A bit of an update to this old thread.
While viewing tonight I did a bit of self diagnosis on the level
of astigmatism I have.
It seems to be almost a perfectly matched deficiency or degradation
developing over time, with age, per eye.
The diffraction pattern I see on stars at infinity focus when my eyes
are totally relaxed (and out of focus because I need glasses for distance)
is a match in each eye, only mirrored.
This is caused by hardening/ distorting of the cornea and/or lens due to age
apparently.
I googled a pic of a point source diffraction pattern and adjusted it in PS
to try and describe my astigmatism per eye.
Left eye on left, right eye on right. Original at bottom.
My prescription glasses correct for this and when I have them on, a star
at infinity is a pinpoint, both eyes relaxed.
But when I use an eyepiece I hate using my glasses, and so the
Airy disk is distorted.
Sometimes it is also good to read up on the theory of the basics, just as
a refresher.
This page covers quite a lot of interesting theory about magnification,
point source versus extended objects and an explanation of just what
causes diffraction.
http://starizona.com/acb/basics/observing_theory.aspx
Steve