[Han Tzu]

Sometimes it is not best to use pneumatics for telescoping arms. A rack and pinion, cable and pulley, and of course pneumatics are all devices you can use to do some sort of system like that. The hard part is figuring out what kind of hook-up you want to use with that device. For any of the above, you could directly hook it to the bottom of the shaft, or top if it suits you, use a wire that acts on a system of pulleys if you're spanning a large distance, or you can use a slider and gravity. Whichever you think your team will be able to build and handle is up to you.
Pneumatics is a wonderful thing for rookie teams who dont have a lot of experience. Their limited range of motion are offset by the fact that they are simple to assemble, program and are very good at working (and by working I mean not breaking).