Simple telescope (lenses)

[Alan Anderson]

Originally Posted by sanddrag

So that's really all there is to it? Let me see if I've got thsi straight. If you go down the telescope, here's what happens. The big objective lense gathers light and eventually focuses it to a sharp point. Then the light leaves this point invetered and the rays are going away from each other to make an enlarged image. Then a short distance away, they hit the eyepiece, where they are re-bent to be straight and focused again.

Does that sound right?

It's close. The "sharp point" focus is only valid for a point source of light like a distant star or a laser. The more general description is that the light from each individual point on the object is focused to a separate point in the middle of the telescope, forming an inverted image there.

(By the way, I was wrong before -- it's a "real" image, not a "virtual" image. It can be projected onto a piece of paper at the focal point.)

The second lens doesn't quite "straighten out" the light; it just bends it in the manner of a typical magnifying glass. What you see is then a virtual (I got it right this time) image that's a larger version of the first real image.

Since the eyepiece magnifier doesn't invert anything, the final view is indeed reversed in all directions, just as if it had been rotated 180 degrees. More complex optics, typically mirrors and/or prisms, can re-invert the image to make it match the real-world orientation; that's how fancy binoculars and so-called "terrestrial" telescopes work. (Many reflector telescopes have a "diagonal" mirror that reverses the image in one direction, and some refractors also include an optional "star diagonal" to make it easier to observe things high overhead and which incidentally does the same single-axis reversal.)