Visit advrider.com. You may have to register to see it but click the Hack thread. There's a sticky on there on how to make a sidecar. You CANNOT just build a rig, bolt it up, and expect it to work. There is a great deal of detail in getting the dimesions and positioning right. If you wing it, it could be extremely dangerous once you get it on the road.
A cheaper version would be to look into panniers (saddlebags) and maybe a rear utility rack.
I have the 36L Caribou cases on my bike and I can fill up two handbaskets of groceries in both cases. When I go on a weekend jaunt, I can fit my sleeping bag, sleeping mat, stove, a couple days of food, and a spare set of clothes with room to spare. Then the tank bag holds the camera. Here's a pic of what it looks like.

I built a rear utility rack to hold a duffel for extra crap for my longer rides. In your case, you could place your telescope on there.
It's a different bike but I'm just trying to pass the concept onto you.
In your case, you'll need a set of custom brackets fabricated to bolt to your bike but I can guarantee you that'll still be cheaper than building a side car. Some have spent a couple grand to just for the frame, links, brackets, suspension, brakes, spindle, and the wheel. That's not even including the tub.