|MadSci Network: Physics|
The easy way to think about this is to remember that light rays going through the centre of the lens are undeviated (to good approximation). So a telescope looking at a whole scene will form an image of that scene one focal length away from the lens (providing the scene is all fairly distant). So what happens to rays coming in at other angles (from the edge of the scene as it were) is that they will form an image providing the angle isn't so great that they hit the side of the tube before they reach focus. Of course, the eyepiece is then used to view this image, and generally will not see the whole of the image of the scene formed inside the tube. If the telescope is well blackened (and baffled) inside, then the rays hitting the side of the tube will have no effect. If not, then they could be reflected back into the field of view causing "ghosting" effects in some circumstances.
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