Which are two main points important for astronomical telescope ?

Explanation

The main considerations with an astronomical - telescope are its light gathering power and its resolution or resolving power.
The former clearly depends on the area of them objective.
With larger diameters, fainter objects can be observed.
The resolving power or the ability to observe two objects distinctly, which are in very nearly the same direction, also depends on the diameter of the objective. So the desirable aim in optical telescopes is to make them with objective of large diameter.
The largest lens objective in use has a diameter of 40 inch (- 1.02 m). It is at the Yerkes Observatory in Wisconsin, USA.
Such big lenses tend to be very heavy and therefore, difficult to make and support by their edges.
Further it is rather difficult and expensive to make such large sized lenses which form images that are free from any kind of chromatic aberration and distortions.