Thanks, I was going for a professional part.

John- I do have some nice machine tools and engineering software. I'll have to cut Dennis and Pavel in on the action. Couldn't you just see the look on Pavels face when big royalty checks start rolling in?

I do think I have an understanding of how the design works, and why you can't just scale it for other size scopes.

The axis is centered for the "left" side and has a precise light block of 50%. The "right" side has a 20 degree angle that is centered to blocked area of the left. The right side has a light block of 40%. The axis of the right open area must align to the center of the left blocked area. If it centered, then the spikes you see when in focus will align correctly.

I don't read Russian, so I had to look a photos and calculations that Pavel posted.

If this works like I think, then it will be simple for me to draw this design for any size scope.

darn clouds