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In regards to the Orion thin OAG, I think the secret is to use thin spacers to adjust the focus and to leave the grub screws and the stalk alone so it's tight against OAG body.
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Rick is totally on the money. Get the prism height right, so that it does not shadow the sensor (mine is quite high) then lock those grub screws down HARD and never adjust them again. No wobble then, it holds its position beautifully. Then, use spacer rings between the guide camera and the OAG mount to find focus, after working out the approximate distance required from the light path to each sensor (imaging camera and guide camera). Obviously, focus is found when the distance is the same.
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...even then the stars were astigmatic seagulls
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That is totally expected from an OAG. You are picking off light from the edge of field! The key is, it doesn't matter a jot to guiding. The guide program will find the centroid in the seagull's head, and that will be stable.
