why does the objective of an optical telescope have larger focal length and long aperture . also explain why does a deflector give better image of a small portion of the celestial body.
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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.
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The larger the telescope's aperture, the more light the telescope can gather making the image brighter, sharper, and able to produce more detail. A longer focal length will produce a larger image and a shorter focal length will produce a smaller image. 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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