Kepler’s design for an improved refractor (1611)

Any spherical lens brings light rays to many focal points, resulting in blurry images. Kepler envisioned a less curved spherical primary lens with a longer focal length than Galileo had used. The less curved lens would make light rays travel farther before they crossed, bringing the focal points closer together. This would reduce, but not eliminate, image fuzziness.

The convex eyepiece lens would give the telescope the additional benefit of a wider field of view.