The telescope at a small observatory has objective and eyepiece focal lengths re

Question

The telescope at a small observatory has objective and eyepiece focal lengths respectively of 18.4 m and 12 cm. If this telescope is used to view a 1620 m diameter lunar crater on the surface of the moon

from the surface of the Earth, determine the following.

(a) angular magnification of the telescope


(b) size of the first image
m

(c) length of the barrel of the telescope
m

(d) angle subtended at the unaided eye by the lunar crater
rad

(e) angle subtended at the eye when the lunar crater is viewed through the telescope
rad

Explanation / Answer

We know for telescope, magnification is
M = Fo/Fe = B/A
where
B = angle subtended by final image at eye
A is angle subtended by object at eye

M = 18.4/12 = 153

For objective lens Hi/Ho = V/U = 4.21*10-10
or Hi which is inverted = 162 * 4.21*10-10 = 6.82 * 10-7 m

tanA = Hi/V = 6.82 * 10-7/ 18.4 = 37*10-8

or A = 2.12 * 10-5 radians

B/A = 153
or B = 3.24*10-6 rad

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