Update on Astigmatism of the Eye
In the June 2005 issue of Night Times, I had an article about how our eyes operate and the resulting effects on observing. I recently came across a discussion of astigmatism, along with a test for this common vision defect. Even if you're not affected by astigmatism to the point where you need corrective glasses, it is still of interest to most observers. It is said that almost everyone suffers some level of astigmatism, but it's only noticeable in everyday situations for the more serious cases.
If stars seem to be elongated as we look at them in a telescope, it would be the result of our own astigmatism if the "tails" on the stars change direction as we move our head. For those of us with only mild astigmatism, the most noticeable effect may be that it becomes somewhat harder to reach a perfect focus when looking at an object (especially a star) through a telescope. To check your own level of astigmatism, stare at the very center of the test chart. If the set of lines on one or two axes appear slightly sharper than the others, that indicates you have a certain amount of astigmatism. The reason for fixing your gaze at the center of the chart is that astigmatism first becomes apparent in peripheral vision, hence this vision defect would show up in the spokes of the chart.