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Hi Eric, personally I wouldn't put any grease on the diagonal - just loosen the locking ring when you want to change the orientation of the diagonal. The diagonal can slip from the locking ring being loose or from the nosepiece unscrewing from the diagonal body - check that it's tightly screwed into the body, but don't overdo it since the threads in the diagonal are a bit sloppy and you might damage them.
Collimate with the diagonal in place then remove it to see if there is any difference without the diagonal; there probably won't be. You can check for astigmatism introduced by the diagonal by checking for oval shaped diffraction patterns on either side of focus. (remembering to check for astigmatism in your eyes as well)
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