Telescope counterweights and equatorial mounts

 

I'd always wondered what the bits sticking off the side of an astronomical telescope were, but up close they turned out to be counterweights to stop gravity pulling the telescope down. This doesn't happen on my birdwatching telescope because it sits directly over the top of the tripod. So why is an astronomical telescope off to one side? To simplify the answer, stars move over time because the Earth is turning. You need a drive to make the telescope move to compensate for this. There are two axes for movement - stars move sideways across the sky but also up and down. By aligning one axis with the axis of the Earth that never changes, the other motion can get a constant speed drive. I've not yet worked out why being off centre makes that possible. It is called an equatorial mount.