Which part are you referring to exactly? To unlock the celestial altar, just play through the mod. It's the final upgrade of your AS crafting table. The constellation papers can then be crafted on this celestial altar, after which you can insert them into your Astral Tome. This does not require you to have seen the constellation through the telescope beforehand. Once you have the constellation paper for the constellation you're interested in, you can use a telescope to draw it, which will unlock the full information for that constellation.