How To See Faint Things in the Night Sky
Using Averted Vision
An overlooked skill among new amateur astronomers is the art of seeing faint objects, whether with your unaided eye or with optics. The fact is, if you just head out the back door walk up to a scope and look at a faint nebula or galaxy in the eyepiece like you’re looking out the window, you may very well not see anything at all.
To see the subtle detail in faint celestial objects, you have to condition your eye and learn to use it to expose the most sensitive part of your retina.
First, a little background on how your eye works…
The retina of your eye has two types of light-detecting cells: rods and cones.
Cones detect color under well-lit conditions and are densely packed in the fovea, near the center of your retina. Cones help you see color and fine detail, which is why you look directly at objects you want to see well, like books, movies, and faces.
Rods are mostly away from the center of your retina. You see less detail and no color with the rods, but they are much more sensitive to light.
A diagram of the rods and cones on the retina. Rods detect faint light, cones detect bright light and color.
The way your eye is structured means you see the faintest objects if you look 8 to 16 degrees off center. The exact angle is a little different for each person. This only works if the object you’re looking at is on the nose-ward side of your eye. So look slightly rightward with your right eye and leftward with your left eye. This technique is called “averted vision”. It exposes the most sensitive part of your eye and lets you see much fainter objects. But averted vision only works if you avert your eyes in the right direction. Do the reverse and you’ll expose the blind spot of your eye and you won’t see a thing.
If you’re using both eyes, as with binoculars, looking only sideways makes one eye more sensitive at the expense of the other. The solution? Look up. That uses another rod-rich part of your retina above the fovea.
With a little practice, averted vision reveals objects 20-40x fainter than direct vision. That’s a huge difference. You will be amazed how much you can see.
And one more thing… The rods in your eye are most sensitive to blue-green light, but your optics nerve and brain are not wired to detect color when only your rod cells are exposed to light. That’s why faint objects in a telescope appear grayish-white. You will not see color in galaxies and nebula like you see in the photographs from big telescopes.
The sensitivity of rods and cones to light of different color.
Your eye evolved to operate in two modes, photopic for seeing in well-lit conditions and scotopic for seeing faint objects in the dark. As you learned above, your retina has two types of cells, rods and cones. In photopic mode, the cones detect bright light and colors. But in scotopic mode, the rods detect faint light. Both types of cells contain dyes that undergo a chemical change called “bleaching” when hit by light.
In light-adapted or photopic mode, the dyes in your rods are fully bleached, so they can’t detect faint light… they’re out of action, and that’s very bad if you’re trying to see faint objects in the sky. Turn the lights off and the rods to return to dark-adapted mode, but it takes a long time, about 20-60 minutes. That’s why astronomers get so angry when someone carelessly shines a white light in their eyes… they have to wait a long time to recover their dark-adapted vision.
Going from a dark to light adapted state happens much faster, in only a few seconds.
Each eye reacts separately to light, so you can keep one eye dark adapted while using your other eye to read star charts and slew your telescope. An eye patch is ideal. You can also keep unwanted streetlights out of your eyes by throwing a towel over your head when looking through the eyepiece of your scope with your dark adapted eye.
You often see astronomers using bright red LED flashlights when looking at star maps and gear around the telescope. That’s because red light cannot bleach the dye in the rods if the wavelength is > 620 nanometers. So the chemical structure of the dye in the rods is completely unaffected, while the dye in the cones still enables scotopic vision. You will want to get yourself a red flashlight as soon as possible to help you see your star maps and telescope in the dark without ruining your dark-adapted vision.
An interesting fact… your body cannot by itself make the dyes for the rods and cones in your retina. It needs an external chemical-beta carotene-to synthesize the dyes. A good source of beta-cartone? Carrots. So carrots really can be good for your eyesight.
Where To Go From Here…
So, we come to the end of the One-Minute Astronomer 5-day mini course. Where do you go from here?
As in most things, knowledge is power. If you want to learn more about amateur astronomer, the next step is to read more and to keep practicing where and how to find things in the night sky. The practice is up to you.
But here are 6 excellent books to help you learn more. These are superb books, and you should get your own copies (new or used) or borrow them from your local library.
• Nightwatch, 4th ed. (Terence Dickinson, 2006). This is the gold standard for beginner’s astronomy guides. You get easy-to-follow star maps, advice for equipment selection, guides on how to find and see common objects, and a tour of the known universe. Read this book cover to cover and follow its advice, and you’ll be well on your way as an amateur astronomer.
Perhaps the best guidebook for beginners: Nightwatch, by Terrence Dickinson
• The Stars: A New Way to See Them (H. A. Rey, 1952). Written by one of the creators of Curious George, this slim guide combines simple prose and delightful artwork that shows you how to find the major constellations of the northern and near-southern skies. If you want to learn the constellations, then start here.
• Turn Left at Orion (Guy Consolmagno, 2000). If you just bought a small telescope and are wondering what you can see, this is the book for you. It presents a very intuitive way of finding your way around the sky and presents dozens of fascinating objects that can be seen in small scopes. This is perhaps the most insightful starter book available for beginners with a small telescope.
• Sky Atlas 2000 (Tirion and Sinnott, 2007). No matter what your level of expertise, you need a good set of star maps. This is one of the best. You get 26 star maps covering 2,700 deep-sky objects and more than 81,000 stars. Alternative: A more compact version is the Pocket Sky Atlas by Roger Sinnott. Also very good.
• Backyard Astronomer’s Guide, 3rd ed. (Terence Dickinson and Alan Dyer, 2008). A comprehensive guide for the amateur astronomer on buying and using equipment, including telescopes, mounts, eyepieces, and accessories. If you’re bewildered by the choice of astronomy equipment out there, this book will help immensely.
• Beginner’s Guide to Digital Astrophotography (Jerry Lodriguss, 2009). An excellent guide to taking your own images of the night sky with a DSLR camera. Written by one of the world’s finest astrophotographers. No previous experience necessary.
• Celestial Sampler (Sue French, 2005) Can’t think of anything good to see with your telescope tonight? This crisply-written compilation will help you find and enjoy hundreds of objects including double stars, odd asterisms, nebulae, star clusters, and galaxies. Organized by month, each bite-sized “sampler” shows you where to find each object and what you can expect to see with a 4-inch telescope. A delight to read.
And finally, we invite you to learn more in the pages on One-Minute Astronomer. You can return here any time to browse the archives and learn more about astronomy from our short articles. Or, if you haven’t already, you can subscribe here to get new articles delivered by RSS or email.
Whatever you do, remember to keep looking up at the stars.