Before and after a corrector plate cleaning on my 10" scope:
When it comes to cleaning the corrector plate on a telescope (such as a Schmidt-Cassegrain, Maksutov-Cassegrain, or Rowe-Ackerman Schmidt Astrograph), the primary lens of a refractor, or the primary mirror if the telescope doesn't have a corrector plate (e.g., Newtonian, Ritchey–Chrétien, or [Modified] Dall-Kircham), the best advice regarding when it is time to clean is:
Procrastinate
Procrastinate
Procrastinate
It's the one thing in life where procrastination is usually a good thing.
A little dust on the objective doesn't help the contrast, but other than that doesn't negatively impact the image as much as you might think. The reason for the procrastination is to lessen the chance of scratching the sensitive anti-reflective (AR) coatings on the lens or mirror. Less frequent cleanings mean less chance of scratches.
And when you do clean, do it
carefully. As in
really, really carefully.
The
before picture above represents about a year and a half to two years of successful procrastination.
(I plan to finish up a couple of targets with this scope and then take it down for some planned maintenance soon anyway, so I figured now would be a good time to clean the corrector plate.)
) similar enough 