Points to consider when choosing a telescope:

•  What do you want to use it for and where?
•  How portable must it be?
•  How much can you afford?

What do you want to use it for and where?
Broadly speaking, there are two main areas of interest for telescopes: planets and deep sky. For planets you need something that will give good views at high magnification, while for deep-sky objects magnification is less important than large aperture. Many people don’t want to specialise, but want something general-purpose. Most telescopes will actually show you both types of object, but often the choice is determined not by their preferences but by their local conditions, which is why ‘where’ is so important.

If you live in a heavily light-polluted area, you can more or less forget most visual deep-sky observing. While there are many objects that you can observe, the views from the city are not usually very rewarding. There is little point in getting a telescope ideally suited to deep-sky work if all you can see are planets. Light pollution is no bar to observing the Moon and planets. Out in the country, this is less of a problem and you may choose what you want – though if your garden has a poor southern horizon because of trees, the planets may be hard to observe unless you have a portable instrument. You may be unlucky enough to live in a place where the seeing is always bad, for reasons of local geography, in which case you are more likely to want to observe deep-sky objects than the planets.

For planetary work that demands high magnifications, that is over about 150, it is important to have a telescope with a long focal length and good quality optics. Usually this means a comparatively long focal ratio – f/8 or longer. Refractors are often cited as being ideal for planetary work, but this is not necessarily true these days as many refractors are designed to be portable and have shorter focal ratios, around f/5 or f/6, which makes them hard to use at high power.

In my view, the main advantages of the cheaper short-focus refractors (meaning anything that does not use a special glass such as fluorite), are that they are fairly maintenance-free and do not need realigning and recoating from time to time. They are OK for occasional stargazing, or for wide-field photography, but not for high magnifications. You do get a brighter and more contrasty image from a 4-inch (100 mm) refractor than from a 4-inch reflector, but the optical from a good reflector should be similar if not better because it will be free from false colour.

The deep-sky observer prefers large aperture and, usually, fairly short focal length. These conditions are met by short-focus reflectors, and the bigger the better. But if you must have portability, a short-focus refractor will do the job but with a smaller aperture.

For photography and CCD imaging, a good mounting is essential. If you have a small, portable instrument do not expect it to be good for imaging. It may be possible, but it won’t be easy and the results will be mediocre at best.

How portable must it be?
These days there are many portable telescopes, the most obvious being the Meade ETX instruments, which pack away into a fairly small space and yet have a great deal of functionality. But the sad fact is that no amount of cleverness can make a small telescope perform as well as a large one. If you want to observe while on holiday in a dark-sky location once a year, and you buy a portable telescope just to fulfil this requirement, you will be spoiling your chances of getting good views for the rest of the year, and probably won’t see as much as you had hoped when you get to your dark-sky site. Views through small telescopes are not as good as views through larger ones, it’s as simple as that. And in any case, if all you can observe when you are in your dark-sky site is the planets, well, you might as well be in your back garden anyway.

In my view, the only reason for having a portable instrument is if you don't have room in your house for anything larger. If you live in a town and need to travel to get a dark sky, take something worthwhile. A 150 mm reflector, for example, doesn’t take up much room in the house or your car, and is many times more useful than a small refractor.

There is a lack of an instrument from a major manufacturer that offers largish aperture and light weight so that it will fit on a photographic tripod. Such instruments are possible – telescope wizard Horace Dall used to travel the world with a six-inch reflector in his coat pocket, for example – but not readily available.

If, on the other hand, you need something that you can store indoors or in a shed because you don’t have a permanent observatory, that’s another matter. You can put up with a bit of effort lugging a mounting outside in return for better results. But if you are no longer as young as you were, take into account the fact that many instruments require you to lift fairly heavy weights. The tubes of the popular 8-inch (200 mm) Schmidt-Cassegrain Telescopes (SCTs), for example, have to be lifted up onto their wedges before use. You must lift not just the tube but the fork mounting as well. You would be as well off with a conventional Newtonian in a PVC tube, maybe even of larger aperture, that doesn’t need to be lifted so far onto its mount.

