If the wind is from the direction of that coal I'd be keeping the mirror covered. Coal contains sulphur - mixed with dew you might have acids eating away at the coating. I live near an open cut mine and am a little paranoid about my mirror, so I use Baader turbo-film on the front of my 10" solid tube dob - probably not practical on a large truss dob. The scope and shed look like a military installation
