We know that resolving power

R.P. = λ/d λ nN

and dispersive power D.p. dθ /D λ =n /(a+b) cos θ _{n}

Therefore, λ /d λ= nN= N (a+b ) cos θ_{n} n /(a+b) cos θ_{n}

λ/ d λ =Ax dθ /d λ

Resolving power = total aperture of telescope objective x dispersive power.

The resolving power of a grating can be increased by

(i) Increasing the number of lines on the grating N.

(ii) Increasing the sides of spectrum n.

(iii) Increasing the total width of grating ‘w’, for which one has to make use of whole

aperture of telescopes objective.