Question

Why do you need to adjust the second lens of a telescope to create a sharp image? What are you
actually trying to do?

Answer 1

To make a telescope you can actually look through, you'll need to add another lens. This eyepiece lens magnifies the image formed by the large objective lens and directs the light to your eye. Basically, the eyepiece works a lot like a magnifying glass; it enables your eye to focus much more closely than you normally can. The eyepiece on a typical telescope allows you to inspect the image formed by the objective lens from a distance of an inch or less. The diagram below shows how the objective lens and eyepiece work together in a simple telescope.