Amateur Astronomers Association of Princeton

by Rex Parker

An astronomy field trip to spice up the summer family visit back in the home state… destination: merely the birthplace of modern astrophysics!

I was bound for Yerkes Observatory, home to the world’s largest refracting telescope, the Clark 40-inch. Playing my AAAP, Princeton astronomy connections rather well it seems, I arranged for a custom personal tour graciously hosted by Yerkes expert and author, Dan Koehler. Dan agreed to meet me, my wife and two good friends from Indy, for a personal evening tour of the Observatory and telescopes on August 2. Yerkes is about 75 miles NW of Chicago, yet my enthusiasm wasn’t dampened when I realized this was a 260 mile drive from Indy! We arrived mid-day at beautiful Geneva Lake, which is pretty famous as a resort for well-to-do’s from Chicago. After a lake swim and lunch in town, we hopped on over to Yerkes just as a violent Midwestern lightning storm hit, complete with one-inch hail and 50-mph gusting winds. As suddenly as it started, the storm subsided, and we strolled the beautiful park-like grounds (designed by John Olmsted, brother of NYC Central Park’s famed landscape architect Frederick Law Olmsted). Dan Koehler arrived, and we embarked on a wonderful, informative four-hour guided tour back into time and space.

Dan Koehler is the executive producer of the well-received PBS video “400 Years of the Telescope”.

He is an amazing historian and astronomer who knows Yerkes inside-out, and is now writing a book on the life of George Ellery Hale and Yerkes. He has dedicated years of his life (and a lot of blood sweat and tears) to preserving Yerkes facilities, telescopes, and lore. We learned how Hale, as a young man fresh from MIT and now professor of Astrophysics at the new University of Chicago, in 1893 convinced the famed opticians, Alvan Clark and Sons, to figure a 40-inch objective lens from French blanks, which had become available only when USC abandoned plans to make the world’s largest telescope. Hale and William Raney Harper, the first president of the university, talked industrialist Charles Tyson Yerkes into paying for the immense and beautiful observatory structure to house the 40-inch in 1895. The choice of Geneva Lake as the site is an interesting one. History and scientific legends drip from the ceilings and domes. The architecture by Henry Ives Cobb is stunning neo-classical with hundreds of figures and gargoyles melded into the structure.

Through the years, astronomical greats conducted cutting-edge research here. George Ellery Hale first detected carbon in the sun’s spectrum here, long before he moved on to become the force behind Mt. Wilson. Gerard Kuiper began his studies of solar system objects at Yerkes. Sherbourne Burnham and Edward Barnard conducted their binary star and stellar mass observations here. Frank Schlesinger developed the parallax methods using the 40-inch which led to precise distance determinations of nearby stars. Otto Struve developed his stellar spectroscopic techniques using the 40-inch, and Chandrasekhar (Nobel 1983) started his work on white dwarfs at Yerkes. William Morgan conducted studies of luminous blue stars here. The list goes on, true to the claim of being the birthplace of modern astrophysics.

If the stories weren’t enough, we were then taken into the main dome housing the 40-inch – what a gigantic yet stately telescope it is.

It became clear that the statement, “no one will ever build a larger refractor”, is very likely true (limited as much by the fluid properties of glass as much as money). Noble instrument indeed! The 65-foot long, f/19 telescope hangs in perfect balance on the four-story tall mounting edifice. It was built like a battleship using iron thick enough to stop an incoming six- inch shell. I personally activated the drive to open the dome, and with my pulse quickening I slewed the telescope in RA with push button controls. The immensity must be seen to believe. The entire floor rises up and down as a giant elevator to position the observer at the instrument/eyepiece end of the refractor! Amazingly, the huge original DC drive motors, which slew the scope and drive the dome, are still in place and working perfectly. While it has been upgraded to accept modern detectors and instruments, all components of the 40-inch scope are essentially original and working. The pictures give a glimpse of the uniqueness of this instrument. If you ever find yourself in the Chicago / Milwaukee area, take the time to visit this wonder of the astronomy world.