Telescopes focus light down to a point to increase the light gathering capacity of the astronomer’s eye. The optimal shape for such focus is a parabola, either a parabolic mirror, or a refracting lens of parabolic shape. Unfortunately, parabolic lenses do not have the same curvature everywhere the way spherical lenses do, making their construction without computer controlled equipment very difficult. Seventeenth century astronomers avoided this issue by constructing very shallow spherical lenses because at small angles, the spherical lens approximates the shape of a parabolic lens.
The lack of a single focal point from a spherical lense is called “spherical aberration.”
The problem with this arrangement is that the extremely shallow lenses had extremely long focal lengths, and therefore the telescopes during this era were extremely long.
While Christiaan Huygens was able to discover Titan with a 12-foot long refractor, astronomers of the time saw no reason to stop there. Such thinking led to the 150-foot long telescope constructed by Johannes Hevelius pictured in the drawing. Unfortunately, at these extreme lengths, the telescope was very difficult to move and susceptible minor changes in weather, but it was these limitations that led to the improvement of reflector telescopes that are primarily in use today.