The 15-inch Great Refractor at the Harvard-Smithsonian in Cambridge is much smaller, but still in working condition, and open every Thursday night to visitors. It was the largest in the U.S. in 1847. (There is also a public lecture.) The original bearings for the dome were iron cannonballs, but they were melted down during World War II, and replaced with roller bearings. Two of the cannonballs are still on display, worn elliptical by 93 years of use.
The great refractor was a pioneer in astrophotography.
https://pweb.cfa.harvard.edu/about/about-harvard-college-observatory/about-great-refractor
For a number of years I lived a block away from “The Ob” as it was known then. It’s nice to know that the instrument is preserved, but with all the artificial light in the city there’s not much to see.
Respect must be paid to Alvan Clark, who learned to grind lenses in middle age, and produced quite a few for observatories around the country. The 40 objective for the Yerkes was his largest. Working without computers and lasers, it was quite a feat.