Simple sky-watching with the unaided eye is great fun, but there comes a time when everyone wants to look through a telescope – so how to choose a telescope? Saturn is a brilliant yellow ‘star’ with the eye alone but even a small telescope will reveal it for the ringed wonder it really is.
If there is an astronomy club in your locality then joining them on an observing night might be enough to satisfy your telescope curiosity. But once hooked on the sky, the chances are you will want a telescope of your own.
If you are careful in making your choice, there has never been a better time to buy one – they are cheaper than ever in real terms. Some, including many from the major players in the market, Celestron and Meade, have computerised mounts meaning that in theory they will find your targets for you. You can find ind some independent telescope reviews at the Cloudy Nights site.
You can also find a bargain telescope or accessories such as eyepieces on eBay though you should remember the adage “buyer beware” and research the model you see advertised and take appropriate precautions before handing over your money.
Don’t forget that binoculars are a great value observational tool too – essentially they are a pair of low-power telescopes.
The array of telescopes on offer may seem bewildering but they boil down to two basic types – the refractor and the relector, plus so-called catadioptrics which marry the two forms.
The refractor – or refracting telescope – uses a lens to collect light from a star, planet or whatever you happen to be looking at. The reflector – or reflecting telescope – collects the light with a curved mirror.
The bigger the lens or the mirror, the more light is collected from your target, and the fainter the objects you can see. You can read more about the different types of telescope in our article here.
The light that is collected by the lens or the mirror is directed to a smaller lens (usually a group of lenses) called an eyepiece which magnifies the image and is adjusted to bring the object you are observing into focus. The path through a refractor is shown in the cutaway diagram above and through a reflector in the cutaway below.
On astronomical telescopes you will usually have a selection of interchangeable eyepieces so that you can view objects at different magnifications.
The distance from the lens or mirror to the point where the image comes to focus is called its focal length. Sometimes you will hear mentioned the telescope’s focal ratio or f-ratio – this is the focal length divided by the diameter of the main lens or mirror.
An f-ratio of 4 or 5 is preferred by amateurs observing deep sky objects such as galaxies an nebulae as the observing field is wide and bright. Such telescopes are often known as Rich Field Telescopes.
Planetary observers prefer long focal lengths, with corresponding narrower fields of view, because this allows them to see more detail on their Solar System targets.
When choosing a telescope, bear in mind your personal circumstances. Apartment-dwellers might go for something light and portable such as a small refractor or Meade’s ETX range. If you have a big garden and can leave your telescope outside permanently, you could go for a large reflecting telescope.
For more extensive, excellent advice, which can help you learn more on how to choose a telescope, read this article by Robin Scagell, a leading amateur astronomer in the UK and vice-president of the Society for Popular Astronomy. He has written a classic book, Stargazing With A Telescope, that is a mine of information and comes highly recommended.