Astronomy Hacks

Learn to See DSOs

Seeing a black cat in a coal bin at midnight is easier than seeing some of these dim objects.

One of the major challenges that every beginning astronomer faces is learning to see. That sounds stupid, we know. After all, you’ve been using your eyes every waking moment all your life. How hard can it be?

Very hard, as it turns out. In daily life, you look at brightly lit, colorful, high-contrast objects that are familiar to you. When you observe DSOs (deep-sky objects—nebulae, galaxies, and so on) you look at unfamiliar, gray, dim objects with almost no contrast. You have to retrain your eyes, your brain, and your way of thinking if you want to see dim astronomical objects.

Tip

Imagine a gray scale that runs from 0 (pure black) to 255 (pure white). In daily life, you see objects that span most of that range. In DSO observing, you may be trying to tease detail from an object with an average brightness of perhaps 2 or 3 against a sky background of 1 or 2. Under light-polluted skies, it’s even worse because the background sky may be nearly as bright (or brighter) than the objects you are trying to see. That’s why you need to get to a dark site to observe galaxies, for example.

Beginners are often flabbergasted at just how dim and low contrast many DSOs are. Even under dark skies, newbies often literally cannot see a “bright” DSO even if an experienced observer has centered it in the eye-piece. For example, one very clear, dark night we were with a group of several experienced ...