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

Larger aperture because that creates a larger light collecting area and provides better resolution of small details. Longer focal length makes it easier to make the lens or mirror, since optical elements with really deep curves require a lot more precision in fabrication. Longer focal length also allows for greater magnification.
The reflector telescope uses a mirror to gather and focus light. All celestial objects (including those in our solar system) are so far away that all of the light rays coming from them reach the Earth as parallel rays. Because the light rays are parallel to each other, the reflector telescope's mirror has a parabolic shape. The parabolic-shaped mirror focusses the parallel lights rays to a single point. 
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