Choosing a telescope – a beginner’s guide
It is said that all amateur astronomers want a telescope – and the bigger the better. In my experience that isn't always true. Meteor observers prefer the naked eye and Comet hunters prefer large binoculars. A large scope is difficult to handle and may get rarely used. A smaller scope may not catch as much light, but one will see 'more' as it will be used more often.
|A better truism is, "the best telescope is the one that
will be used".
For most a better starting instrument are binoculars, often cheaper and better value than small telescopes. With these one will see many interesting astronomical objects as well as finding regular daytime use.
To go with the binoculars, one should gather together
Philip's Planisphere circa 1910
However, most of us want to see details on the planets, closeups of the Moon and galaxies galore. For that one will need a telescope. The good news is that if you choose wisely, you can now get an excellent telescope for a reasonable sum. One can get a scope that will perform well for around £200, and a computer-controlled one that will find objects for you for under £300.
Ask 10 astronomers, 'which is the best telescope?' will gather 10 different answers, none of which is wrong. Many seasoned astronomers will have more than one, indicating that there is no simple answer.
There are two main types of telescope:
|Refractors (the traditional/classic type) which use lenses to collect and focus light; and|
|Reflectors, which collect light with a mirror.|
|There are also telescopes combining the two principles, known as catadioptrics, combining the two principles to create a more compact instrument.|
To choose the right telescope, one needs more information.
Telescopes are compared by their aperture — the diameter of the main lens or mirror. When astronomers refer to a ‘small’ telescope they mean one with a small aperture. The aperture dictates how much light the telescope collects – and the more light it collects, the more one can see.
Therefore to see more one buys the telescope with the largest aperture one can afford.
Well no! The truism above makes plain that there are other considerations , such as portability.
The smallest refractors, of 50 to 70 mm (2 to 3 inches) aperture, will show the Moon’s craters, Saturn’s rings, Jupiter’s cloud belts and its four main satellites, some attractive double stars, and the brighter nebulae and galaxies.
The views will not be bright, but with care and in good, dark skies there’s a lot to see.
However its probably best to start with something with more light gathering power.
Beware!. There are some small refractors which are not all they appear. They may have a 50 mm lens but at the front end there is a disc with a hole behind it which cuts the aperture down to around 25 mm. This allows the manufacturer to use a simple lens rather than the more expensive achromatic variety. The view is very dim, and of very little use. From the outside, they look impressive, but in practice they are so bad that most people give up on them straight away.
They are often sold in toy shops or on the Internet, and even in some prestigious department stores, not necessarily very cheaply.
Incidentally, many refractors show some false colour unless you go for the much more expensive variety with what are called ED or apochromatic lenses. But reflectors and catadioptrics are usually free from it.
Reflecting telescopes are generally made in larger sizes. A typical small reflector is 100 – 130 mm aperture, (a refractor of this aperture would cost considerably more).
With catadioptric telescopes you view in the same direction as the object, as with refractors. They generally cost more than a refractor or reflector of the same aperture, as you are paying for a complex optical design to make the instrument more compact.
Again beware of excessive Magnification claims.
My first lesson on reading books by Patrick Moore on telescopes was
'The maximum useful magnification of a telescope is twice the aperture in millimetres (or 50 times for each inch of aperture). '
Those with large telescopes rarely use magnifications much more than about x250 because our own shimmering atmosphere (‘bad seeing’, when the image is fuzzy and indistinct). And low magnifications are every bit as useful as high ones. When finding an object, start wide then increases the power if circumstances warrant it. Many diffuse objects can only be appreciated at lower magnification. One of the commonest eyepieces I used on an 80mm 480mm focal length refractor was a 30mm eyepiece that's just 16x.
Focal length and the f-number
This bit is important.
The focal length is the distance between the lens and the eyepiece. A telescope with a focal length of 900 mm would be just under a metre long.
The f-number is the focal length divided by the aperture and tells you whether the telescope is short and fat or long and thin. A telescope with a small f-number (such as f/6) is short and fat and gives bright images of low magnification, while one with a large f-number (such as f/12) is long and thin and will give dimmer images at higher magnification, assuming that the same aperture.
Catadioptric telescopes are different, as their optical systems provide a long focal length and high f-number in a short tube, which makes them more portable for their size than would otherwise be the case.
|In many ways this can be more important
than the scope and way more expensive. To use a telescope one needs a
sturdy, vibration free mount. If not, one will easily tire of using the
scope. Camera tripods are invariably unsuitable as vibration makes even
moderate magnification unusable.
The simplest type of mounting is the altazimuth design, which allows one to move the scope up and down and from side to side.
Then there is the equatorial mount, which needs more care and time to set up with the polar axis pointing to the north celestial pole, near Polaris. An equatorial mount has the advantage that objects can be kept within the field of view as the Earth rotates by turning the telescope around the poleward-pointing axis only.
Before buying, I believe it is essential to try both types of mount. A sturdy altaz mount is often easier to use than a cheap equatorial.
One example is the Dobsonian mount, (right) used for reflectors of all sizes. These can allow one to move the telescope just by pushing it. There are motorised auto-tracking mounts which, like equatorials, do need to be aligned on the sky before they will work, so you really need to know the names of the brighter stars at least. And then there are Go To mounts, available as both altazimuth and equatorials, which will find any object in their databases once they are aligned on the sky. Some, such as Celestron’s Sky-Align system, don’t require you to know any of the names of the stars or even where north is.
Which one for you?
In recent years I have seen many recommendations that a first scope should be a Dobsonian. They are cheaper, don't require an expensive mount, are often more comfortable to use, come in a variety of apertures and focal lengths and use the same eyepieces that one will use with the next scope.
I am inclined to agree and would probably start with one if I was beginning again.
Downsides: moving it into position, collimation (optical alignment of the mirrors), lack of portability for larger ones. Mounted on a firm trolley on good quality castors such a scope can be wheeled out of the garage or store and be ready immediately.
Townies living with light pollution, are better off with a larger f-number enabling one to push up the magnification and get detailed views of the planets which are not affected by city lights. It will also give good views of many deep-sky objects such as nebulae, star clusters and galaxies.
Out in the country, a shorter f-number will give one bright, wide-field views of nebulae and galaxies, but may struggle to give high magnification views of planets unless it is of excellent quality.
Reflector, refractor or catadioptric?
A small telescope for occasional astronomy and also for terrestial (needs erecting prism) use should be a refractor. It’s maintenance-free and straightforward to use. An altazimuth mount is all one needs.
To see fainter objects, a larger refractor or a reflector is probably better.
You get more aperture (light) for your money with a reflector.
Go To telescopes sound great but one won't learn much about the sky.
If going for photography, don't until you have some experience. You will need a good scope and an even better equatorial mount and plenty of cash.
Where to buy
Don't buy from a catalogue shop, a supermarket,or high street store who don't specialise in scopes. They will generally be poor and the quickest way to dissappointment.
Use a specialist store (see adverts in monthly astronmy magazines). The specialists all have a presence online.
Join a club and ask for recommendations. Attend a star party and ask for a look! Most astronomers are only too willing to show off their kit. They are often proudest of the cheap bit that does a better than expected job.
Original text by Steve Tidey of the Association for Astronomy Education, revised by Robin Scagell, 2014.
Further revised by Alan Banks 2015
© National Astronomy Week and AAE. This information may be copied freely for non-commercial purposes.