Gnomon's Guide

Astronomy, the study of the stars, planets and other objects in the night sky, is a science that can be enjoyed by amateurs with a minimum of equipment. Anyone with a clear view of the night sky can join in. There's quite a lot to see in the heavens with the naked eye or binoculars, but your enjoyment can be increased by getting an astronomical telescope. One of the puzzling features of such telescopes is that they come with a range of different eyepieces - it can be quite tricky picking the right eyepiece or deciding whether you will benefit from buying another one. This entry attempts to make all this a bit clearer.

An astronomical telescope can be thought of as a tube with a big lens at one end and a small lens at the other. In fact, the big lens may be a curved mirror rather than a lens, and the lenses may have more than one piece of glass in them, but this can be ignored for most purposes.

• The big lens is called the objective, and its purpose is to gather light. It has two important numbers associated with it: the diameter, or 'aperture', and the focal length. Both of these are normally given in millimetres, although sometimes the aperture is quoted in inches¹ (for example, an 8-inch refractor telescope). The bigger the aperture, the more light the telescope can collect and the better an image it can produce, and as a result the more expensive it will be. The focal length is a measure of how much the lens bends the light that goes through it - the shorter the focal
  length, the more the lens bends the light. It is used in calculating the magnification, so you should know the focal length of your objective. It may be displayed on the side of the telescope or you may have to delve into the manuals to find it.

• The small lens is called the eyepiece, because it's the bit you look into. Its purpose is to convert the light coming from the objective into an image, and to magnify it. If you want to change the magnification, you remove the eyepiece and insert a different one. Telescopes normally come with a few eyepieces, but you will probably want to buy a few more so that you get the most out of your telescope. This can be confusing, because there are lots of different sizes, and there are different prices too.

When choosing an eyepiece, bear the following in mind:

1. The eyepiece has a metal tube at one end, which slots into a slightly larger metal tube in the telescope; the eyepiece is normally held in place by finger-tightened screws. In most telescopes this tube is 1¼ inches (32mm) in diameter, but some more expensive ones take a 2-inch (51mm) tube. This is important: there's no point in even considering eyepieces that are the wrong size for your telescope. Some really expensive eyepieces can be used with either, being provided with interchangeable tubes. In general, however, if your telescope only takes 1¼" eyepieces, then those are the ones you should look at.

2. Other than the tube diameter, just about any eyepiece will fit any telescope - you don't have to confine yourself to eyepieces made by the same manufacturer - good makers are Meade and Celestron, but there are plenty of other eyepiece makers.

3. Each eyepiece has a number of specifications, but by far the most important one is its focal length. Eyepiece focal lengths vary from about 3mm up to about 40mm. In general, the smaller the focal length of the eyepiece, the higher the magnification. The magnification of a telescope is given by the focal length of the objective divided by the focal length of the eyepiece.

   M = F(objective) / F(eyepiece)

   So for a typical objective with a focal length of 600mm, an eyepiece of 40mm will provide a magnification of 15 (normally written 15×) while an eyepiece of focal length 3mm will give a magnification of 200×.

   You might think that the purpose of a telescope is to make distant things look close and that you should go for as high a magnification as possible. There are two reasons not to. First, the amount of light gathered by the objective is fixed, so if you magnify the image, you lose brightness and clarity. High magnification images tend to be dim and hard to see. Secondly, there are many sights in the night sky which are quite big - but we can't see them because they are very faint. Open star clusters such as the Pleiades are a good example. What you want here is not high magnification but lots of light, so low magnifications where the image is brightest are best.

   As a result, it's a good idea to have a range of eyepieces that give you different magnifications from 15× upwards to the highest magnification possible on your telescope.

4. The highest magnification practical on your telescope can be calculated by multiplying the aperture of the objective in millimetres by 2. So a 100mm aperture will allow a practical magnification of up to about 200× with the appropriate choice of eyepiece. Trying to boost the image any more than this will result in a dim, blurred image. Telescope manufacturers say that you can push the magnification as far as 2.4 times the aperture in millimetres, which would give a maximum magnification of 240× for the same 100mm telescope. This is the equivalent of multiplying the aperture in inches by 60. Most amateur astronomy sites agree that this is being optimistic - the viewing conditions would have to be very good for a clear view at this magnification.

5. If you work out that the maximum magnification for your telescope is 200×, then you probably want a selection of eyepieces which will give you magnifications over the range from about 15× to 200×.

   For example, you could have four eyepieces with magnifications of 15x, 35x, 85x and 200x. These cover the range nicely and each one is 2.4 times the magnification of the previous one. For a telescope with an objective focal length of 600mm, these would correspond to eyepieces of 40mm, 17mm, 7mm and 3mm.

Other Numbers

Field of view - the apparent field of view is the size of the image when you look into an eyepiece. It ranges from about 30° to about 80° depending on the design of the eyepiece. A small field of view will mean that you see a small circular image surrounded by blackness. You might not see very many stars in it. With a large field of view, you'll see a much bigger image, so you'll be able to see more stars at the same time. Most lenses have an adequate field of view, so don't worry about it too much unless you read in reviews that it is too small.

Eye Relief - this is the distance between the front of the lens and your eye at the ideal viewing distance. If it is too small, your eyelashes will hit against the lens. If it's too big you may not be able to find the viewing point easily. Again, this is not likely to be an issue unless you read in reviews that there is a problem with a particular lens.

A Few Oddities

The Barlow lens is not an eyepiece. You slot it into the telescope and put a standard eyepiece into it. This has the effect of doubling the magnification while reducing the available light. This can be useful.

Zoom eyepieces have an adjustable focal length, so they can be used like the zoom in a camera. While this gives more flexibility, the extra glass needed to achieve this will reduce the light and therefore the clarity of the image slightly.

Moon filter - if you look at the Moon through a telescope you will be dazzled by the brightness of it. You won't be able to see any detail because your eye will be trying to close itself. You need a moon filter - a piece of grey glass that you attach to your eyepiece. It's like wearing moonglasses.