. There are several options. The basic division is between altazimuth (altaz) and equatorial mounts. Altaz mounts have two axes, allowing you to move the telescope up-and-down (in altitude) and from side-to-side (azimuth). What more do you need than that, you might wonder.

A 130 mm reflector on a German-type equatorial mount with motor drive to track objects once you have aligned the mount and found the object yourself

However, the fact is that celestial bodies don’t move in this way, but more usually move through the sky at an angle unless you are at the Earth’s pole or equator. So an equatorial mount has one axis aligned parallel to the Earth’s axis, so it can counteract the movement by turning one axis only. This is extremely useful when observing, as it means you just have to move the telescope around the single axis to follow an object through the sky. It is well worth having a motor drive, which then does the work for you so that your chosen object remains in the field of view for as long as you want.

At least, that’s the theory. In practice, beginners often have trouble setting these mountings up, in which case they make the job of tracking objects even more difficult. There is a school of thought that says that beginners should be banned from buying equatorial mounts, but the fact is that very many telescopes are supplied on them.

Altaz mounts are, however, now equally widespread with motorised and computer-controlled mounts, known as Go To mounts. These have transformed observing, as once you have entered your location, time and date, and aligned the mount on two or three stars, the clever handset knows where all the other stars and planets are. Those elusive deep-sky objects that required ages to find them now present themselves in the field of view at the press of a button or two. Again, at least that’s the theory. 

Sky-Watcher 130 mm reflector on computer-controlled altazimuth mount using SkyAlign. Once aligned, the drive tracks objects automatically

The alignment system varies from manufacturer to manufacturer. Most require some user-input, even if it only knowing where north is. Don’t assume that everything is done for you. Probably the easiest system to use is Celestron’s SkyAlign, which requires you to simply drive the telescope to any three bright celestial objects. There are now actually some Go To mounts that contain GPS, and carry cameras that recognise the star patterns, so you just need to plonk them down and switch them on, then let them twirl around for a few minutes while they get their bearings. Once they have done that they are properly aligned on the sky, all being well, and you can dial up the objects for the night’s observing. But while this is very useful, there’s a danger that you will quickly get bored, as it removes the challenge of finding your way around the sky.  Yes, there’s a place for them, but it’s a bit like setting a satnav in a driverless car and letting it go to the address you tell it to. You will have no idea how you got there, or where you are when you get there. And of course you are totally dependent on battery power. There’s a lot to be said for an apprenticeship of learning the sky and knowing where everything is.

A Dobsonian telescope gives you the largest aperture for your money, though you have to find all objects by yourself and tracking them requires practice

At the other end of the scale is the Dobsonian mount (known as a Dob). This is a very simple altaz mount for reflectors, designed so as to be easy to construct, originally for home telescope makers. It remains the cheapest way to get a large aperture telescope. You can get a 250 mm Dob for about the same price as a 127 mm Maksutov on Go To mount, for example.

Each instrument has its strengths and weaknesses. The big Dob is ideal for use in the country for finding deep-sky objects under your own steam and will give good views of planets, but you have to keep on shifting the scope so detailed study of the planet is tricky. And you need a fair bit of space to store it and a it’s a bit of effort to lug around. The Mak, on the other hand, is ultra-portable, so you could take it on holiday on a plane without making too much of a dent in your baggage allowance, and will find objects by itself. It will still give good views of the planets, and while not as bright as the image in the Dob, they will stay put in the eyepiece for as long as you want while you study them.

Your telescope choice

So how do you make the decision about what to buy? The above will give you some ideas about where to start, and the pros and cons of each type. You’ll also have to think about how much you want to spend, and whether this is something that you’ll be using a lot or just from time to time when the fancy takes you.

Regarding cost, you can get perfectly reasonable telescopes for around £200, with simple but nevertheless usable telescopes costing even less. But if you want the computerised versions then you need to start at around £400. A lot of people these days want to be able to take photos through their telescopes, but do bear in mind that while snapshots of bright objects such as the Moon are quite easy with most telescopes, photos of nebulae, galaxies and other deep-sky objects are much more difficult. In particular, the drive systems that keep an object in view are not good enough for long exposure times. If you want to go down this route, be prepared for a significant outlay, of the order of £1000, for decent results.

We have a video explaining in greater detail the choice between refractors and reflectors.

Where to buy a telescope

There are many outlets selling telescopes, from camera shops to specialist suppliers and of course Internet suppliers. The trend is for even the specialist suppliers to operate online only, as showrooms where you can go and look at the goods before buying are costly to run. Some do still have showrooms, generally out in the country where space is cheaper than in towns. It may be that they have to charge more than the people who operate out of their garage and keep small stocks, so play fair and order from them if they have helped you by demonstrating their wares.

While your local camera shop or even superstore may have a few telescopes in stock, they don’t always know what they are selling and we’d recommend consulting a specialist supplier before buying. An alternative to going to a showroom is to visit an astronomy fair such as Astrofest (held in London in February) or the International Astronomy Show (usually in Warwickshire in the summer or autumn) where you’ll see a large range of goods from suppliers large and small, often at show discounts.

And of course there is eBay, where you can find a wide range of goods, often from suppliers with unfamiliar names. All we can say is, do your research among astronomy forums and see if they have a track record. Some suppliers have, sadly, gone bust with no warning so it’s hard to cover yourself completely.

Finally, don’t overlook the secondhand market. Telescopes don’t necessarily deteriorate too much, though mirrors may tarnish and screws get lost or even the threads stripped, but many secondhand scopes perform as well after 20 years as when they were new.  The SPA Forum has a classified section, and there is also www.astrobuysell.com/uk/ which often has bargains.