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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?

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    $\begingroup$ Perhaps a related question is what, if any, measures were incorporated into Hubble's design to protect the mirror from such damage? And what, if any, operational measures are or have been taken to protect the mirror? $\endgroup$
    – Anthony X
    Commented Jun 5, 2014 at 2:18

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From here: http://www.spacetelescope.org/about/faq/

How do you protect and clean the lens of the Hubble Space Telescope? Hubble doesn’t have a lens. Like all large telescopes, Hubble uses a curved mirror to focus starlight. This mirror is located deep inside the telescope, protected by its long tube-like structure. As there is no atmosphere around Hubble, there is no risk of dust or corrosion reaching inside.

Also, from here: http://www.nap.edu/openbook.php?record_id=11169&page=12

Hubble was designed with an anticipated 15-year lifetime based on the expected integrity of the main mirror. It was believed that over HST’s 15-year life the space environment in low Earth orbit would cause sufficient degradation of the mirror that the telescope’s light-gathering capabilities would be severely damaged by cosmic rays and orbital debris. To date, since the first shuttle servicing mission’s correction for a significant aberration in the mirror, there has been no measurable degradation.

From these two sources, I'd say no, there is no measurable degradation of the mirrors.

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    $\begingroup$ Related: dx.doi.org/10.1109/tns.2002.805384 'Characteristics of the hubble space telescope's radiation environment inferred from charge-collection modeling of near-infrared camera and multi-object spectrometer darkframes' (2002). $\endgroup$
    – Deer Hunter
    Commented Dec 12, 2014 at 9:49
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    $\begingroup$ "To date, since the first shuttle servicing mission’s correction for a significant aberration in the mirror, there has been no measurable degradation." Aberration is associated with low-order Bessel modes of deformation of the lens, and those are associated with plastic strain due to thermal cycling. Those are interesting to know, but is not what I'm interested. My concern is about the microscopic degradation of the mirror surface. Aberration doesn't detect that, only reductions in Strehl ratios and scattering leaking can estimate those, and there is no practical way to measure the latter $\endgroup$
    – lurscher
    Commented Dec 12, 2014 at 19:33
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    $\begingroup$ It seems that Hubble has well surpassed its projected lifetime. Is there an updated lifetime, with/without assuming that it can continue to be boosted/serviced by some means? $\endgroup$
    – Anthony X
    Commented Feb 13, 2016 at 23:06
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    $\begingroup$ Another point to considers is that space junk is usually in orbit around the earth and the tube of the Hubble Space Telescope is pointed away from the earth. So most of the space junk would would hit perpendicular to the tube and not so much perfectly straight down the tube, which would a require that the piece of space junk have a highly elliptical orbit to make it inside the tube and actually hit the mirror. $\endgroup$
    – Zac Helmberger
    Commented Feb 25, 2018 at 4:24

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