Monday, 30 May 2011

Observing 101: Averted Vision

Averted Vision: Getting The Most From What Nature Gave You

Using averted vision means looking slightly off to one side rather than straight on. It exposes the most sensitive part of your eye and lets you see much fainter objects. If you’ve never tried this before, you’ll be amazed at how much more you can see, with or without a telescope.
The Basics
• 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.
• 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. Do the reverse and you’ll expose the blind spot of your eye and you won’t see a thing.

A cross section of the human retina, showing rod and cone cells.
A Deeper Look
• 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 grayish-white.

The sensitivity of rods and cones to light across the spectrum
Good To Know
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.
Personal View
I usually begin with a short lesson on averted vision when showing faint objects to beginners. When they try it, they usually gasp at the subtle detail that suddenly appears.

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