Happy birthday to a grand old telescope

Posted by Daniel Pendick
on Friday, December 12, 2008

Harold McAlisterCalifornia’s Mount Wilson Observatory (MWO) is celebrating the centennial of its famous 60-inch reflecting telescope. Upon its completion in 1908, the “60-inch,” as astronomers call it, was the largest telescope in the world. Pioneer astronomer George Ellery Hale commissioned the project under the auspices of the Carnegie Institution of Washington. The 22-ton behemoth saw first light December 13, 1908, and is considered the grandparent of all modern research telescopes.

Harold McAlister, MWO’s current director, took time to talk to us about the 60-inch telescope’s place in astronomical history and its current scientific activities.

Pendick: What was the first thing an astronomer looked at through the 60-inch telescope?

McAlister: So far, this question has stumped me and several of my colleagues knowledgeable about 60-inch history. MWO Superintendent Dave Jurasevich pointed out that Saturn was in the sky on the evening of December 13 when “first light” was obtained, and it seems inconceivable that it wasn't observed.

Pendick: Could you give us a very bare-bones overview of the effect the 60-inch had on astronomy in its day?

McAlister: The 60-inch was a turning point in telescope technology, launching all subsequent glass-based and precision-controlled telescopes. Even the dome was revolutionary with its attention to heat management and seeing preservation through its double-walled construction.

The telescope had an early and tremendous impact on astronomy through the discovery by Harlow Shapley that the Sun was well away from the galactic center — contrary to the then-conventional wisdom — and, on top of that, the galaxy was huge. This was a sort of ultimate conclusion of the Copernican Revolution, although Shapley believed that the Milky Way dominated the universe.

The first inkling that he was wrong also came from the 60-inch, which produced spectra of the Andromeda Nebula that resemble those of the Sun. This suggested that this "nebula" is composed of stars and not just gas.

Photographs of nebulae showed starlike "condensations," and the 60-inch found evidence of reddening through interstellar absorption from distant nebulae. The conversion of these objects to "galaxies" had to await Hubble and the 100-inch telescope, but the 60-inch paved the way for that ultimate revolution that showed the universe is a vast assemblage of galaxies.

Pendick: What are MWO’s major scientific activities nowadays?

McAlister: Mount Wilson continues a very active science role with daily observations of the Sun’s magnetic and sunspot properties being carried out at the 150-foot Tower Telescope by a University of California, Los Angeles, group directed by Roger Ulrich.

Helioseismology is the focus of University of Southern California professor Edward Rhodes' activities at the 60-foot Tower.

Two interferometers are pursuing high-resolution studies of stars and their immediate environs. The first of these is the Berkeley Infrared Spatial Interferometer, led by professor Charles Townes, which is pursuing the shapes of evolved giants and supergiants and the circumstellar material surrounding them.

The second is my own project, Georgia State University's CHARA Array, which has the world's longest operational interferometric baselines and is active in a variety of areas dealing with fundamental stellar astrophysics. These projects receive funding from the National Science Foundation, NASA, and other federal agencies, and are continuing George Ellery Hale's tradition of 24/7 astronomy.

The 100-inch is available for scientific programs, and, although there are no active users at present, the telescope has supported several efforts from the Jet Propulsion Laboratory and the U.S. Naval Observatory during the last couple of years. The 60-inch is routinely rented to individuals and groups for an unrivaled viewing experience, and is the largest telescope in the world made exclusively available for public viewing. Links to each of these programs can be found at our web site, www.mtwilson.edu.

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