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Hi Stu,
I have a 16" Newt in my observatory, it is about 16 years old and I washed it as per Mikes article, for the first time last year.
When you observe with your scope the image that you see comes to focus at the optical elements within the eyepiece that you are using. The distance this is from the surface of your primary, when at focus, is the FL of your particular mirror.
It is amazing how dirty a mirror can look and still produce knock out images at the eyepiece. I would go as far to say that the collimation of your optical system is of far greater importance than how dirty the surface of the primary is, regarding the quality of the image you see through the scope.
Regards
Trevor
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