Originally posted by Ron Kruger I clean my sensor whenever spots show up (macro) with a kit sold for the purpose. Was nervous about it the first time or two, but perform it now as simple, easy and quick maintenence. I also developed a number of spots on my mirror, which were annoying, but didn't effect IQ. Tried also cleaning the mirror with the sensor cleaning kit, but they must have been behind the mirror. Didn't realize mirror assembly was simple as well until I bought a Katzeye and replaced the original. If--more like when--I get spots on the mirror, I won't hesitate to take it out and clean it. Really pretty simple. However, with the replacement mirror kit is a small, plastic, tweezers-like tool that makes the process simple and cuts down on the possibility of scratching the mirror.
If you will search, you can find sensor cleaning kits and mirror replacement kits online with instructions about how to do it.
I assume you're talking about the
focus screen here, and not the mirror.
The focus screen, up at the top of the body cavity and against the viewfinder prism (or pentamirror), is indeed removable, cleanable, and easily replaced.
The mirror, on the other hand, is the item in front of the sensor that swings away when you take a shot. It's permanently installed, delicate, and probably pretty expensive to replace!
It's true that you can get spots on the mirror. But be careful cleaning them -- the mirror is silvered on its front surface and very easily scratched. I'd leave the mirror alone for the most part, use a no-contact blower when necessary, and maybe wet-clean with sensor fluid on rare events when something big 'n' ugly is stuck on there.