Walther Bauersfeld documented the treatment of the theoretical and design requirements for the development of the first projection planetarium in writing in his laboratory journal "Star Projection for the Munich Museum". The 603 numbered pages are preserved in the ZEISS archive. They provide information about essential ideas and development steps in the design process, which lasted several years.
The entry from May 1920 is the earliest evidence of the design of the two axes for the diurnal and annual motions (polar axis and ecliptic axis) and their intersection in the center of the star sphere. This solution was implemented in the first two projector models (Model I) and later also in the ZEISS ZKP 1 small size planetarium.