The Clay Telescope was built on the roof of Harvard's Science Center in 2007. Since then, its primary mirror has never been cleaned. A small group of graduate students, including myself, and one of the scientists at the Center for Astrophysics decided to spend a couple hours one afternoon to clean the Clay's primary mirror.
In some cases, telescope mirrors have their thin coating of aluminum completely "cleaned" (removed) and are re-aluminized to reproduce their original reflectivity.
We just used soapy water and cotton swabs.
Before that, though, we had to dismount the telescope's camera and take the mirror off the main body. There are some pictures below of the dismounted camera and of the secondary mirror seen through the bottom of the telescope.
The process was pretty straightforward and beside a few streak marks, the mirror looks great (or at least much better than it did before).
A before and after shot of the Clay Telescope's mirror.
Hopefully the next cleaning will be sooner than 2027.
:^)