Star Test Interpretation Help (1 Viewer)

I have a Zeiss 85mm. I've always been disappointed with it at any power above about 40X even on clear days with good viewing conditions. Above 40X it just gets murky with little if any increase in resolution. Although I don't have any experience or knowledge about them, I decided to try a star test using hints and intructions I picked up from this and other forums.

I used 60X on a very clear night on a nearly vertical bright star. After focusing on the star, going towards infinity resulted in totally circular concentric rings with the outermost ring being much brighter then inner ones. Focusing towards the near side resulted only in a bigger blob of light with no apparant rings - - it looked more like a kalidascope.

I'd appreciate any comments or ideas.

Gordon,

Sometimes when you use a real star a layer of unstable air can destroy the diffraction pattern outside of focus (nearer focus). If you saw a lot of rapid motion in the kaleidoscopic pattern then unstable air could have been causing the star test to look worse than it should. I actually prefer to use an artificial star, like a Christmas tree ornament reflecting the sun at about 100'. The air still needs to be steady so I often do this over grass in the early morning with the scope and the target as high off the ground as possible.

If air turbulence is definitely elliminated then a complete lack of rings outside of focus suggests considerable undercorrection for spherical aberration. In a near APO scope like the Diascope it's probably a combination of straight forward SA and spherochromatism. Unfortunately once the undercorrection is bad enough so that no rings are visible outside if focus it becomes difficult to say just how bad it is. As the undercorrection gets worse the center of the blob outside of focus will get brighter compared to the edges and on the other side of focus the inner rings will get dimmer and the outer rings brighter, but that wouldn't be easy to interpret.

Maybe you should also try a resolution test, which can be done at the same time with a dollar bill set up next to the artificial star. Instructions are in this thread:http://www.birdforum.net/showthread.php?t=92034

The best Diascope I've tested was good enough to show definite but not very strong rings outside of focus. I've seen one unit that I considered pretty poor in which the only defect I could see in a star test was excessive SA.

Henry