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# Astronomical telescope

An astronomical telescope is used to get the magnification of distant astronomical objects like stars, planets, moon etc.

Astronomical telescope

An astronomical telescope is used to get the magnification of distant astronomical objects like stars, planets, moon etc. The image formed by astronomical telescope will be inverted. It has an objective of long focal length and a much larger aperture than the eyepiece as shown in Figure 6.87. Light from a distant object enters the objective and a real image is formed in the tube at its second focal point. The eyepiece magnifies this image producing a final inverted image. ## Magnification of astronomical telescope

The magnification m is the ratio of the angle β subtended at the eye by the final image to the angle α which the object subtends at the lens or the eye. The length of the telescope is approximately, L = fo + fe

### EXAMPLE 6.43

A small telescope has an objective lens of focal length 125 cm and an eyepiece of focal length 2 cm. What is the magnification of the telescope? What is the separation between the objective and the eyepiece? Two stars separated by 1’ will appear at what separation when viewed through the telescope?

### Solution

fo = 125 cm; fe = 2 cm; m = ?; L = ?; θi = ?

Equation for magnification of telescope, m = fo/ fe

Substituting, m = 125/2 = 62.5

Equation for approximate length of telescope, L = fo+ fe

Substituting, L = 125+2 = 127 cm = 1.27 m

Equation for angular magnification, m = θi0

Rewriting, θi = m×θ0

Substituting,

θi = 62.5×1′ = 62.5′ = 62.5/60 = 1.04º

Tags : Optical Instruments , 12th Physics : UNIT 7 : Wave Optics
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12th Physics : UNIT 7 : Wave Optics : Astronomical telescope | Optical Instruments