Q.What is meant by the resolving power of an astronomical telescope? State the factor on which it depends, and explain why astronomical telescopes are built with objectives of as large a diameter as possible.

By the same Rayleigh criterion applied to the circular objective aperture, a telescope of diameter $D_{ap}$ can just resolve two point objects separated by an angle $d\theta=1.22\lambda/D_{ap}$. Resolving power $=1/d\theta=D_{ap}/(1.22\lambda)$ increases …