Make a very large, clear balloon, fill the bottom portion of it with water, and presto, you have a huge lens. Then make the smaller lenses in the appropriate shapes to complement the odd shape of the larger lens, and presto, you have a gigantic telescope.
Or, make the balloon and air balloon and coat the bottom of the inside of it with silver, then put the complementary mirrors inside of it. Presto, a gigantic reflecting telescope. You could even float it up to high in the atmosphere.