King Frederick II granted Tycho an estate on the island of Hven and the funding to build Uraniborg, an early research institute, where he built large astronomical instruments and took many careful measurements, and later Stjerneborg, underground, when he discovered that his instruments in Uraniborg were not sufficiently steady. On the island (where he behaved autocratically toward the residents) he founded manufactories, such as a paper mill, to provide material for printing his results. After disagreements with the new Danish king, Christian IV, in 1597, Tycho went into exile. He was invited by the Bohemian king and Holy Roman Emperor Rudolph II to Prague, where he became the official imperial astronomer. He built an observatory at Benátky nad Jizerou. There, from 1600 until his death in 1601, he was assisted by Johannes Kepler, who later used Tycho's astronomical data to develop his three laws of planetary motion.