On Jan 13, 2008, at 12:27 AM, John R Pierce wrote:
> Noted a very odd distortion, where bright stars would go from -- to
> | as I shifted in and out of focus, but never resolve to a point.
> Yet, the same eyepiece on my barlow gave excellent images. Anyone
> have any idea what might cause this? Is f/4.5 just too fast for
> this eyepiece?
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.
--David Kingsley
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Received on Sun Jan 13 16:40:32 2008