The bigger the convex lens or the concave mirror, the more light it can collect. The more light it collects, the fainter the objects it allows us to see. The human eye has a convex lens, but the lens is small, and can only collect a small amount of light. Telescopes allow us to see more because their mirrors and lenses can collect more light than the human eye can collect on its own.
To create a clear image, telescope lenses and mirrors bring light rays to meet at a single point, the focal point. If the light rays don’t meet at the same point, the image is blurry. The shape of the lens or mirror is designed to make light rays meet at a single focal point.
Once an image forms, it must be recorded for research. Originally, people drew the images they saw through telescopes. Later, photography allowed people to take pictures of the image on film. Today, astronomers use charge-coupled devices (CCDs), electronic light-sensing devices like those in digital cameras, to record images.