Magnification is the biggest mis-seller of telescopes ever. How many adverts have you seen that give unrealistic magnification figures for telescopes? Fortunately this practice is confined to the department store type of telescope. The more serious manufacturers and retailers rarely mention it.

Its worth remembering that the magnification of a telescope or binoculars is only one consideration and, in my opinion, by and far the least important factor. So beware!

Nevertheless, this is how magnification is obtained by a telescope. The thing to remember is that the telescope itself does not actually magnify. In general the telescope is designed to gather light and the eyepiece magnifies the image produced by the telescope. But certain parameters of the telescope will affect the magnification that the eyepiece can provide.

Basically the longer the focal length of the telescope, the higher the magnification a certain eyepiece will provide. For example, if your telescope has a focal length of 500mm and you use a 10mm focal length eyepiece, this set-up will provide 50 times magnification.

To work it out we need to go back to the maths lesson!

Magnification equals focal length of the telescope divided by the focal length of the eyepiece.

So our telescope has a 500mm focal length, our eyepiece is 10mm. 500 divided by 10 = 50! So the magnification for this arrangement is said to be 50 times.

If however, we have a longer focal length telescope, the magnification will be higher for the same eyepiece. The same formula still applies though.

The amount of magnification you may need will depend upon what celestial object you are interested in viewing. Basically if your interest is "deep sky objects", then a long focal length eyepiece, providing low magnification will provide the best results. However if you want to view the planets, then you will need a short focal length eyepiece to provide a high magnification.

For example, with our telescope of 500mm focal length we want to view Jupiter. We would need to use something like a 5mm focal length eyepiece, to obtain 100 times magnification. But if we wanted to view the Pleiades star cluster with our telescope, we would need to use something like a 20mm focal length eyepiece, to obtain only 25 times magnification.

So as a rule of thumb, high magnification for viewing the planets and low magnification for viewing deep sky objects such as star clusters and galaxies.