So he could not determine the functional dependence of the gravity force on the separation distance of the two objects. It also varied, he noted, with the product of the masses. To do so he had to invent the integral calculus. His inverse square law tested out very well within the bounds of the uncertainty of the lunar-earth distance. Another astronomical advance was Newton s invention of the reflecting telescope.
Optical telescopes come in two major categories now refracting and reflecting.
The former uses lens, the latter mirrors. Refracting telescopes require lens which are difficult to manufacture the slightest imperfection throughout the volume of the glass can corrupt the image. The dispersion effects of light cause Chromatic Aberration in which colored rings circle the image. The lens can only be supported at its periphery since light must pass through it. The mirror on the other hand can be supported on the entirety of its backside.