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I find collimating my C8 to be very difficult using the doughnut technique spelled out in my manual.
Instead I use a 5mm eyepiece to give me 400X on a bright star, and look at how concentric the diffraction rings are, and adjust if they are not. Only problem is I might have to wait for one or two weeks before I get a night when the tiny diffraction rings stand out well rather than being a mushy blur around the star.
For cleaning the front corrector plate I use distilled water mixed with the proper isopropal alcohol (not the foot rubbing kind), together with lots of little lens cleaning tissue or DSLR CCD cleaning tissues, gently dragging the tissues out from the centre - with the plate being nearly perpendicular to the ground.
Regards,
Renato
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