Checking Your Telescope (Star Testing)

There are a two simple tests OZScopes recommend you perform on your newly bought telescopes to test its quality.

1) Tap the tube lightly while looking through the telescope. The image shown should take no more than three to five seconds to stop shaking. To improve its stability and damp down vibrations, some people may hang a weight from the centre of the tripod.

2) Star testing is the most useful quality assurance test for optical systems. Before this test is conducted, the telescope should be left outdoors for approximately an hour for it to reach the equilibrium temperature. Also, star testing should only be done on a night when the seeing is excellent.

Using the optimum magnification (approximately 2x aperture as a rule of thumb, see our guide on magnification and using eyepieces. Examine the image of a moderately bright star. When perfectly focused (intra-focus), it should be a point or a very small disc of light surrounded by faint rings.

Defocus the image on both sides of the focus point (extra-focus). In both cases it should become an evenly illuminated and circular disc, although in the case of a reflector you will see the shadow of the secondary mirror in the middle. If it is impossible to achieve perfect focus, and the image appears as a short line on either side of focus, the telescope is suffering from the optical defect called astigmatism.

For more information, refer to our guide to ‘Interpreting what you see in Star Testing’