Refracting Astronomical Telescope
An astronomical telescope is used to view heavenly bodies. The simple astronomical consists of two lenses. The objective is a convex lens of long focal length and forms a real image of the distant object. The eyepiece is a convex lens of small focal length.
Normal adjustment: The telescope is in normal adjustment when the image is formed at infinity and the distance between the objective and the eyepiece is equal to the sum of their focal lengths. Fig. illustrates the formation of the final image at infinity when the telescope is used in normal adjustment. The image AB is formed at the focus of the objective and OA is equal to the focal length of the objective. The point A is also the focus of the eyepiece and the final image is formed at infinity.
To calculate the angular magnification M of the telescope, it is assumed that the eye is very close to the eyepiece. Moreover, the distance between the objective and the eyepiece is very small compared with the distance of the object from the objective or the eyepiece. The angle 
subtended at the unaided eye by the object can be taken as the angle subtended at the objective by the object.
OA = focal length of the objective = F
AE = focal length of the eyepiece = ƒ
In the Δ ABE,
In the Δ OAB
Thus the magnification is equal to the ratio of the focal lengths of the objective and the eyepiece. To increase the magnification of the telescope, the objective should be of large focal length and the eyepiece should be of short focal length.
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