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| Reflecting Telescope |
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| Refraction Telescope |
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| The objective lens is replaced by a concave parabolic mirror of large aperture. The images in such telescopes are brighter and have a high resolving power compared to astronomical telescope. |
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| Cassegrainan Reflecting Telescope |
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| where O-objective, B-convex mirror, E- eye piece |
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| This consists of an objective lens O, a convex lens of large focal length and aperture and an eyepiece (E), a concave lens of a short focal length and small aperture separated by a distance equal to the difference in their focal lengths. The parallel beam passing through the objective would have converged at a distance f0 to form the image A1B1. But the eyepiece intercepts these rays and converts them into a parallel beam. |
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| The magnifying power is given by |
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| Telescope is given by |
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| L = fo - fe |
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| The advantages of this telescope is |
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 The final image is erect |
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The length of the telescope is small |
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| The disadvantages are: |
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The field of view is small |
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Cross wires cannot be used |
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