With a star test, you'll be able to judge how well made or collimated (aligned) the lenses or mirrors are in your telescope. You can then use the results to fine-tune the optics and squeeze the most out of your scope for perfect views.
To carry out a star test you need to observe a bright star with your telescope and a high magnification eyepiece, looking at the star when it's in focus when its inside focus and when its outside focus. The patterns of concentric rings that the star makes reveals the state of your scopes optics.
The star should be high in the sky so that atmospheric effects don't cause it to twinkle too much. If the stars are twinkling heavily, don't bother with a star test because you won't get good enough results. Wait for a night of good seeing instead. A tracking mount is handy for keeping the test star in the field of view through the night. If you don't have a tracking mount, you can observe Polaris as it hardly moves. On the flip side, Polaris isn't particularly high or bright.
Whatever the Weather
Alternately, you can do a star test in any kind of conditions using an artificial star. This is a piece of kit that uses a white LED and a small length of fibre optic cable to makes a star like source of light. Place it 25 to 50m from the end of the telescope and you can star test even when it's cloudy.
Another way to get around an unforgiving atmosphere is to make you star test observation with a webcam and then take as long as you want to analyse them on a computer indoors. A freeware program such as K3CCDTools can extract images from a webcam movie file so you can grab the images quickly when for instance the sky briefly clears during a night of passing cloud.
Before you begin star testing, make sure the telescope has cooled down to the ambient air temperature. This will take at least an hour after setting up. You'll be able to tell if the scope needs more time to cool down if you can see streaks and streamers bleeding away from the out of focus view of your star. These are caused by air currents inside the tube of the telescope distorting the view.
If you've got a reflecting telescope, you also need to make sure that it's collimated. You'll know that your reflector is properly collimated when the out of focus concentric rings of a bright star are perfectly aligned. If the concentric rings are grouped to one side then you telescope will need adjusting.
You are now ready to begin star testing. Adjust the focuser so that it's at the inside focus, then move it through the point of focus to a similar point outside focus. Note the pattern of the concentric rings that appear at the inside focus. In a perfect star test, the pattern the rings make should be identical to that outside focus.
This shows that your telescope if free of aberrations. You should also look at how clear the rings are because this shows how smooth your mirror is. If the telescope has ideal optics, the rings should be clearly defined.
You're looking for the same things whether you've got a reflector or refractor. The only difference is that reflectors show a dark ring near the centre of the concentric ring pattern which is due to the secondary mirror.
Common Star Test Problems
A perfect star test rarely happens, its likely that your scope will show signs of aberrations.
One of the most common causes is astigmatism. You'll know that the objective mirror or lens of your scope has this if the concentric ring pattern looks elliptical rather than circular and its long axis moves through 90° as you move through the point of focus. This is often caused by the mirror clamps being too tight which distorts the mirror. Ease them off a bit to fix the problem.
A star test also reveals a feature of the optics of a telescope known as spherical aberration. It happens because the outer parts of the mirrors or lens don't bring light rays to the same point of focus as those hitting the centre.
Some telescopes have this aberration - usually cheap poor quality ones - making faint objects hard to see. If this effect is bad it can make pin sharp views impossible.
There are two types of spherical aberration over correction and under correction. Overcorrection is when the concentric rings inside focus are diffuse and not well defined, while those outside foci are sharply defined. Sharp rings inside focus, together with misty rings outside of focus, shows under correction. This bigger the inside and outside focus patterns, the great er the aberration.
If the outer ring has little spikes, like whiskers, radiating from it, it shows that the mirror has a turned down edge. One way of combatting a turned down edge is to place a circular mask around the end of your telescope blocking iff the outer few millimetres. This will remove the effect from the pattern you see.