GREGOR: Optics redesign and updates from 2018–2020

The GREGOR telescope was inaugurated in 2012. In 2018, we began a complete upgrade, involving optics, alignment, instrumentation, mechanical upgrades for vibration reduction, updated control systems, and building enhancements, and in addition, adapted management and policies. This paper describes all major updates performed during this time. Since 2012, all powered mirrors except for M1 were exchanged. Since March 2020, GREGOR observes with diffraction-limited performance and a new optics and instrument layout.