Seeing Faint

How to see more in the night sky

In my last stargazing story, I said experienced observers can generally see objects about a magnitude fainter than the average person sees.

You, too, can learn to see fainter objects by applying five simple tricks:

  1. Let your eyes adapt to the dark. When you move from a bright place into dark, your pupils dilate to let in more light. But this isn’t the only way your vision adapts. Chemical changes in your retinas gradually increase sensitivity to light at the expense of color perception. This process, called dark adaptation, takes about twenty minutes to complete. But there’s an important caveat: even a momentary flicker of white light destroys dark adaptation. If that happens, you must wait another twenty minutes to regain it. When stargazing, try to find a location free from lights. (Alas, that’s not always possible in the modern world.) If you must use a light, use a red one. Red light doesn’t much affect dark adaptation.
  2. It’s easier to see things when you know where to find them, so learn the layout of the sky, beginning with the brightest stars and major constellations. Use a basic star chart like Sky & Telescope’s interactive star chart. You can also get star chart apps for your cell phone or buy star charts in varying levels of detail from planispheres (the most basic) to star atlases. But remember that white light destroys averted vision. Use the night mode on your app, which will display charts in red light, or use a red flashlight to illuminate printed charts.
  3. Don’t look straight at a faint object. Your retinas contain light-sensitive structures called rods and cones. Rods are sensitive to lower light levels, while cones like brighter light and are color-sensitive. Cones are more concentrated near the center of the retina, which means the density of rods there is lower. In the dark, it’s therefore easier to see a faint object if you look a bit off to its side. Astronomers call this technique averted vision. Averted vision takes a little practice, but it’s not too hard. Find a star just barely visible when you look straight at it, then look slightly to its side. Slowly move your gaze around until you can see the star clearly. Resist the temptation to look straight at it. You’ll catch on soon enough. With averted vision, you can see some objects that are invisible when you look straight at them.
  4. Be patient. Just because you can’t immediately see a faint object doesn’t mean you won’t ever find it. Give yourself time. Use averted vision and scan the immediate area. If you don’t find it, try again later, or on another night. Seeing conditions change from night to night, hour to hour, and sometimes even minute to minute. Patience pays off.
  5. Practice, practice, practice. The more you look, the more you’ll see.

The above tips are equally useful for observing with a telescope. If you have one, you can try two other techniques to locate faint objects. First, try moving the field of view a little, either by tapping the telescope or slewing it slightly. Moving objects are often easier to see than stationary ones. Second, in some cases you may be able to locate a faint object by messing with the focus. Gradually turn the focuser in and out and see if anything pops out at you.

However you’re observing, give these techniques a try and soon you’ll find yourself seeing more than you ever thought you could.

Photo by Jake weirick on Unsplash