Diffraction rings

I heard that to check proper collimation was to out focus your eyepiece on a star, and if the difraction rings are circular, then you have proper collimation. I did this and all I see is a big blur with the shadow of the vanes and the secondary in it. How do you get the difraction ring effect? thanks

Suggestions: You need good (or at least OK) seeing.
Try both sides of focus. Only need to go a tiny
bit out of focus so you see 3 - 5 rings. Use high
power eyepiece that gives 25x - 50x per inch of
aperture.

If all you want to do is check collimation, then even a big blur will do. Center the out-of focus bloated image of a star or planet and see that the central obstruction shadow is also centered within the image. I always do this 1st when I bring the scope out for the night. Temperature stabilized optics & lack of turbulance (local and atmospheric) together with high power is needed to define the rings within this bloated image, and their similarity between the outside-of-focus and inside-of-focus images indicates optical quality. If you need to adjust collimation, place a finger in front of the scope and use it's shadow to help identify the proper screw. Ron

One more thing to consider. Don't use a very bright star if you want to see diffraction rings. I'd say you'd want to look at something between 2nd-3rd magnitude. I never see diffraction rings on anything brighter.

Happy diffraction hunting,
Brock