Bright star test
Find a good bright star reasonably high up. A first magnitude star is ideal, particularly a white one such as Vega. Train your telescope on it and focus it as carefully as you can: you should be able to get it to a single very tiny disc of light (assuming that you haven’t chosen a double star!). Use a moderate magnification, of the same order as the aperture in millimetres (that is, 150 for a 15 cm – 6 inch – instrument).
You may have read that a good telescope will show what are called diffraction rings around star images. This is not always the case: it depends on the aperture and the magnification as well as the seeing. A small telescope (say 50mm) will show these delicate rings surrounding star images quite readily if it is any good. However, a larger instrument may not show them unless you see a fairly high magnification and look at a fairly faint star. But if the seeing is bad all you will see is an animated blur, even if the telescope is perfect. So diffraction rings are not a good guide to performance.
Once you have found the best focus point, move the eyepiece slightly outwards and inwards, equal amounts on either side of the focus point, and compare the defocused star images. What should happen is that the point of light opens out to a disc, getting larger and fainter as you go further from the focus point. This disc should be evenly illuminated and perfectly circular. In the case of a reflector which has a secondary mirror, you will see the image of this as a shadow in the centre of the disc.
The defocused disc should be identical on either side of the focus point. If it isn’t there is something wrong. You may find, for instance, that on one side of the focus the disc has a bright edge, which indicates a fault in the mirror or lens. Small deviations from the ideal should not affect the images too much, however.
A very common fault which the bright star test will reveal is astigmatism – where the optics have a slightly different focal length in one direction. You will find in this case that the star image is never perfectly round, and that as you move very slightly inside focus it elongates into a short line. The other side of the focus, it turns into a line at right angles to this one instead. If you detect this fault, it’s worth checking that the eyepiece isn’t the cause. Rotate the eyepiece but not the telescope: if the short line moves as you turn the eyepiece, then it is at fault. Your eyes, too, can produce the same effect and you may have to try rotating your head to check this (it’s not easy!). But if the effect is not in the eyepiece or your eye, then the problem lies with the rest of the optics.
Incidentally, as you move outside focus you are actually focusing on comparatively nearby points in space – the atmosphere a few hundred metres away from you. You may well see streams of turbulence against the defocused disc, indicating that this may be the cause of the bad seeing.
The next test will use the same bright star, but is much more difficult to carry out.