Question

(a) Draw a labelled ray diagram of a refraction type telescope in normal adjustment. 

(b) Give its two shortcomings over reflection type telescope. 

(c) Why is eyepiece of a telescope of short focal length, while objective is of large focal length? Explain.

Answer 1

(a) Refraction type telescope in normal adjustment.

(b) Shortcomings —
(i) Very difficult and expensive to make large sized lenses, which can form images that are free from any kind of chromatic aberration and distortions.

(ii) By lenses of diameter 1.02m and above tend to be very heavy and therefore, difficult to make and support by their edges

(c) If eyepiece of short focal length and objective lens of large focal length are used in constructing the telescope, magnifying power increases, as per relation M = f₀/f_e.
This telescope of high magnifying power can see path of stars of actual separation of one minute arc and even less values of arc.