Averted Vision, How And Why

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Averted Vision, How And Why

Postby Dion » Wed Mar 30, 2011 11:43 am

You will often hear the term 'averted vision' used in astronomy, what is it and how does it work?

Well, see below a simple diagram of the eye :-

Image


Contrary to popular belief, the optic nerve doesn't come out directly at the back of the eyeball, it comes off to one side, simply because, there are no sensors at the point the optic nerve connects so if it was directly at the back, we would have a blind spot in the middle of everything we looked directly at. The blind spot doesn't really affect us in astronomy but if you're interested, a search on google will show you how to find your blind spot, so, your blind spot is off to one side.

So, the construction of the eye, basically, light enters the eye, hits the retina, and then signals of what it sees are sent to the brain, imaging a camera on a telescope, the scope is the eye, the camera is the brain. Now the eye is designed in humans from way back, when we were just mammals. The retina is coated with two different sorts of sensors, these are called 'RODS' and 'CONES'.

CONES work best in daylight and are what interpret colour

RODS work better at night and only see in black and white

Notice how when it's dark, you don't see colours too well?

The cones are more concentrated directly at the back of the eye, meaning, in daylight, you see better by looking straight at an object.

The rods are more concentrated on the edges of the eyeball, meaning your peripheral vision is more sensitive to the dark, remember we said, the eye was designed when we were simple mammals? A mammal is more prone to predator attacks at night, and the majority of predators do not attack face on, so, it's designed to help us avoid predators at night!

How does this affect our telescope vision?

Well, remember, it''s dark, so we wont see colours too well, but we do want to see more detail, so the rods are best for this job, if you look directly at an object in the scope, you will be using more cones, not as good, so look to the side of your desired object, you will see the object become clearer because you are now using more rods.

You can also practice this with the naked eye, a good target is the Pliedes cluster, look directly at it, and make note of how much detail you can see. Now look to the side of it and notice how it's brightness increases. This is averted vision.

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