Re: what happens to light that enters a telescope lens not parallel to the tube

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.