When astronomers first used telescopes, they looked through them and drew or described what they saw. However, with the invention of the camera, it became possible to take pictures through a telescope.
This means that not only can astronomers have a record of what they saw, but by using longer and longer exposure times (i.e. collecting more light), they can make measurements of fainter and fainter objects.
Nowadays, telescopes can have a whole host of different instruments, each for making different sorts of measurements.
These instruments include: