Newton has been generally credited with constructing…

Newton has been generally credited with constructing in 1668 the first practical reflecting telescope.

It uses a spherically ground metal primary mirror and a small diagonal mirror in an optical configuration that will come to be known as the Newtonian telescope.

Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the speculum metal mirrors being used at the time means it will it take over one hundred years for these devices to become popular.