Why doe the objective of an optical telescope have large focal length & large aperture? Also explain why does a reflector give better image of a small portion of the celestial body?
Answers
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If you want to see a small portion of a celestial body, like details on a planet, a long focal length is required to get enough magnification in the final image. Diffraction by the aperture formed by the telescope’s objective determines the resolution obtainable by any telescope. There must be sufficient magnification so that the eye can see the detail or so that the detail is larger than the sensor resolution of the camera. Typical effective focal lengths used for detailed planetary images in amateur telescopes range from 2500 to 5000 mm. Objective diameters of up to about 200 mm can be effective before atmospheric seeing limitations dominate diffraction limits.
Reflectors have a secondary mirror which introduces additional diffraction which limits the resolution and contrast of planetary images. Refractors do not have this limitation. Refractors are often preferred for the sharper and higher contrast views that they offer at higher magnifications with the same size objective.
Reflectors have a large cost advantage at moderate to large sizes and that is why they are used, not because of a superior image.
On the earth’s surface atmospheric seeing (turbulence) limits resolutions at high magnification. Large observatory instruments at high altitudes with adaptive optics and space based telescopes can profit from the resolution of larger objectives. Refractors are impractical at sizes above 1000mm because of the difficulty of supporting a heavy unobstructed lens. This is well above the size at which blurring due to atmospheric turbulence dominates diffraction limitations.