Fundamentals of Stargazing Stargazing Tools

01 – Visual Observing Basics

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Visual Observing – How the Eye Works Whether you look through $50 binoculars or an a $10,000 telescope, whether you observe a single star or a galaxy of 500 billion suns, all the ancient starlight you hope to see passes through the tiny 7 mm-wide-lens of your eye and falls on a delicate retina just 1.5 mm across. Your eyes are a work of art in themselves, the culmination of 100 million years of evolution, exquisite tools to help you see the the beauty and danger and opportunity of our own world. But they are not optimized for stargazing.

With their larger and more sensitive eyes, cats and hawks and eagles, among others, would have a far better view of the night sky than us. But we have the brains to understand what we see, so we need to understand how to extract as much detail and sensitivity from our eyes as nature will allow.

The retina of your eye has two types of light-detecting cells: rods and cones (see image below). Cone cells detect color under well-lit conditions and are densely packed in the fovea, the area on the back of your eye near the center of your retina. Cones let you see color and fine detail when you look at, for example, books, movies, and faces. To get the most detailed view of bright objects, you must look directly at objects to expose the part of your retina where the cones are most densely concentrated. That’s why you can only read the words on this page when you look at it directly. Look slightly off to one side, and you can’t see enough detail to read.

Cross-section of the retina

Rod cells also lie on your retina, but away from the fovea. As it turns out, because of the distribution of the rods on your retina, you can 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. Do the reverse and you’ll expose the blind spot of your eye

Fundamentals of Stargazing -3- and you won’t see a thing. This technique of looking off to one side to see faint objects is called averted vision. With this technique, you can objects 20-40x fainter than if you look straight on. That's a huge increase. When you first try averted vision, you will be shocked at the subtle detail that suddenly appears.

If you’re using both eyes, as with binoculars, you can still use averted vision. Of course, 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. Rods are most sensitive to blue- green light, but your optic nerve and brain are not wired to detect color when only your rod cells are exposed to light. That’s why faint objects appear grayish-white through all but the largest telescopes.

Before you can use averted vision, however, you must ensure your eyes are prepared to see faint objects. Your eye actually operates in two modes, scotopic and photopic. In photopic mode, the cones are optimized to detect bright light and colors. But in scotopic mode, the rods are set to detect faint light in dark conditions. For astronomy, you need to get your eye in scotopic mode, a state also known as dark adaptation.

Both types of cells in your retina, rods for bright light and cones for faint light, 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’re out of action. Turn the lights off and the rods to return to scotopic mode, but it takes a long time, about 20-60 minutes. That means you can't just walk out of a bright room and expect to see much through a telescope, whether you use averted vision or not. You must wait at least 10 minutes (preferably more) for your eyes to begin to enter scotopic, or dark-adapted mode.

Going from a scotopic to photopic state happens much faster, in only a few seconds. That's why astronomers get quite angry when someone carelessly shines a bright white light in their eyes… they lose their dark adaptation almost immediately, and they have to wait a long time to recover it.

Each eye reacts separately to light, so you can keep one eye dark adapted while using your other eye to read star charts and move your telescope. An eye patch is ideal to keep one eye completely dark adapted. 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.

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Spectral sensitivity of rods and cones. Notice cone cells are not sensitive to light with wavelengths longer than 650 nm

You often see astronomers using bright red LED flashlights when looking at star maps and gear around the telescope. Doesn't the bright red light ruin their dark adaptation? No... because red light cannot bleach the dye in the rods if the wavelength is > 650 nanometers (see above). So the chemical structure of the dye in the rod cells is completely unaffected by red light, while the dye in the cones still enables scotopic vision.

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. Grape-seed extract is also supposed to help night vision.

Dark adaptation is not critical for casually observing bright stars and constellations. But you will always see more if you give your eyes time to adjust to the dark, and keep even brief flashes of bright light out of your eyes.

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A red flashlight for astronomy, a critical tool for seeing in the dark without ruining your sensitive night vision.

Fundamentals of Stargazing