Choosing a telescope – a beginner’s guide


Perhaps you have always promised yourself a telescope for a bit of stargazing, and now that Mars is close this is your opportunity to do something about it.

The good news is that there are now many telescopes of good quality readily available. If you stick to specialist UK suppliers rather than taking a chance on something off the internet, you shouldn't go too far wrong nor pay over the odds.

The only downside is that there are so many different models and systems that choosing the best for you can take a lot of research. So this article aims to do some of that work for you.

Your expectations

Let's get a couple of things out of the way to start with. If you have a vision of taking top-quality photos of nebulae and galaxies through your telescope, accept that you are embarking on a steep and costly learning curve so don't expect that you can get much out of a cheap telescope. And don't expect your views through even a fairly large telescope to be like the photographs. Our eyes are no match for modern cameras, which can bring out colours and details by building up the image over a long period. This is covered in a separate article.

Image of Mars
Even a small telescope will show detail on Mars at its closest

But don't be put off – even budget telescopes can give you good views of the Moon and planets if you use them carefully. You'll see the reddish colour of Mars, make out some of the dark markings that people once thought were vegetation, and maybe even glimpse its white polar cap and bluish clouds. You'll see the rings of yellowish Saturn and the ochre-coloured belts of Jupiter. The Moon becomes a world of craters and mountains and amazing detail.

You can see wreathes of gas in the Orion Nebula, pick out glittering stars in the Seven Sisters star cluster and witness with your own eyes the ancient light from the Andromeda Galaxy as a misty blur. You won't see colour in any of these objects, but the thrill comes from tracking them down and appreciating that you are viewing them for yourself.



The costs

Telescopes of reasonable quality begin at under £100. You don't need anything very large to show you some details on the Moon and planets, but telescopes at this end of the market are usually on rather flimsy mountings that make it hard to find the object in the field of view and track it smoothly as it moves through the sky as a result of the Earth’s rotation. You might be surprised by how hard it is to find even the Moon through a small telescope, how much the telescope shudders when you just try to focus, and how tricky it is to keep it in the field of view as you change magnifications. The cheaper instruments put demands on your patience and manual skill.

Photograph of lunar crater Petavius
The lunar crater Petavius photographed with a webcam through a budget 70 mm refractor

So while a basic scope is OK for a quick look, and can live in its box under the bed for most of the time, if you want a more satisfying view you should aim to spend over £200. This will get you a larger and somewhat more versatile instrument, although still limited in its capabilities.

In the price bracket you can now get instruments to which you can attach a suitable recent smartphone to help you find objects in the sky. The phone's camera first takes a photo of the sky, then tells you which way to push the instrument to find a wide range of objects in its database - always a problem for beginners.

But for a true ‘GoTo’ instrument with motors that will find objects for itself you need to spend around £300 or more. This will get you a motorised telescope that will both find and track any object in its large database. From there on upwards you can pick your price bracket and get more and more useful instruments, and start to get into the region of serious amateur astronomy. The larger the telescope, the more solid the mounting and the better its drives and computer control, the more you can do with it. Versatile instruments start at around £1500-£2000.

Image of Celestron's StarSense Explorer Telescope
Celestron’s StarSense Explorer system uses your smartphone to tell you which way to turn your telescope to find any object in its large database

Understand the jargon

First, a few terms that you may need to get to grips with, and their implications for your choice.
Refracting telescopes (or refractors) are the classic telescopes, with a lens at the top end and the eyepiece down at the bottom. Good beginners’ telescopes, but they do suffer from false colour fringing around objects unless you pay a lot for special lenses using ED glass or apochromat designs. Usually limited to the smaller sizes, with apertures (see below) under about 120 mm (4½ inches).

An image of the Sky-Watcher Mercury 707 telescope
A basic 70 mm refractor costing under £100, the Sky-Watcher Mercury 707

Reflecting telescopes (or reflectors) use a mirror system instead of a lens, and you need to view the image at the top of the tube, sideways on to the object. The mirrors overcome the false colour problem, but can lose their reflectiveness over time. However, almost all telescopes over about 150 mm (6 inches) aperture are reflectors of one sort or another. The basic reflectors are of the Newtonian design.

Basic Newtonian-type reflecting telescope
This very basic Newtonian-type reflecting telescope has 100 mm aperture. A mirror at the bottom of the tube reflects light to the top, where it is reflected sideways to the eyepiece by a second mirror. This model suitable for low-magnification views


Catadioptric telescopes combine mirrors and lenses so as to give a compact telescope, through which you view from the bottom as with a refractor. There are two basic types – Schmidt-Cassegrain telescopes (SCTs) and Maksutov-Cassegrains, usually called Maksutovs or Maks. Better suited to viewing planets and other objects requiring high magnifications; not as suitable for wide-field views of extended objects such as nebulae.


Image of a Maksutov-type catadioptric of 100 mm aperture on a GoTo motorised mounting
A Maksutov-type catadioptric of 100 mm aperture on a GoTo motorised mounting, able to give high-magnification views of a wide range of objects

Aperture is the diameter of the telescope’s lens or mirror. The smallest useful aperture for astronomical use is about 70 mm, and many amateur astronomers have telescopes with apertures in the range 200-300 mm (8-12 inches)
Focal length is basically the length of the telescope, although in catadioptrics the tube length is reduced by folding the light path. Photographers will be familiar with this – a telephoto setting might be up to 200 mm, whereas telescopes are 500 mm focal length upwards.
Focal ratio is also familiar to photographers, and refers to the image brightness and scale (it’s actually the focal length divided by the aperture). A typical f/6 telescope can give a fairly wide field of view and is good for use in dark skies, while the f/10 of refractors or f/14 of catadioptric telescopes are more suited to the planets and for viewing in cities, but this is not a hard and fast rule.

