The telescope was one of the central instruments of what has been called the Scientific Revolution of the seventeenth century. It was the first extension of one of man's senses, and demonstrated that ordinary observers could see things that the great Aristotle had not dreamed of.
Although the magnifying and diminishing properties of convex and concave transparent objects was known in Antiquity, lenses, as we know them, were introduced in the West at the end of the thirteenth century. But the telescope was not the invention of scientists; rather, it was the product of craftsmen. For that reason, much of its origin is inaccessible to us since craftsmen were by and large illiterate and therefore historically often invisible.
Today you can buy a better telescope than Galileo's for under a hundred dollars, and a careful replica of his telescope for a great deal more. An ordinary pair of binoculars will show just about all that he saw