Through-Lookers,
It is often said that, in most eyepieces, off-axis astigmatism is the dominant aberration in visual observing. Naglers and a few other high-toned many-lensed designs are the exceptions. My experience agrees with this.
It is also well known that astigmatism in the eye will cause stars, especially at eye-filling exit pupils, to look wonky.
I have enough astigmatism in my favorite eye to notice when the exit pupil gets large. But, tonight I discovered something cool, that other people may have noticed but I was unaware of. I was looking at Alberio through my 10", f/6 Newt, using a 40mm UO 70-degree Konig, a dated design, with only fair edge correction. By moving Alberio to a spot about 1/3 way to the edge, towards the lower right, I was able to eliminate the wonkyness that I see in the center, and got the best view of my life at this power--neat, tight, round spots. Whee!
This was not inspired by, but reminds me of, passages on the ATM-ing of achromatic lenses that describe how astigmatic crown and flint elements can be rotated to reduce or almost eliminate net astigmatism, by playing one lens's aberration against the other's.
Those of you with mild astigmatism who don't like wearing glasse to observe, try this trick using your lowest power eyepiece. Of course the direction and distance from center where you get cancellation will depend on your own eye, so you'll have to experiment. I'd like to hear if anybody else can make it work.
Ron
It is often said that, in most eyepieces, off-axis astigmatism is the dominant aberration in visual observing. Naglers and a few other high-toned many-lensed designs are the exceptions. My experience agrees with this.
It is also well known that astigmatism in the eye will cause stars, especially at eye-filling exit pupils, to look wonky.
I have enough astigmatism in my favorite eye to notice when the exit pupil gets large. But, tonight I discovered something cool, that other people may have noticed but I was unaware of. I was looking at Alberio through my 10", f/6 Newt, using a 40mm UO 70-degree Konig, a dated design, with only fair edge correction. By moving Alberio to a spot about 1/3 way to the edge, towards the lower right, I was able to eliminate the wonkyness that I see in the center, and got the best view of my life at this power--neat, tight, round spots. Whee!
This was not inspired by, but reminds me of, passages on the ATM-ing of achromatic lenses that describe how astigmatic crown and flint elements can be rotated to reduce or almost eliminate net astigmatism, by playing one lens's aberration against the other's.
Those of you with mild astigmatism who don't like wearing glasse to observe, try this trick using your lowest power eyepiece. Of course the direction and distance from center where you get cancellation will depend on your own eye, so you'll have to experiment. I'd like to hear if anybody else can make it work.
Ron