An image comparing two different telescopes. The telescope on the left has a shorter focal length than the one on the rights.
Short and long focal ratios compared. The f/5 telescope at left is very compact and is good for wide-field views but will not give quite as good high-magnification images of the Moon and planets as the f/10 instrument at right

Altazimuth mounts have simple side-to-side and up-and-down movements. This usually means adjusting two controls to follow an object through the sky. This is not a major problem for the motorised tracking mounts, though it has its limitations for long-exposure photography. 
Equatorial mounts need to be aligned with the Earth’s axis so as to allow tracking using one motion only. Most beginners find them tricky to set up, but they do have their advantages. If you just want to do occasional casual viewing you may find them more trouble than they are worth. 

Image of a 130 mm Newtonian reflector on an equatorial mount
A 130 mm Newtonian reflector on an equatorial mount. The bottom axis first needs to be aligned on Polaris, then
objects can be tracked as they move through the sky using the other axis only. Requires a heavy counterweight which adds to the weight of the system
OTA means optical tube assembly: just the telescope, not the mounting. The more advanced instruments are available as tube only with a dovetail fitting and you can put them on different mountings to suit your needs.

Some other important basics

  • Most astronomical telescopes give an upside-down or even a mirror image, because that is the simplest optical system and it doesn't really matter which way up you view the objects. So they are not suited to terrestrial viewing unless you have an adapter of some sort, and Newtonian reflecting telescopes are particularly tricky to use by day.
  • You vary the magnification by using different eyepieces. The eyepiece with the longest focal length (usually around 25 mm) has the lowest magnification, the exact value of which depends on the focal length of the main telescope. Most telescopes come with two eyepieces and maybe a Barlow lens, which typically doubles the magnification of any eyepiece by inserting it between the telescope and the eyepece. But the Barlow lenses of cheap telescopes are rubbish and are only put in to imply impossibly high magnifications.

Your choice

There are several factors that will influence your choice:

  • Your level of interest in astronomy and what you want to get out of it
  • How versatile you want it to be - just for visual observing, or for full-blown photography
  • How good your local skies are
  • How much room you have
  • Whether you want to take it around with you - such as on holiday
  • The cost

So here are a few options with their pros and cons.

A small refractor on an altazimuth mounting
The smallest refractors you can routinely get are around 70 mm aperture these days. The cheapest camera-shop and catalogue shop refractors are best avoided as they are made down to a budget. Short-focal-ratio f/5 instruments are compact but will not perform well at the high magnifications (100 or more) which you really want for seeing detail on planets, although they are good for taking on holiday to get nice wide-field views of deep-sky objects. For viewing Mars, get a longer-focal-ratio-instrument such as f/10. Cost: £100 upwards.

A small reflector on equatorial mounting
Sizes start at 114 mm but 130 mm is preferable. These can give nice sharp views of planets and deep-sky objects but the equatorial mountings often mean trouble for beginners - a problem, because they are usually sold on these.  If it includes a motor drive and you can be bothered to set it up properly, however, it can be a worthwhile instrument at comparatively low cost, which may even be adaptable for some forms of photography.
Avoid small Newtonian reflectors with short tubes but long focal lengths (around 1000 mm). These are achieved by putting an extra lens into the focuser which reduces the optical quality. Cost: £140 upwards.

A Maksutov on driven altazimuth mounting
Here, small means 90 mm upwards to about 130 mm. The smaller units are generally Maksutovs at about f/14, which makes them very portable and provides high magnification, so they are good for travelling and for viewing the Moon and planets as well as the smaller deep-sky objects such as galaxies and planetary nebulae. Motorised telescopes can be run off small battery packs, or larger power tanks or car batteries. Cost: £400 upwards.

Larger reflector on Dobsonian mounting
There is a very basic type of altaz mount known as a Dobsonian mount, and Newtonian reflectors mounted in this way are called Dobsonians. Although they start at 130 mm aperture, they are the cheapest means of getting a large telescope for your money, and 200 mm to 300 mm Dobs are popular with committed amateurs who want to observe visually and don't mind their cumbersome nature. Not really suited to imaging. Cost: £150 upwards.

Image of a Dobsonian telescope
This 200 mm no-frills Dobsonian costs about the same as a 70 mm refractor on GoTo mount but is very much more powerful if you are prepared to learn the sky and find objects for yourself.

A Schmidt-Cassegrain on driven altazimuth mount
Catadioptrics larger than 130 mm are usually Schmidt-Cassegrains at about f/10. These are still good for a wide range of observing but you are now into more heavyweight instruments and heavy mountings and tripods. The ubiquitous amateur instrument is a 200 mm (8-inch) SCT, invariably motorised with GoTo. Cost: £550 upwards.

An image of the Celestron CPC-800 telescope
The Celestron CPC-800 is a Schmidt-Cassegrain telescope on GoTo altazimuth mounting that can be converted to anequatorial for long-exposure imaging. Cost over £2000.

Where to buy

There are specialist astronomical suppliers around the country, but very few of these have showroom facilities. Those that do have showrooms usually have to charge somewhat more for their instruments, so if you want to see before you buy, play fair and buy from them. Many others can still offer expert advice over the phone. If telescopes are just a small part of their range, you may not get as knowledgable advice as from a specialist.

Look at the astronomical magazines and get to know the established brand names, but bear in mind that at the budget end of the market even some top manufacturers who make highly regarded equipment for discerning observers still sell some pretty poor stuff.

If you buy something from an unknown name off the internet you are on your own!

Last Updated: 2020-11-08 15:44:50