Question

a) State two main considerations taken into account while choosing the objective of astronomical telescope. 

b) Draw a ray diagram of reflecting type telescope. State its magnifying power. 

c) State the advantages of reflecting type telescope over the refracting type?

Answer 1

(a)The two main consideration taken into account while choosing the objective of astronomical telescopes are:-

(i) light gathering power, which depends on the area of the objective.

(ii) Resolution or resolving power, which depends on the diameter of the objective.

(b)

when the image is formed at infinity:

magnifying power , m = \(\frac{f_o}{f_e}\) 

(c) Advantages of reflecting type telescope over the refracting type telescope.

(i) No chromatic aberration in the mirror,

(ii) If parabolic reflecting surface is chosen, spherical aberration is also removed,

(iii) weighs much less

(iv) can have a large focal length in a short telescope.

(v) image formed is much brighter.

Answer 2

(a) choice of objective 

(b) ray diagram of reflecting type telescope Formula of magnifying power

(c) stating two advantages

Answer 3

(a) (i) Diameter of the objective should be large so as to allow the maximum amount of light to pass it.

(ii) Resolving power

(b) 

When the final image is at infinity

\(\mathrm{m=\frac{f_0}{f_e}}\)

(c) Two advantages of reflecting Telescope over refracting Telescope are given as-

(i) No loss of intensity of incident light, therefore we get a bright image in reflecting telescope.

(ii) In a reflecting telescope, spherical aberration is removed by using parabolic mirrors, so there is no spherical aberration in reflecting telescope.