Stargazing guides often recommend that you get an expert to check a telescope before you buy it. This may be excellent advice but it can be difficult to do in practice.

If you are just starting out, you may not know anyone else interested in astronomy. And even if you do, the chances are that they know little more about telescopes than you do!

Then of course there’s the biggest problem of all – how do you get your expert to the telescope on a clear night before you’ve bought it?

So here is another way: a series of simple checks which you can carry out for yourself on a clear night, and which should at least tell you how good your instrument is, once you’ve bought it, plus where any faults lie.

No complex test equipment is needed, just plenty of time, including allowing the telescope to settle down to night-time temperatures. Don’t take it straight out of a warm room.

In the shop

Unfortunately, there is no quick way to test an astronomical telescope during the day in the shop. All you can do is to look through it, but there are so many other factors which can affect the image that this tells you little.

It may seem poor, yet be ideal for astronomy, or it may seem to give a bright, sharp image by day and dismal images at night. Only if you can see strong, false colour around the edges of objects silhouetted against a bright sky (such as TV aerials or chimney pots) can you be sure that the fault will be worse on astronomical objects. A small amount of false colour is inevitable anyway in all but the most expensive refracting telescopes.

A lunar check-up

The Moon is an excellent object to begin your checks with, especially when it is high up and bright. The bright lunar limb should appear perfectly sharp and free from false colour under low powers. If you can detect ghost images of the limb, or cannot get it sharp, then there is something definitely wrong.

Increased magnification will start to show up any faults, but you can easily be misled by atmospheric turbulence – seeing – which has the effect of blurring the image and sometimes even producing its own double images as air cells act as lenses in their own right.

If your instrument fails the Moon test, you will want to find out why. If it passes, then you will still want to test it further. So in the next guide, we will show you how to test on a bright star.