Why Upside-down?

[gareththegeek]

This may be a stupid quesiton but something that has always puzzled me about optics is why the image needs to be upside-down and back-to-front.

My understanding is that the lens at the aperture causes the light to converge in a cone shape, the light is then allowed to cross-over before reaching the eye piece lens, which makes the edges of the light cone become parallel again.

.......___ aperture

\..../

.\../

..\/___ cross-over

../\ ___ eye piece

.|.| ___ eye here

What I don't understand is why the eye piece lens can't just cause the light to diverge before it reaches the point of cross-over, giving a magnified, in focus and correctly oriented image. Wouldn't you just need the lens to be cancave instead of convex (or the other way around, I get confused ).

......___ aperture

\..../

.\../ ___ eye piece

.|.| ___ eye here

I'm sure there must be a reason otherwise telescopes would work this way but what is it?

[brianb]

Yeah, a Galilean scope uses a concave lens at the eye end and has an erect image.

The disadvantages are legion: very small field of view being the most important. The concept of using a positive lens at the eye end goes back to Kepler, the inverted image was a small price to pay for the larger field of view once the magnification became large.

Modern opera glasses working at about x2.5 still use the Galilean principle. Other modern instruments with an erect image use extra mirrors and/or prisms and/or lenses to re-invert the image. For astronomy the image orientation makes no difference but it helps if you know what it is ...

[reddoss]

The main reason for not introducing erector lenses to make the image the right way up in astronomical telescopes is that they will attenuate some of the incoming light. They may not cut out much, but in astronomy the more light that forms the final image, the better.

The length of an astronomical telescope is the sum of the focal lengths of the Objective and the EP. The Objectve lens forms a real primary image in it’s 2nd focal plane

The primary image coincides with 1st focal plane of the eyepiece

If the EP were to be placed before the 2nd focal point of the Objective (whiich is what I thnk you mean by the cross over), the final image would not be in focus.