Yerkes Observatory, in Williams Bay, Wisconsin, houses the largest refracting telescope ever built for astronomical research, with a main lens that's 40 inches (1.02 meters) in diameter. 

 

The observatory is a facility of the University of Chicago and is housed in an ornate Romanesque-style building located on a 77-acre park-like site near Lake Geneva, Wisconsin, about a two-hour drive northwest of downtown Chicago. It opened in 1897 and was closed to almost all activity in 2018.

The Yerkes refracting telescope was gigantic for its time, with a main tube that's 60 feet long (18 m). A refracting telescope is one of the two main types of astronomical telescopes. A refractor has the familiar spyglass design, which operates by gathering light through a large len at one end of a long tube and focusing the light on an eyepiece, camera or other instrument at the opposite end. 

According to space.com

 

The observatory is a facility of the University of Chicago and is housed in an ornate Romanesque-style building located on a 77-acre park-like site near Lake Geneva, Wisconsin, about a two-hour drive northwest of downtown Chicago. It opened in 1897 and was closed to almost all activity in 2018.