When I focus on a star, I find that near the perfect focus, the star changes shape from a fuzzy horizontal to a fuzzy vertical blob. Why is this? David Kingsley, a TAC member, wrote this reply:

Flipping from “–” to “|” as you travel from one side to other side of focus is a classic description of astigmatism.

It is very likely in your eye, not the eyepiece. (To test this, try changing the orientation of your head as you look through the eyepiece. If the direction of star elongation changes when your eye orientation changes, the aberration is in your eyes, not the telescope or eyepiece).

Many people have some astigmatism in their eyes that only become apparent with large exit pupils, (which pass through a larger surface area of the cornea). Using a 35 mm eyepiece in a f/4.5 scope gives an exit pupil of 35/4.5= 7.8 mm. That may “waste” some light. Most people’s pupils don’t open larger than about 7 mm (less as you get older). Not wasting light is the origin of the recommendation not to use an eyepiece whose focal length give an exit pupil larger than 7 mm (Dickinson’s “scope focal ratio x 7” rule). However, a large exit pupil should not introduce any inherent aberrations in the image. It simply samples a larger amount of the aberrations that may be present in your own eye, thus producing image defects that you don’t see with barlowed or short focal length eyepieces that produce small exit pupils.

My own eyes have astigmatism that is readily apparent whenever I use a scope/eyepiece combination that gives an exit pupil larger than 1 or 2 mm. That corresponds to eyepieces longer than 9 mm in my f/4.3 scope, since exit pupil=eyepiece focal length divided by scope focal ratio. With smaller exit pupils, stars look pinpoints. With larger exit pupils, stars flip from slightly elongated “–‘s” to “|’s” no matter how much I focus. The orientation of the astigmatic image clearly moves with my head position. And I can easily get the pinpoint stars back, even when using long focal length eyepieces, if I wear glasses to correct for my astigmatism when observing.