Instruments on telescopes are used to make measurements of objects in space.
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: