In this astronomical "holy land", look for the interstellar glow

Author: Du Lian

Audit│Ding Yi

Editor: Zhao Jingyuan

Mount Wilson Observatory | Image source: Mount Wilson Observatory

Mount Wilson Observatory is a famous observatory on Mount Wilson near Pasadena, California, USA. In 1904, American astronomer George Ellery Hale built the observatory with the support of the Carnegie Institution of Washington.

George Ellery Hale | Source: Mount Wilson Observatory

Mount Wilson Observatory currently has two telescopes available to the public - the 60-inch telescope and the 100-inch Hooker telescope. These two telescopes once produced brilliant scientific results. Although they are basically no longer used for scientific research, they have become a public benefit, bringing the public an unparalleled observation experience.

60-inch telescope

The construction process of the 60-inch telescope was very tortuous. Its 1.5-meter diameter mirror blank was donated to the Yerkes Observatory (an observatory established by Hale in 1892) by Hale's father, William Hale, in 1896. William promised to fund the polishing of the telescope. However, things are unpredictable. Two years later, William died, and Hale had to raise funds for the 60-inch telescope again. In 1902, Hale applied to the Carnegie Institution to build a new observatory on Mount Wilson. Two years later, the Mount Wilson Observatory was established. Hale obtained sufficient funds and the development of the 60-inch telescope officially began. After many setbacks, the 60-inch telescope was finally opened in December 1908 and took the first photo. Before the 100-inch Hooker Telescope was built in 1917, the 60-inch was the largest optical telescope in operation in the world.

A tourist uses a 60-inch telescope to observe | Source: Mount Wilson Observatory

The 60-inch telescope has many important innovations, one of which is that by adding and changing the secondary mirror, the telescope has multiple focuses: Cassegrain focus, Newton focus, and folded axis focus. This is the first large telescope to use a folded axis focus, which is of great significance in telescope design.

During the decades of scientific research, the 60-inch telescope has made a lot of scientific discoveries. For example, in 1918, American astronomer Harlow Shapley used it to discover that the sun is not the center of the Milky Way, breaking the knowledge at the time. Now, although the 60-inch telescope has been retired, it has also assumed a new role: showing the beauty of the universe to the public.

100-inch Hooker telescope

While building the 60-inch telescope, Haier had greater ambitions. With funding from American entrepreneur John Hooker, he planned to build a 100-inch (2.5-meter) telescope. Casting of the mirror blanks began in 1906. Due to their huge size, the process was not smooth. There were bubbles in the first mirror blank, the second and third mirror blanks shattered when cooled, and the fourth mirror blank was too thin and was likely to deform during use... After suffering many setbacks, Haier and others decided to try using the mirror blank with bubbles. Finally, in 1917, the 100-inch diameter telescope was built and named after its sponsor Hooker. Before 1949, it was the world's largest optical telescope, and its primary mirror is still the largest solid glass.

The Hooker Telescope primary mirror filled with bubbles | Image source: Mount Wilson Observatory

On the night of November 1, 1917, the night of the opening ceremony, people were still full of doubts about this imperfect primary mirror. After dark that night, people turned the Hooker telescope to observe the target, and then they were stunned because they saw six or seven overlapping images instead of the normal one. Then Hale and others thought that the dome might have been opened when the telescope was installed and debugged during the day, which caused the temperature inside the dome to rise, and the huge primary mirror might have been deformed. So all people could do that night was wait for the mirror to cool down. In the early morning of the next day, Hale and others aimed the Hooker telescope at another target, and they observed a clear and bright star point. The telescope was finally successful!

100-inch Hooker Telescope dome | Source: Mount Wilson Observatory

The Hooker telescope has been used by many famous astronomers and can be regarded as a world heritage scientific instrument. In 1920, American physicists Albert Michelson and Francis Pease used the Hooker telescope to measure the diameter of Betelgeuse, the first time that humans measured the diameter of a star other than the sun. American astronomer Edwin Hubble used it to determine that "spiral nebulae" are actually galaxies like the Milky Way, which are located outside the Milky Way, ending a long-standing debate. Hubble also used it to discover that the redshift of galaxies is proportional to the distance. This conclusion shows that the universe is expanding, which changed people's understanding of the nature and size of the universe and triggered an astronomical revolution that continues to this day. In the winter of 1931, American physicist Albert Einstein visited the Mount Wilson Observatory. With Hubble and the director Walter Adams, Einstein observed the universe through the Hooker telescope. Now, this historic telescope is open to the public. Imagine: wouldn’t it be cool to personally observe celestial bodies using the telescope used by people like Michelson, Hubble and Einstein?

