John Hadley's Reflector.
The First Modern Newtonian.
Isaac Newton's design for a reflecting telescope made possible a whole new breed of powerful instruments with exciting possibilities. But the reflector was not yet a practicality, because Newton's mirror was spherical, and not very well polished. Only 16% of the light was transmitted, and only low powers could be used.
John Hadley solved several of the design's most serious problems when he produced, with the help of his two brothers, a 6" f/10 Newtonian with a novel yet steady mounting. The instrument was first demonstrated to the Royal Society in 1721. The Society was so impressed that they gave it to Edmund Halley (for detailed examination, of course) and commissioned another from Hadley.
When Halley turned in his report he said that the little telescope was in every way the equal of Christian Huygens' 123-foot aerial refractor (this was before the days of achromats). Halley reported that he was able to use powers as high as 200X, and that he had observed the shadows of two satellites on Jupiter's disk. He experienced no difficulty in detecting Cassini's division in Saturn's ring. He later saw the shadow of one of Saturn's satellites on the disk, a very difficult observation, even today.
But the most important aspect of Hadley's accomplishment is that he published his methods of polishing and parabolization, and instructed others in them. The reflector was launched on its journey from toy to mountaintop glass giants.