I'm interested in understanding what effect does long-term radiation and microdust exposure would do to the surface roughness of a high-precision mirror like the one on the Hubble. Since it has been on orbit for 24 years, it provides an unique opportunity to estimate accumulated environmental damage to high-quality mirror surfaces in general. Since surface roughness is directly related to scattering losses and Strehl ratio, I figure that a measurable decay of image quality and total luminosity would enable an estimate of this. Do astronomers working on the Hubble track for optical degradation of the optics? if yes, what techniques do they use? what fraction of that degradation is actually related to mirror degradation?