My experience is that if they want to nick it they will. My friend is a sculpter and uses a lot of copper and bronze, he buried it all 3 feet underground in a field, they dug it up and nicked all of it!
My opinion is to keep things to yourself about what your doing, don't advertise that you have expensive equipment knocking around. Keep it indoors and wheel it out in the dark is the best bet. I was going to build a proper observatory but have had second thoughts because of advertising what is in it.
Some people are either after your money or what they can nick from you.
I'm beginning to think the best way around all of this astronomy lark, with all the cost involved in equipment, is to subscribe to professional telescopes. In the long run I think it is much more efficient, cost effective and less risky on the theft side.