How Does an Astronomical Telescope Form an Image at Infinity?

[rishch]

In astronomical telescopes, they use a convex mirror to from a real image, which is formed at the focus of the eyepiece lens, effectively forming an image at infinity. But how can it truly be at infinity? If it was truly at infinity then how could you see it? Also they say that image at infinity means less strain on the eye. Why? I think that by infinity they mean very far away.

 

[Drakkith]

In astronomical telescopes, they use a convex mirror to from a real image, which is formed at the focus of the eyepiece lens, effectively forming an image at infinity. But how can it truly be at infinity? If it was truly at infinity then how could you see it?

As you move the focuser in and out you modify the way the light cone comes out of the eyepiece. Focusing at infinity means that the light cone is pretty much a cylinder instead of an expanding cone when it comes out of the eyepiece, which allows your eye to focus it without flexing its lens, IE your eye focuses for infinity.

Normally the light entering your eye from everyday objects forms an expanding cone as it enters. Your eye focuses this light by making it into a converging cone of light. Closer objects have a steeper angle cone and you have to flex the lens in your eye more and more as an object gets closer and closer because the angle of the light cones becomes steeper and steeper. Objects "at infinity" can be said to have parallel rays entering your eye, or as I said earlier the light is no longer a cone but a cylinder.