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Galileo Telescope

This telescope was first constructed by Galileo at Padua in 1609, which provides an erect image of an object with the aid of only two lenses. It consists of an objective which is a convex lens of large focal length and an eyepiece which is a concave lens of short focal length. The length of the telescope

OE = F – ƒ,

where F is the focal length of the objective and ƒ is the focal length of the eyepiece. The image of the distant object in the objective is formed at A where OA = F. As the eyepiece is at a distance ƒ from A, the rays falling on it are refracted and emerge parallel. The exit pupil R is virtual.

The angular magnification

Thus, the angular magnification is given by the ratio of the focal lengths. For high magnification, an objective of long focal length and an eyepiece of short focal length are required.

Final image at finite distance: When the distance between the objective and the eyepiece is less than (F – ƒ), the final image is formed at a finite distance and is erect.

(a) Let the distance between the objective and the eyepiece be diminished by x.

Then, F_eF_e = x

and AE = ƒ + x

OA = F

Angular magnification

(b) Let the final image PQ be formed at the near point.

Then, EP = D, the least distance of distinct vision

∴ v = –D, u = AE, and ƒ is –ve

Angular magnification

The exit pupil or the eye ring in a Galileo telescope is virtual and is situated between the objective and the eyepiece. The observer’s eye has to be placed close to the eyepiece because the eye ring is virtual. Thus, the field of view is restricted in this telescope. Moreover, cross wires cannot be used in this telescope because the eyepiece is negative.