Those older telescopes had to focus on nebulae, where the interstellar gas is most concentrated (a familiar one, visible through binoculars, is the gas cloud around the stars that make up Orion's sword).

以往的望远镜必须关注星云，那里是星际气体最集中之处。一个可用双筒望远镜看到的典例是气体环绕恒星所形成的猎户座的”剑”。