Einstein observed using the Hooker telescope. From left to right: Einstein, Hubble, Adams | Source: California Institute of Technology

Other Facilities

In addition, Mount Wilson Observatory has many facilities, including the Snow Solar Telescope built in 1905, the 60-foot Solar Tower built in 1908, the 150-foot Solar Tower built in 1912, and the Center for High Resolution Astronomy Array (CHARA) put into use in 2004. Some of them are briefly introduced below.

The Snow Solar Telescope was the first telescope installed at Mount Wilson Observatory and the world's first fixed-site solar telescope. The telescope was donated to Yerkes Observatory by Helen Snow of Chicago and later brought to Mount Wilson by Hale as a formal scientific instrument. Its primary mirror has a diameter of 61 cm and can be used with a spectrometer to conduct spectral studies of the Sun and brighter stars.

The 60-foot Solar Tower uses a vertical tower design, which makes its solar imaging and spectral observation resolution much higher than the Snow Telescope. The "60 feet (18 meters)" in the name refers to its focal length. Soon after it was built, Hale used it to record the Zeeman effect in the spectrum of sunspots, which was the first time that the existence of a magnetic field was discovered outside the earth. In the 1960s, American astronomer Robert Leighton used the 60-foot Solar Tower to observe the complex oscillations on the surface of the sun for the first time, thus pioneering the field of helioseismology.

The 150-foot Solar Tower has a primary mirror with a diameter of 30 centimeters. The "150 feet (46 meters)" in its name refers to the focal length, which was once the world's longest focal length solar telescope. The facility uses a tower-within-a-tower design, with the inner tower supporting the optical components at the top and the outer tower supporting the building structure and dome. This design protects the optical components from wind effects, thereby obtaining a stable image. Astronomers use it to study solar rotation, sunspots, solar magnetic fields, and helioseismology.

The top of the 150-foot Sun Tower │ Source: Wikipedia

The High Resolution Astronomy Center array consists of six 1-meter telescopes arranged along three axes with a baseline of 330 meters. It was once the largest optical interferometer in the world. In the near-infrared band, its resolution can reach 0.5 milliarcseconds; in the optical band, it can reach 0.2 milliarcseconds. Thanks to its excellent resolution, astronomers directly measured the diameter of an exoplanet for the first time in 2007; and in 2013, astronomers observed sunspots on distant stars for the first time.

One of the telescope domes of the High Resolution Astronomy Center array │ Source: Wikipedia

Today, Mount Wilson Observatory is open to the public for various activities, including telescope observations, observatory concerts, astronomy lectures, and art events, making the observatory no longer out of reach for the public and astronomy no longer out of reach. I personally agree with this approach, allowing professional facilities to continue to carry out astronomical observations and research, while making "retired" telescopes gradually approach the public. This is a good way to popularize science.

100-inch Hooker Telescope Dome Concert│Source: Mount Wilson Observatory

References

1. Mike Davis, Crystal City: A Peep into the Future of Los Angeles, translated by Lin He, Shanghai People's Publishing House, 2010.1

2. Mount Wilson Observatory History, Mount Wilson Observatory, https://www.mtwilson.edu/history/

3. Telescope Observing, Mount Wilson Observatory, https://www.mtwilson.edu/observe/

4. Mike Simmons, Building the 60-inch Telescope, Mount Wilson Observatory, https://www.mtwilson.edu/building-the-60-inch-telescope/

5. Mike Simmons, Building the 100-inch Telescope, Mount Wilson Observatory, https://www.mtwilson.edu/building-the-100-inch-telescope/

6. Li Geng, Chasing the Dream of a Giant Mirror, Chinese Academy of Astronomy, 2023.2

7. Events, Mount Wilson Observatory, https://www.mtwilson.edu/events/

8. Sixty-inch Hale Telescope at Mount Wilson Review, http://www.scopeviews.co.uk/MountWilson60.htm

9. Mount Wilson Observatory, wikipedia, https://en.m.wikipedia.org/wiki/Mount_Wilson_Observatory

For more questions about Mount Wilson Observatory, please leave a message in the comment area and Xiaoxing will answer them for you one by one~

Follow the Toutiao account/Science Popularization China/WeChat public account/Sina Weibo "Xingming Observatory" and walk through the stars with Xiaoxing to harvest romance~