The first collection of star maps engraved on a grid so that star positions could be read off to fractions of a degree. It was also the first to use the modern system of identifying the stars within a constellation by the use of Greek letters for the brighter stars and Roman letters for the fainter ones. Bayer's main sources for star positions were the recent observations of Tycho Brahe and of Pieter Dierkzoon Keyser (with regard to the southern hemisphere). The artist Alexander Mair clearly found some inspiration in the Jacobo De Gheyn engravings in the Aratea published by Hugo Grotius in 1600, but most of Bayer's constellation figures are quite different from De Gheyn's, and generally more attractive. Many have no known prototype.