I've been filling up the Telescope with oxygen for a while now in a test world, while having a duplicant research some stuff. So far it's past 600 kg of oxygen and counting! That seems like a really excessive amount of oxygen to store, even worse than the exosuit dock and especially since it will all vanish into space if the player discovers their telescope is poorly placed and needs to deconstruct it.
1. Connect gas pipe to telescope 2. Feed oxygen into telescope 3. If necessary, queue a research project that requires interstellar research