They don’t get dirty.
There is nothing in the vacuum of space to collect on the mirror.
Orbital debris is a potential problem, but experience with Hubble shows that it’s not too serious. But if the mirror does get hit, it’s not something you’ll be able to clean off…it’ll be a hole the size of a quarter.
Hubble’s biggest problems with debris has been impacts to its solar panels:
But Hubble is in a moderately low orbit – because that’s as high as the crappy Space Shuttle could get it.
These days, we’d put it MUCH farther from the Earth—far from the places where debris is common.
The James Webb Space telescope isn’t even going to be orbiting the Earth—it’s going to be parked in a Sun-centered orbit at the Earth/Sun L2 point.
Credit : Quora
Picture Credit : Google