Dioptrice

In the preface, Kepler highlights several examples of the close links which, then as now, exist between optics and astronomy. His excitement about Galileo's new observations is palpable, especially the discoveries of the four moons orbiting Jupiter and the phases of Venus. Both discoveries lent crucial support to the Copernican system, of which Kepler had been an ardent supporter since his days as a student in Tübingen. 

The Dioptrice is available here in a complete English translation for the first time, along with an introduction and numerous notes to help guide the reader through Kepler's text.