The Mirror is Tweaked
By the middle of the eighteenth century, reflecting telescopes with primary mirrors up to six inches in diameter had been made. It was found that for large aperture ratios, f/10 or more, the difference between spherical and paraboloidal mirrors was negligible in the performance of the telescope. In the second half of the eighteenth century, in the hands of James Short and then William Herschel, the reflecting telescope with parabolically ground mirrors came into its own.