Accumulated environmental damage to Hubble main mirror

[Ganesh Ujwal]

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?

 

[Drakkith]

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.

 

[Chronos]

It would be splendid if the Hubble could be retrieved when it is finally retired, It would not only look splendid in the Smithsonian, but, I expect engineers would have a field day assessing and understanding material effects [or lack thereof] of long term exposure in low Earth orbit. Perhaps we could boost it to a more permanent orbit until retrieval became feasible. It would be a shame to turn it into a funeral pyre crashing back to earth.