Mount Wilson Observatory

Within fifteen years of its founding in 1904 Mount Wilson held the world's largest telescope, and within twenty-five it had shown that the Milky Way is one galaxy among many and that the universe is expanding.

The telescope builder

George Ellery Hale left Yerkes in 1904 to found a new observatory on Mount Wilson, above Pasadena, with funding from the Carnegie Institution. The first telescope was a solar one; the 60-inch reflector saw first light in 1908, and the 100-inch Hooker telescope, paid for by the Los Angeles businessman John Hooker, on 1 November 1917. In 1908 Hale also identified magnetic fields in sunspots from the splitting of spectral lines, the first evidence of magnetic fields beyond the Earth.

The Hooker was the largest telescope in the world until 1948, and its mirror was cast in thick glass by a French glassworks after an earlier attempt failed.

The size of the universe

In 1920 Albert Michelson and Francis Pease used an interferometer mounted on the 100-inch to measure the angular diameter of Betelgeuse, the first stellar diameter ever measured. In 1923 Edwin Hubble found Cepheid variable stars in the Andromeda nebula and used them to show that it lies far outside the Milky Way. In 1929 he showed that the more distant a galaxy, the faster it recedes, the observational basis of the expanding universe.

The measurements depended on a calibrated relationship between a Cepheid's period and its brightness, established by Henrietta Leavitt at Harvard, a good example of how a standard candle needs standardised scales.

Light pollution and renewal

As Los Angeles grew, its light reduced the usefulness of the large telescopes, and Palomar took over the biggest projects. The Carnegie Institution reduced support, and in 1989 operations passed to the non-profit Mount Wilson Institute. The site found a new use for high-resolution work in the CHARA Array of Georgia State University, six 1-metre telescopes combined as an interferometer, fully operational in 2004. The solar tower telescopes, built in 1908 and 1912, are still used to track the sun's magnetic field.

Measuring a universe with borrowed scales

Hubble's discovery depended on the work of others: Henrietta Leavitt's relation between the period and luminosity of Cepheids, Vesto Slipher's measurements of galaxy velocities and Milton Humason's spectra of faint galaxies, taken at Mount Wilson. The first values for the expansion rate were too high by a factor of several, because the calibration of the distance scale was imperfect, and refining it took decades, with Hubble's own telescope eventually used to fix it. The episode shows that a discovery is often a measurement plus a calibration, and that the calibration can take longer than the measurement.

Instruments and technologies

  • 100-inch Hooker telescope A 2.5 metre mirror with a focal length of about 12.9 metres in a Cassegrain or Newtonian arrangement, with a precision-engineered mount and drive.
  • Stellar interferometer A 6-metre beam carrying two mirrors on the telescope's top; interference fringes disappear when the star's diameter is resolved.
  • Solar tower telescope A tall tower with a coelostat at the top feeding a vertical spectrograph in a pit, which isolated the instrument from ground heat and vibration.

The people

  • George Ellery Hale Founder and first director.
  • Edwin Hubble Showed that Andromeda is a separate galaxy and that the universe is expanding.
  • Albert A. Michelson Physicist who measured the diameter of Betelgeuse with an interferometer.

Timeline

  • 1904 Observatory founded with Carnegie funding.
  • 1908 60-inch telescope sees first light.
  • 1917 100-inch Hooker telescope sees first light.
  • 1920 Michelson and Pease measure the diameter of Betelgeuse.
  • 1923 Hubble shows the Andromeda nebula lies outside the Milky Way.
  • 1929 Hubble shows that the universe is expanding.
  • 1989 Operations pass to the non-profit Mount Wilson Institute.
  • 2004 CHARA array becomes fully operational.

Key lessons for observation today

  • Calibrated scales unlock discoveries Hubble's results needed the Cepheid period-luminosity relation. Standard references, such as calibrated candles and reference stars, are infrastructure.
  • Cities move into the sky Light from Los Angeles dimmed the site. The electromagnetic and optical environment of a site changes, and it must be monitored.
  • Combine the instruments you have The CHARA array combines six telescopes into one. Federating instruments extends capability without building one big one.

Sources

  • Wikipedia, Mount Wilson Observatory
  • American Physical Society, historic site: Mount Wilson Observatory
  • Astronomy, Nov. 1, 1917: The Hooker Telescope sees first light