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Big Gratings: The Ruling Engines at Mount Wilson

“When Rowland died in 1901, production of gratings ceased. In 1908, Nobel laureate physicist Albert Michelson stepped in and built a ruling engine at the University of Chicago, but it did not perform up to expectations. Nevertheless, a few of his gratings were used at Mount Wilson. By 1912, Hale decided he needed to do something to secure better, and hopefully bigger, diffraction gratings for the spectrographs for his telescopes. Larger gratings meant better resolution of the absorption lines, both for their positions and widths. In the plans for the new Mount Wilson Observatory’s offices, he included a special room below the basement to house what became known as Ruling Engine “A.” Carnegie funds were allotted to the effort. He invited physicist John Anderson on extended stays to advise on its construction. Anderson had recently revived and improved Rowland’s machines at Johns Hopkins. In 1916, Anderson was hired permanently. Hale also recruited master instrument builder Clement Jacomini from Italy to do the machining. Before “A” even started producing gratings, Hale promised one to physicist Robert Millikan (who needed one to advance his research in the ultraviolet) to lure him from the University of Chicago to the newly founded Caltech. Along with Millikan came Ira Bowen, who later became the first director of the combined Mount Wilson and Palomar Observatories. Mostly between 1920 and 1934, the “A” machine ruled some 85 gratings, many of which were used to make significant discoveries in astrophysics and atomic physics.

Ruling Engine “A” (1922)

Above: Inside the climate-controlled booth of Ruling Engine “A” as it looked in 1922. Master instrument maker Clement Jacomini focuses on the diffraction grating. Above it, a carriage moves back and fourth on twin rails, cutting a line with the diamond and then lifting it for the return stroke, about 10 times a minute. Meanwhile, after each stroke, the grating is advanced on its platform, as precisely as possible, by moving the gear on the left, which turns a screw an imperceptible amount.

What’s the Ruling Engine Doing (Below)?: The diamond moves away from the viewer, leaving a burnished groove in the thin layer of metal on the substrate. Somewhat like a plow cutting furrows in a field, the diamond completely reworks the original surface of the blank by plastic flow of the metal, creating the sawtoothed “blaze.” For scale, the sine wave below represents the wavelengths of green light. For comparison, a typical human hair would be about 100 microns in diameter. Reproduced from Physics Today, July 1986, p. 37, with the permission of the American Institute of Physics.

In 1928, Anderson became the executive officer of the 200-inch telescope project at Palomar Mountain and Harold Babcock took charge of the gratings effort. The original ruling engine had been built to make the largest gratings yet. But in this case, Hale overreached. “A” had too much flex in its large frame. This would affect movement of the main screw and thus the spacing of the lines. Floating the machine on mercury helped greatly, but it was still not good enough. And friction was a problem. From 1929 to 1934, the more compact Ruling Engine “B” was designed and built with the help of Francis Pease, Clement Jacomini, Elmer Prall, and Edgar Nichols. Many innovations were incorporated to reduce each source of error. More attention was given to maintaining a constant temperature around the machine — to within a tenth of degree Celsius. To remove stresses in the frame, it was thermally cycled for two years. Exotic materials were used. Parts made of nitralloy, a low-stress steel, could be hardened to reduce wear. Graphitar was used to reduce friction on the rails. A monorail was adopted for the diamond carriage, and a new mechanism was designed to advance the grating after each stroke. The diamond moved in repeated straight lines for hundreds of thousands of grooves with a tolerance of only a few angstroms — it was one of the first nanotechnologies.

Ruling Engine “B”

Above: Ruling engine “B,” with its much more rigid frame, was a great success. The small mirror mounted to the grating carriage and the optics to the left monitored the line spacing with great precision and made needed corrections as the machine operated.

The four perfectly matched gratings used in the Palomar coudé spectrograph for 33 years. The gratings were set in a single mount to handle the wide light beam of the new 200-inch telescope on Palomar Mountain.

In the subbasement of the Mount Wilson Observatory offices in Pasadena (now headquarters of Carnegie Observatories) rest two marvels of 20th-century technology: Ruling Engine “A” and Ruling Engine “B.” Astronomer Allan Sandage compared the two machines to Egyptian sarcophagi hidden away and preserved in separate burial chambers. A bank calendar on the wall, frozen on March 1964, records when the operators set down their tools and oil cans for the last time.”

Reference: Robert Anderson, “The Diffraction Gratings Laboratory,” Reflections, September 2017, Mount Wilson Institute.