Question

(i) Draw a schematic labelled ray diagram of a reflecting type telescope. 

(ii) Write two important advantages justifying why reflecting type telescopes are preferred over refracting telescopes.

(iii) The objective of a telescope is of larger focal length and of larger aperture (compared to the eyepiece). Why? Give reasons.

Answer 1

(ii) Advantages : 

(a) Parabolic mirror is used to remove the spherical aberration. 

(b) No chromatic aberration in mirror. 

(c) Light mechanical support is required, because mirror weighs much less than a lens of equivalent optical quality. 

(iii) In normal adjustment, magnifying power of the telescope M = F₀/Fe. 

(a) If focal length of the objecctive lens is large in comparison to the eyepiece, magnifying power increases. 

(b) Resolving power of the telescope RP = a/1.22λ. To increase the resolving power of the telescope, large aperture of the objective lens is required.