Why are concave mirrors used in a telescope instead of using a plane mirror?

1. A telescope is a device used to see and observe distant objects.
2. The concave mirror is a converging mirror because it converges the light falling on the surface of mirror.
3. Since it is a converging mirror, it converges the light from a far-off object(infinity) to a point on focus.
4. All mirrors in telescopes are to collect/concentrate and focus the received light at focus.
5. Plane mirrors do not converge the light.

Hence, concave mirrors are used in a telescope instead of using a plane mirror.