I'm new to these forums and telescope ownership, and have a question about the scope I have bought. It's a SkyWatcher 12" GOTO Dobsonian which I am still familiarising myself with, and am curious as to why there are 2 positions that the tube can be opened up to? One is approximately 2 inches short of full extension. Does it have any effect on the focal length, and which is the correct position? I've only used it on 3 or 4 occasions so far, and have always had it fully extended. I'm waiting for another opportunity to try the scope with the tube extended to the shorter position, but just wondered if anyone else would know?