The early evolution of the telescope presents a paradox: on the one hand, in 1611, the Galilean telescope perfected through successive attempts revolutionised the world; on the other, in that same 1611, the theoretical application of the laws of optics to combinations of lenses delineated a new type of instrument, which however was not constructed. In his Dioptrice (1611), Kepler showed in fact how to construct a telescope - known today as the Keplerian - replacing the concave ocular lens of the Galilean version with a convex lens.

The convex ocular lens solved the problem of the narrow field of view characteristic of the Galilean telescope, due to which the instrument could not magnify objects more than thirty times, a result already attained by Galileo. However, the combination of two lenses worsened the aberration that not only dimmed the clarity of telescopic images but also showed them upside-down. To eliminate the latter defect - astronomically insignificant - Kepler understood that it would be necessary to introduce at least a third convex lens; but this would have multiplied the aberrations of the two-lens system.

It was thus the inadequate quality of the lenses produced in the early 17 th century that caused the evolution of the telescope to proceed along the path indicated by Galileo.