One: Gather as much light as possible from faint things like galaxies. Two: Create very sharp images, so they can do things like find planets around a distant star. Most astronomical telescopes start with that first target, acting like light-buckets.

What do u look through a telescope?

The Moon, Jupiter, Saturn and the Orion Nebula and are terrific to see with smaller telescopes. To see the more distant and fainter objects such as galaxies and other nebulae, you might find you need a telescope with a larger mirror.

Why is looking through a telescope seeing back in time?

Because light takes time to travel from one place to another, we see objects not as they are now but as they were at the time when they released the light that has traveled across the universe to us. Astronomers can therefore look farther back through time by studying progressively more-distant objects.