You can spend thousands of dollars on a big, advanced telescope, hundreds more on the best eyepieces money can buy, and still not see much of anything at all. Unless, that is, you know you to extract maximum light and detail with your eye. There are two concepts every stargazer should know to get the best visual views: averted vision and dark adaptation. Here’s how it all works…
Averted Vision
Averted vision is a technique in which you look off to one side to expose the most sensitive part of your eye to better see much fainter objects. If you’ve never tried this before, you’ll be amazed how well it works, with or without a telescope.
Here’s why averted vision 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, the area 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 with the rods, as you notice when, for example, you try to read a magazine with your peripheral vision. Rods do not detect color, they are far more sensitive to light.
Your eye is structured such that you see the faintest objects if you look 16-20 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 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.
Rods 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 appear greyish-white.
The blinking nebula, NGC 6826, is an object that most dramatically demonstrates averted vision. Stare directly at this blue-green planetary nebula and you see only the dim central star. Look slightly to the side and the faint nebula around the star appears suddenly. When you switch from straight on to averted vision, the nebula appears to blink on and off. It’s darned impressive.
Dark Adaptation
The human 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 in the last article, 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. 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 move your telescope. An eye patch is ideal for this.
You can 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 >650 nanometers. So the chemical structure of the dye in the rods is completely unaffected, 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.
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