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Mike,
From my understanding, the diffraction rings grade into a brighter ring centre then grade out. That is, there is no definite boundary. On a smaller aperture, the rings are larger and the gaps more noticeable. With the 8" DOB the rings are smaller and closer together; therefore not as clearly separated.
Also, are you comparing the rings at the same high power? If you are using the same eyepiece in both scopes there will be a difference in focal length and hence, magnification.
If seeing conditions are not so good, the rings will not be as clearly defined. Have you tested each scope at the same time, in the same seeing conditions? The seeing will also affect the clarity of the DOB's rings more as they are closer together.
Finally, have you checked the collimation by using a laser collimator or cheshire collimator?
Regards, Rob
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