I took my new scope out last night and pointed a flashlight down the tube. My mirror looks really dusty. What should I do?
The answer is simple: Don’t shine a light down the tube at night! Seriously, it only makes the dust look a lot worse than it is. If you clean your mirror every time you notice dust, you’re likely to end up with scratches in your mirror coating.
Cut down on dust accumulation between observing sessions by capping your focuser and by covering both ends of the tube with bags or shower caps. If you store your tube sideways rather than upright, less dust will settle on the primary mirror. If possible, turn the tube so that the secondary mirror faces down.
When your mirror looks dirty even in the light of day, you might want to consider cleaning it. Barring unusual circumstances such as a curious child getting fingerprints on your mirror (yes, it happened to me), an annual cleaning is sufficient. Mirror coatings are delicate, so be sure to get directions for safe cleaning.
— Sue French