How To Buy A Telescope
A Guide for the Novice Astronomer
Practically everyone with an interest in astronomy wants to own a telescope, preferably the biggest one they think they can afford! However, a big telescope can be expensive or difficult to use and a small cheap one can be virtually useless, despite the glossy brochures. Members of the Hampshire Astronomical Group are often asked which is the best telescope to buy? A glib reply would be ‘the one you’ll use the most’! This may seem self evident, but nowadays there is a huge range of telescopes to choose from, so where do you begin?
In order to help you choose, it is probably a good idea to understand a little about how telescopes work. There are basically two main types of telescope, the refractor and reflector. The refractor is the type that most people will be familiar with, having a lens at the front and an eyepiece at the other end (the traditional ‘spyglass’ or naval telescope). This, as its name suggests, refracts the light through the front lens and brings it to a focus at the eyepiece. The reflector on the other hand uses a mirror to gather and focus the light then reflects this back up the telescope tube to another mirror angled to focus the light at the eyepiece which is usually situated at the side of the telescope tube near the open end.
The important measurement in a telescope is not its magnification but its aperture, that is to say its ability to gather light. The reason; it is hard to see things in the night sky is not because they are small, but because they are faint! So that is why you need a good sized aperture. This aperture measurement is usually given in centimetres or inches. A small telescope may have an aperture of 50mm or 60mm (2-2.5 inches) if it is a refractor, or 102mm (4 inches) if it is a reflector. The smallest refractors should be able to show you the craters of the Moon and other surface details and also some of the cloud belts of Jupiter and the Rings of Saturn. You should also be able to see some double stars and some of the brighter nebulae and star clusters. Refractors of 3 inches and above should show you some fainter objects and a little more detail. This of course depends on their optical quality!
Unfortunately, there are some low budget telescopes on the market that are optically very poor and are sold purely on their ability to magnify alone. If you buy one of these, you are sure to be disappointed! If in doubt, take advice from someone you know to be an experienced astronomer or of course, a reputable telescope dealer – many high street shops selling telescopes are generalists, and may not be able to provide the detailed advice you need, whereas telescope dealers will give you all the advice you need.
Before buying a small refractor, make a few checks if you can. Look down the main lens (the wrong end) and see if you can see a disc with a small hole in it a short way down the tube. If there is, it is probably an ‘aperture stop’ put in to try and reduce ‘colour fringing’ and to sharpen the image. This will also effectively reduce the aperture and therefore the image brightness. It will render the telescope virtually useless for astronomy. However, theses stops should not be confused with ‘light baffles’ put in to inhibit internal reflections and improve contrast. If you can, also take the telescope outside and observe a bright object with it (preferably the Moon at night) and see if the image is sharp and without any obvious or obtrusive colours around the edges. If it has, don’t buy it. However, you should remember that a small (un-obtrusive) amount of colour fringing can be seen even on good quality telescopes; it is a question of degree. I suppose it can be best described with the old saying ‘you get what you pay for.’
Magnification is dependent entirely on the eyepiece used. Small refractors and reflectors are frequently supplied giving low, medium and high powers. These can be of variable quality, but in our experience usually passable. Always try to use the lowest power possible to observe an object clearly. High powers tend to magnify any disturbances on our atmosphere and the apparent speed of the rotation of our Earth and also reduce image brightness. A good rule of thumb is not to exceed 50x magnification for every inch of aperture. Seldom is it worth using magnifications above 300x on any telescope, particularly due to the distorting effects of our atmosphere. Beyond this you tend only to see a useless ‘boiling’ image.
How your telescope is mounted is also very important. It is no good having a decent telescope on a wobbly mount as the image will ‘dance’ crazily in the eyepiece and will be un-viewable! The mount is very much half the system, so try not to ‘skimp’ with this and try to avoid telescopes that already supply inadequate mounts. Again it is worth taking advice on this, as you don’t want a telescope that you can’t move because the mount is too heavy unless you intend to permanently install it in some kind of observatory.
The simplest type of mount has the longest name, that being ‘alt-azimuth’. This is really just a description of how the mount works – it allows movement in altitude (up and down) and azimuth (round and round or side to side). Camera tripods and simple telescope mounts are nearly always ‘alt-azimuth’. The other type of mount is ‘equatorial’. This ‘tips’ one of the axes to be parallel with the Earth’s axis so that you only need to guide the telescope around this axis to keep up with the movement of the stars, or more correctly, the Earth’s rotation.
There are quite a few variations on the equatorial mount theme, but they all answer the same basic requirement of allowing the telescope to track around this ‘polar’ axis and so can keep track with the stars. So which type of mount you go for really does depend on how you think you are going to use the telescope. If you want a simple and easy to set up ‘point and look’ scope, then an alt-azimuth mounted one is certainly the best option. If however, you intend to have a go at astro-photography, or want to do extended study of objects or use the telescope in group viewing situations, then an equatorial mount is a virtual necessity. This is also because it is relatively simple to add a motor drive to an equatorial mount, which will drive the telescope at the same speed that the stars are apparently moving across the sky. So when the next person steps up to the eyepiece the object you were looking at should still be there!
Technology progresses of course, so there is a caveat to the last statement. With modern computer controlled telescopes, it is really no longer necessary to have an equatorial mount to keep up with the sky’s rotation, unless of course you intend to carry out long exposure astro-photography. A computer controlled telescope will allow the mount to keep track of objects even in alt-azimuth set up, as it constantly recalculates where the telescope should be pointing and moves the mount continually to achieve this. You also get the advantage of being able to ‘dial in’ an object and have the telescope slew to it at high speed acquire it and track it. This technology comes at a price however, so you need to ask yourself what sacrifices you are making with regards to aperture for a given price. The prices though, seem to be falling all the time!
So there you have it, a few we hope, useful pointers as to buy your first or maybe next telescope. This does not of course replace good advice from astronomer friends or reputable dealers and your own research.
Whatever telescope you decide to go for we wish you clear skies.