Big in every way

The impressive lens size, which is the glory of the telescope, is also why Yerkes is a sort of scientific dead end. Simply put, there’s a practical limit to how large you can make a lens, and Yerkes’ 40-inch telescope butts against that limit. Lenses are made from a single piece of material, which must be free of defects. (Mirrors only have to be defect-free on the surface, so they can be made much bigger, or even built up out of multiple segments.) Even if it’s perfect after manufacture, changing gravitational stresses as the telescope moves can alter the shape of the lens subtly, leading to distorted images. In addition, the edges of lenses are tapered, which makes them act like prisms, splitting the colors of light from whatever object is being observed.

Finally, the larger the lens, the longer the telescope tube must be to focus the resulting image. The Yerkes refractor is 63 feet (19 meters) long, which demands a correspondingly huge building to house it. While it makes for an architectural marvel, the 90-foot-diameter observatory dome at Yerkes was also an engineering challenge. The entire dome must rotate to allow the telescope to point at various objects, and because the tube is so long, a large section of the floor itself moves up and down to allow it to point at higher angles on the sky. (By comparison, Yerkes Observatory also has a 40-inch reflecting telescope, which is a better instrument and its dome is less than half the diameter of the 40-inch refractor’s.) In 1897, electricity was still relatively novel, so Yerkes Observatory was constructed with its own electrical plant to supply direct-current (DC) power to the motors.

The building is ornate inside and out, in many parts more like a museum than a working science facility. Terra-cotta animals, astrological motifs (ironically enough), and fanciful creatures cover the pillars outside, while the interior is graced with marble floors and high ornate ceilings. The dome for the 40-inch telescope is cathedral in scale, with a great deal of lovely cast-iron and wooden fittings. The machinery — pulleys and cables, huge gears and motors — are veritable works of art in their own right.