Question

why does the objective of an optical telescope have focal length and large aperture?also explain why does a reflector give better image of a small portion of the celestial body

Answer 1

The big lens in the telescope (objective lens) collects much more light than your eye can from a distant object and focuses the light to a point (the focal point) inside the telescope. A smaller lens (eyepiece lens) takes the bright light from the focal point and magnifies it so that it uses more of your retina.