That looks pretty well stuck on too.
You don't want to have to rub hard to get it all off.
That lukewarm bath that Geoff recomended should be for at least a quarter of an hour.
Rinse off the muck and repeat the soaking if needed.
Once clean, I mop up the drippy water on the mirror with a slightly damp, antistatic cloth, that is only used on this mirror, then finish with a hair dryer so it dries quickly and evenly with less chance of getting dried water marks/spots. Which as we all know is one of the most frustrating things about washing these darn things.
But everyone has their own ideas on how to treat their beloved mirrors.
Here's how they clean the huge 8 metre mirror at the Gemini North Observatory.
"Through experimentation with various detergents, technicians have found that the most efficient way to clean it is to mop the mirror surface using natural sponges soaked in a detergent popularly marketed as horse shampoo, which they purchase at a general store in Hilo, Hawai'i. Optics Technician Clayton Ah Hee, who supervised the actual washing, said the soap is used because it is a strong detergent, yet is not abrasive to the multi-million dollar mirror's delicate surface, and leaves no residue. "We've found the combination of the natural sponges and horse soap to be the most efficient way to get the mirror surface completely clean," said Ah Hee. "I know this sounds kind of simple, but sometimes the simplest solutions are best, even with something as leading-edge as the Gemini telescopes. And the horse soap really works.""
Here's the link.
http://www.gemini.edu/index.php?opti...ask=view&id=60