How will you know when you telescope mirrors need re-aluminizing?

I have read that telescope mirrors require periodic re-aluminizing. How will I know when mine needs this?

While it’s true that the aluminized surface of a telescope mirror will deteriorate over time, there is no hard and fast rule about how long this will take. Some coatings last only a few years, while others will outlast you. Indeed, the coating on my very first reflector telescope looks absolutely perfect in spite of being more than 30 years old. When it comes to coating life, the main determinants are the environment in which the telescope is used and how it is stored. In both situations, preventing condensation of moisture from the air is a top priority.

So how will you know when your telescope’s mirror needs recoating? In short, if you have to ask the question, the answer is “not yet.” By the time the coating has deteriorated to the point where it actually affects the telescope’s performance, the mirror will look bad enough that you’ll have no doubt. Until then, the best advice is to stop worrying and enjoy the views.

— Gary Seronik