I imagine any impurity could lead to a compromise in structural integrity. All I know is everything with these fancy telescopes requires precision, or you get shit that is blurry or distorted, which is bad when you are looking at a point of light many light years away. This is because the changes they detect for things like planet hunting is such a small change. Like the back is how you mount the thing, so you need perfectly flat to mate with the flat mount.
I remember awhile ago there was worry we were basically at the maximum for how good terrestrial telescopes were because of the wiggle of light going through the atmosphere. But thanks to new digital techniques, we can compensate for that.
