Astigmatism? (1 Viewer)

Last year I bought an ATS-80 HD scope after 40+ years of using a B&L 15-60 zoom. Aside from not yet getting the hang of aiming over and around the angle, I'm very happy with the view of birds and scenery through the zoom eyepiece and with the quality of the construction.

There is one peripheral concern. Last week I was in suburban Phoenix at the time of the full moon, with circa 115 degree days and very clear evening skies. I set up the scope to look at the moon. Wonderful views. Then I turned it to nearby Jupiter. I did make out the four Galilean moons (one very close to the planet and hard to separate), but the planet disk was disappointing. No hint of crispness at the edge, just a diffuse boundary. When I moved the focus ring back and forth through the "best" point, the shape of the disk varied from ellipse to ellipse, with the longest axis being horizontal or vertical according to which side of the best point I was on. I'm supposing that this is some form of astigmatism---I remember reading about this version once, but no longer remember the name.

I realize that the scope is made for terrestrial, not astronomical, viewing, but my old B&L "always" brought Jupiter in as a fairly crisp disk. There were even times when I detected hints of "banding," even through the air over West Los Angeles. Here are two questions:
1. What is the aberration that I am seeing?
2. Is the aberration a sign of an "out-of-spec" flaw that calls for a trip back to Swarovski service?

Thanks for your comments and advice!

SN

Sounds like astigmatism to me. Try using a star near the zenith rather than an extended object like Jupiter. Roll through focus at 60x. Astigmatism may cause the star to look like a little cross at best focus. As you move 4 or 5 diffraction rings out of focus in one direction an astigmatic diffraction pattern will form an ellipse instead of a circle. When you move out of focus in the other direction the axis of the ellipse will shift 90 degrees just as you saw on Jupiter. You may not see diffraction rings on both sides of focus, but the side without rings should still be a circle rather than an ellipse. To be sure the astigmatism is not in your own eye rotate your head around the eyepiece. If the axis of the ellipse follows your rotation the astigmatism is in your eye. Another possibility is that the extreme heat induced temporary astigmatism by causing a lens cell or retaining ring to pinch the edge of a lens. Scopes need time to adjust to the ambient temperature for aberrations to stabilize. That might take a while or maybe never happen at 115 degrees.

Henry,

Thanks for your suggestions. Here's what I did:

Last night was clear in West Los Angeles (6 miles from the Pacific Ocean) and Jupiter was bright about 45 degrees up toward the south. I set up the scope, found Jupiter. The four Galilean satellites were bright points. The planetary disk was still fuzzy-edged for me, but could be focused to look round. I still got the astigmatic ellipses before and behind the best focuses. This time I made sure that my wife was ready to watch. She focused to a round disk that she said was sharp, although she also could create ellipses by defocussing. Then I followed your advice by defocussing slightly and rotating my head. The ellipse rotated with my head! It was then that I realized that I had left an important bit of information out of my original posting. I have a cataract lens inserted into each eye. These plexiglass lenses have already produced occasional interesting optical phenomena for me under various lighting conditions. Finally, I found a bright star near the zenith and that could be focussed to a sharp point.

My conclusions are that (1) the scope does have some form of astigmatism, but that this is heightened by the condition of my own eyes, (2) the scope works very well for birdwatching and other terrestrial activities, and (3) I should just work through my unjustified alarm and fussiness.

Thanks again for your very helpful advice.

StrixNebulosa