I have taken an SCT abroad to Kenya, and it is a major undertaking. The tube and wedge fit in one large trunk, while the tripod has to go separately. It is transportable, but definitely not portable. But if you want an aperture that large when you are on holiday, the SCT is probably the best way to do it.

What about the range of SCTs and Maksutovs, such as the larger ETX and NexStar? These are designed to be portable and to be set up anywhere, but when you go to the larger and more useful sizes they are still not so portable that you can pack them in with the rest of your luggage. Their main advantage is not so much light weight as ease of use – they are simple to set up. This counts for a lot – it’s better to have a telescope that you like using and have fun with, than one that is so tricky to set up that you can’t be bothered, no matter how good its performance.

So in summary, consider what you mean by portable and how important it is to you. If you need real portability, be prepared to pay for it and put up with a fairly small aperture.

How much can you afford?
Cost is actually not always the most important factor. You can get excellent results with comparatively cheap telescopes, while you can spend a fairly large sum on something glamorous that does not perform as well. Usually, cost is a matter or priorities rather than pocket. We are all prepared to pay for something we really want and will get a lot of use from – our cars, for example. Someone who smokes even moderately will probably pay more for their habit each year than many amateur astronomers would consider spending on new equipment.

Often, the matter of cost is more important to parents buying telescopes for their children than by the more dedicated amateur. In this case there is one important guideline – don’t buy something small and cheap to get their interest whetted. Invariably, the small telescopes you see in toy departments, superstores, mail-order ads and even photo shops are not worth having and may even be so bad that they will put your offspring off astronomy for life. This sounds a bit damning, and there are a few good, cheap instruments to be had but they are few and far between, and not generally available from toy stores! Instead, aim for something that you can sell on if necessary, and buy with caution.

The terminally hard-up should consider kit telescopes, usually Dobsonians. If necessary, buy the optics only and make your own Dobsonian mounting. It is probably not worth trying to make the optics themselves, unless you would enjoy doing it, to save money. The savings, if any, are minimal these days. Or buy secondhand – there are many older telescopes around that still work well and are often given away because they don’t have motor drives and Go To.

The market
Here follows a list of typical commercially available instrument types, with an assessment of their pros and cons. This is based on UK availability, and reflects the world market. In the US there are numerous small manufacturers whose instruments I know nothing about, but you can find their ads in astronomy magazines such as Sky and Telescope.

Refractors
These telescopes with lenses at the top end (rather than reflectors which have mirrors at the bottom end) are most people’s idea of what a telescope should look like. Their advantages are simplicity, lack of maintenance as the lenses need only be kept dust-free and are usually mounted firmly in optical alignment so that adjustments are unnecessary. In the larger sizes, however, they are more expensive than reflectors.

Terrestrial telescopes are generally speaking not suitable for astronomy, though a good-quality instrument should show a bit of detail on the planets and give nice views of some deep-sky objects. They have apertures between 20 mm and 50 mm, and give upright views of the landscape, unlike astronomical telescopes, which invariably give upside-down views. You can pay a lot for some terrestrial telescopes, such as those for bird watchers and shooting , who use them to determine the accuracy of their hits. They are often called spotting scopes, and the more expensive models are generally of good optical quality and will give you some nice astronomical views, though inevitably with a rather small aperture.

Many people have been disappointed by small brass collapsible telescopes sold by mail order. These may not even meet their quoted specifications, and may contain very cheap optics in a brass tube that bumps up the cost.

There are some good Russian terrestrial telescopes that are worth having. The 20 x 50, for example, gives nice views and is excellent value, but the popular Turist-3 is no longer widely available in the UK, though as far as I can tell they are still in production. There is an alternative 20 x 50 named Yukon which I hoped would be a good replacement, but I am sorry to say that the sample I tried had a strongly curved focal plane. As a result, only objects within a small central area were in focus at any one time. This is bad enough for terrestrial use, but for astro use it would be intolerable. The smaller 10 x 30 telescope is still available, though it is too small for satisfactory astro use.

