Question

Draw a labelled ray diagram of an astronomical telescope in the near point adjustment
position
A giant refracting telescope at an observatory has an objective lens of focal length
15 m and an eyepiece of focal length 1.0 cm. If this telescope is used to view the
Moon, find the diameter of the image of the Moon formed by the objective lens. The
diameter of the Moon is 3.48x10°m, and the radius of lunar orbit is 3.8 x 10 m.
​

Answer 1

Here, f0=15cm,fe=1.0cm=10−2m

(a) Angular magnification =−f0fe=−1510−−2=−1500

(b) If d diameter of the image, then from Fig.

angle subtended by diameter of moon θ1=l1r1=3.48×1063.8×108

and, angle subtended by image θ2=l2r2=df0=d15

As thta2=θ1∴d15=3.48×1063.8×108

d=3.48×15×10−23.8

=13.73×10−2m=13.73cm