Optical Instruments: Terrestrial Telescope

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As astronomical telescope forms inverted final images of heavenly bodies like moon and stars which are acceptable. But when terrestrial objects are to be viewed, it is necessary to have an erect final image. The erection of image can be made by introducing a third lens between objective and eye-piece of telescope.
This modified telescope is known as "Terrestrial Telescope" whose magnifying power is just equal to the magnification of astronomical telescope but it just gives erect image.

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Terrestrial telescope contains three lenses as compared to the astronomical telescope. It is also known as the spyglass. As astronomical telescope forms an inverted image of the object so, the main difference between the astronomical and terrestrial telescope is the erection of the final image with respect to the object. The third lens of short focal length f is placed at 2f which forms inverted image of the object. This image serves as the object for the eye-piece. The lens placed in the center of the telescope which actually erects the image is called as the Erecting lens. The resolving power of the telescope can be given by the relations as follows:

$M=-\frac{f_{o}}{f_{e}}\times (-1)=\frac{f_{o}}{f_{e}}$