Astigmatism in optics is a defect in the surface of a lens or mirror where the focus of a light beam varies depending on how far off center the light beam hits the lens or mirror. For example, light hitting the center of an astigmatic lens might focus at 20 inches while light hitting the outside of the lens would focus at 20.5 inches (extreme example). In this case, the actual focus of the lens would vary from 20 to 20.5 inches with no perfect focus. A typical sign of astigmatic optics is that stars have little "tails" on them that switch direction on each side of focus.

Do you have astigmatism in one or both of your eyes? If so, the answer to seeing a good image in your telescope may be to use higher magnification. Here is what California amateur Matt Tarlach says:

"One good thing to know about astigmatism is that it doesn't usually cause problems with high power observing. Most of the defect in the eye lens is in the outer areas, and as you increase power and reduce the size of the exit pupil (of the eyepiece) you only use the central part of the eye which is usually pretty good. Try a little experiment: view a bright star at low power, a 5-7mm exit pupil, with your astigmatic eye and you'll probably notice a "spiked" image. If your astigmatism is as bad as I have in the left eye, you won't be able to focus to a point at all; at best focus the star will look like a short, bright line. Now increase magnification to a 1mm exit pupil or thereabouts...bet you money the star image is a lot cleaner, probably close to perfect. I experimented a little like this to determine that my right-eye astigmatism becomes untroublesome when I get down around 2mm exit pupil. that's why I shelled out for the 12mm Type 4 Nagler to go with my f/6 Dob, instead of the 17 or 22."