A terrestrial telescope is an optical instrument which is used for observing distinctly, the magnified images of distant objects on earth.
In an astronomical telescope, the final image is inverted w.r.t. the astronomical object. This does not matter because astronomical objects are spherical. But in a terrestrial telescope, the final image has to be erect. For this, we use an erecting lens held suitably between the objective lens O and the eye lens E.
The course of rays through a terrestrial telescope is shown in fig.
O is an objective lens of large focal length and large aperture. It forms a real, inverted and diminished image A’ B’ of the distant object, at its principal focus i.e. C1B’ = ƒ0 = focal length of objective lens. L is the erecting lens held on the same principal axis, such that B’C = 2ƒ, where ƒ is focal length of erecting lens. A real inverted image A”B” of same size is formed by the erecting lens L. now, A”B” is erect w.r.t. the object. The eye lens is so adjusted that A”B” lies at the principal focus of the eye lens. The final, virtual, erect and highly magnified image is produced at infinity by the eye lens. This is seen by the eye held close to the eye lens. The final image is erect w.r.t. the object, which was desired.
As the erecting lens produces no magnification, we can show that the magnifying power of terrestrial telescope is
m = ƒ0/-ƒe = ƒ0/|ƒe|,
When final image is at infinity
And m = ƒ0/ƒe = [1 + (ƒe/d)],
When the final image is at least distance of distinct from the eye.
1. The drawback of terrestrial telescope is that its length increases. In normal adjustment, length of telescope = distance between objective lens and eye lens = ƒ0 + 4ƒ + ƒe where ƒ is focal length of erecting lens.
2. Galilean telescope: also serves as a terrestrial telescope. Here, two lenses are used. Objective lens is convex and eye lens is concave. In normal adjustment, length of tube = (ƒ0 – ƒe). But field of view is much smaller in Galilean telescope, because eye lens is concave.
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