which subjects has he worked on?
Mathematics and astronomy
Johannes Kepler
Kepler's primary obligation as imperial mathematician was to provide astrological advice to the emperor. Though Kepler took a dim view of the attempts of contemporary astrologers to precisely predict the future or divine specific events, he had been casting well-received detailed horoscopes for friends, family, and patrons since his time as a student in Tübingen. In addition to horoscopes for allies and foreign leaders, the emperor sought Kepler's advice in times of political trouble. Rudolph was actively interested in the work of many of his court scholars (including numerous alchemists) and kept up with Kepler's work in physical astronomy as well.
io and europa the inner two of jupiter's four largest moons They include also Ganymede, and Callisto what are these four moons collectively known as? The Galilean moons who discovered them? Galileo Galilei are these moons visible from earth? Yes through a telescope are there different type of this instrument? Yes, reflecting telescope, radio telescopes  and infrared telescopes who invented it? Hans Lippershey in what kind of research is the first type that you mentioned used? In astronomy as well as for terrestrial viewing are there any variations of this type of instrument? Yes the Keplerian telescope who discovered it? Johannes Kepler what is his birthplace? In the Free Imperial City of Weil der Stadt has he worked as an advisor? Yes to Emperor Rudolph II
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Johannes Kepler [SEP] Advisor to Emperor Rudolph II
Kepler's primary obligation as imperial mathematician was to provide astrological advice to the emperor. Though Kepler took a dim view of the attempts of contemporary astrologers to precisely predict the future or divine specific events, he had been casting well-received detailed horoscopes for friends, family, and patrons since his time as a student in Tübingen. In addition to horoscopes for allies and foreign leaders, the emperor sought Kepler's advice in times of political trouble. Rudolph was actively interested in the work of many of his court scholars (including numerous alchemists) and kept up with Kepler's work in physical astronomy as well.