Binoculars are always a good starting point for amateur astronomers. They are specified as, for example, 7 x 50, where 7 is the magnification and 50 is the size of the main lenses in mm. The bigger the main lenses, the more light they collect – but the heavier they will be. The brightness of the image also depends on the magnification – the higher the magnification, the dimmer the view. So high magnification binoculars need really big lenses. See my reply to a question for more information on this, and of course I have written a book, Stargazing with Binoculars, in conjunction with David Frydman.

The smallest refractors commonly available these are 60 mm aperture. The cheapest are sold by toy stores and are misleadingly described because they have stopped-down lenses so they are incapable of giving good views at the excessive magnifications they boast. Always be wary of small refracting telescopes, even if they are intended for children. Some of the worst offenders even bear the name of a major and well-respected US organization and are sold by a major UK catalogue company.

This is the area where you have to be wary of what you buy. A reasonable 60 mm refractor will show plenty of craters on the Moon, will reveal the belts of Jupiter and the rings of Saturn with some clarity. The key thing to look for is an achromatic lens – that is, one which corrects (to some extent) for the false colour inherent in a simple lens. Many of the telescopes sold in toy stores and catalogue stores have simple lenses that give a very poor view, despite the detailed pictures shown on the box. They also claim that they have, say, 50 mm or 60 mm lenses when in fact there is a stop behind the lens that cuts the working aperture down to 20 mm or so. This would be like advertising a TV as having a 25-inch screen when in practice only the centre 10 inches gives a picture.

Even a cheap telescope with an achromatic lens is not guaranteed to give you a good view, but at least you will be in with a chance. Lenses (and mirrors) are not made on a production line all identical – each one is individual, so you can’t tell how good it will be without trying it. A good 60 mm refractor will show many deep-sky objects, but you need good dark skies to be able to see anything worthwhile.

Larger refractors
You can get 70 mm and larger refractors, in a variety of focal lengths. In general, the shorter the focal length the more tricky it is to make a good lens, so expect to pay a lot more for quality. Short-focus refractors are popular these days on account of their portability, but only the better ones perform well at high magnification. For planetary observing, get an f/10 instrument or similar, such as the one below, rather than a portable f/5 model.

Fluorite and ED lenses
To make a good lens that corrects out most of the false colour for which refractors are notorious, it’s necessary to use more expensive glass than normal. These materials may deteriorate more with time than the more conventional glasses, though with care they should last a long time. The use of these materials does not guarantee freedom from false colour, but the performance of the most expensive instruments is legendary. They are beyond my price bracket, so I can’t comment. In 2004, budget 80 mm ED refractors came onto the market. I own the 80 mm Sky-Watcher ED (also sold under other names) made by Synta, and am impressed by the optical quality. But even so, it isn't suitable for observing the planets.

Reflectors
Mirrors are free from false colour, but the secondary mirror necessary in a reflecting telescope can reduce the contrast of the image. This effect is often overplayed, and a good reflecting telescope, even one as small as 4 inches (100 mm), can be the equal of a refractor only slightly larger but at a fraction of the cost. Reflectors require more care, as the mirrors tarnish with time and the alignment is more tricky.

Small reflectors
Though there are 62 mm reflectors on the market, the smallest practical ones have 90 or 100 mm (3.5 or 4-inch) aperture. There is, however, a rather nice and cheap little 76 mm reflector, sold in the UK as the Infinity-76, that looks a little like a space shuttle. This has limited performance but gives excellent low-power views, and is a lot better value than the average 60 mm refractor though it won't show you closeups of the planets.

A 130 mm Celestron reflector on equatorial mount

A 100 or 114 mm reflector gives a significant improvement in image over a 60 or even a 75 mm refractor, and to my mind is the smallest telescope worth having for many purposes. You can begin to see enough detail on a planet such as Jupiter that you can readily detect changes, and the number of deep-sky objects visible is also increased noticeably over small instruments.  However, don't be tempted by cheap 114 mm reflectors that claim to deliver 1000 mm focal length with a short tube, such as the Sky-Watcher Skyhawk 114 and the Celestron Astromaster 114 Short. The extra lens that is inserted to give the long focal length degrades the optical quality so as to make them very little use, which spoils two otherwise excellent ranges of instruments.

These days, 130 mm reflectors are becoming very popular as a good compromise between cheapness and aperture. The Sky-Watcher Explorer 130 as it is known in the UK costs just £169 (as of 2012) though I prefer the motorised 130M at £199. The motor makes a world of difference to the usability of the mount.

Larger reflectors The bigger, the better, by and large. A 6-inch (150 mm) Newtonian will show a huge range of objects and will generally show masses of planetary detail, while remaining portable enough to move around easily. As you get to larger sizes of Newtonian, the light grasp and potential planetary performance improves at the expense of portability. But even a 10 inch (25 cm) Newtonian of, say, f/6 is not a particularly cumbersome instrument, though it will need a fairly substantial mounting.

A 150 mm Sky-Watcher Dobsonian

A 200 mm SCT

Dobsonians
The Dobsonian is a cheap means of mounting a Newtonian reflector. It offers a good entry-level instrument at a very low price, and is excellent for deep-sky work, though you need a model with smooth motions and good balance to observe planets easily. But forget the idea of long-exposure photography through a basic Dobsonian – they are not designed for it, though you might get away with basic photography of the planets.

Catadioptrics
These combine mirrors and lenses to give large aperture reflectors that do not have excessively long tubes. The classic type is the Schmidt-Cassegrain telescope, or SCT, but there is also the Maksutov design that tends to get expensive in the larger sizes. Rather more expensive than Newtonians of the same aperture, with poorer optical performance, but with the convenience of the short tube that allows them to be used on a fork mount rather than the more unwieldy German mount.

These instruments in the larger sizes such as 11 inch and 14 inch aperture are very widely used by planetary photographers. Image processing makes up for the slight loss of detail caused by the large secondary mirror. 

Basic Go To instruments are available for under £250 (the Sky-Watcher Explorer 130P SupaTrak Auto) and while the cheaper instruments will not have the Go To precision of more expensive models, they can be excellent beginners' instruments. The model quoted does require you to know one or two stars in the sky when setting up the Go To mount, however.

Best buy?
When people ask me what small telescope I would recommend for a beginner, I used to suggest a TAL-1 Russian 110 mm reflector. You can still get these for under £200 and they are a good buy in terms of what you can see for the money you pay. They are sturdy and give images that are good enough for most purposes – better than a 90 mm ETX (see below), for example. Their very sturdiness is a drawback for some people, because they are heavier than the equivalent Far Eastern instrument, but viewing is definitely easier if the telescope stays put when you touch it rather than shuddering with every breath of wind. For the same price, however, you can get a Sky-Watcher 130M, a motor-driven reflector. This is also excellent value, as the extra aperture is well worth having.

One other drawback with the TAL-1 is that it is less adaptable than Far Eastern instruments. So don’t expect to be able to buy the full range of add-on goodies, such as computer control.

My own view of the Go To craze is that it does not actually help the beginner as much as it should. The ETX Autostar software in particular is really badly designed and I wouldn’t recommend it to any beginner. And even when you do get it working, it is much better to learn for yourself where the objects are in the sky.

One thing which neither the ETX nor the NexStar manual gives due prominence to is that both telescopes should be accurately levelled before use, using a spirit level. It doesn't matter how accurately you point them north and with the tube dead level, if the base is not properly level they will not find their two reference stars accurately.

Click here to read some (genuine) questions answered (and also some more comments about ETX telescopes).

Click here for help in collimating (aligning) the optics of a reflecting telescope