Here Comes the Sun: Historical Instruments for Solar Observation

As we celebrate the summer solstice, the longest day of sunlight in the northern hemisphere, I thought it would be fun to look at some of the instruments scientists have developed to observe and study the Sun using the Emilio Segrè Visual Archives (ESVA). While many of our Photos of the Month posts feature photographs of people, the ESVA has thousands of images of scientific equipment, facilities, and technical drawings. In this post we will explore some of the early ground-based instruments scientists developed to observe, photograph, and harness the Sun’s light here on Earth in the 19^th and early 20^th centuries.

Since many historical solar instruments have similar sounding names, a glossary has been provided at the bottom of this article for reference.

Early Solar Instruments and Spectroscopy

One of the biggest challenges of studying the Sun from Earth is doing so safely. Telescopes revolutionized astronomy, allowing observations of stars and objects far away, however the Sun is too bright to observe through a traditional lens without risking damage to the viewer’s eyes. Thus, scientists were inspired to be creative with tools to find ways of studying the Sun.

During the Renaissance, Italian astronomer Benedetto Castelli invented the helioscope. This device projected the image of the Sun through a telescope onto a piece of paper, which could be traced to record data. The helioscope was further developed by astronomers such as Galileo Galilei and Christoph Scheiner throughout the 17^th century and beyond and helped advance the study of sunspots and other features on the Sun’s surface. Before the invention of solar photography, this type of solar activity was recorded through detailed drawings and illustrations such as in this illustration by Smithsonian astronomer Samuel Pierpont Langley in 1889.

While the helioscope allowed for telescopic study of the surface of the Sun, it wasn’t until the 19^th century that instruments were developed to understand the Sun’s composition and modern solar science really took off.

In 1814, German physicist Joseph Fraunhofer developed a new instrument called the spectroscope which allowed him to precisely measure spectral lines. In the 17^th century, Isaac Newton demonstrated that a prism could split light into its component spectrum of colors, but distinct wavelengths were hard to measure since the spectrum was continuous. To address this problem, Fraunhofer’s spectrometer (pictured below), made use of a narrow slit filter and separate the light before entering a prism to get a discrete spectrum. The spectrometer’s telescopic lens magnified the resultant spectral lines and allowed Fraunhofer to record and measure the first detailed solar spectra.

The solar spectrum recorded with the spectroscope showed dark lines at certain wavelengths, which Fraunhofer mapped out in detail. It would not be until almost 45 years later that scientists identified these lines (now called Fraunhofer Lines) as absorption lines indicating where atomic elements in the Sun’s atmosphere and photosphere absorbed photons emitted by other parts of the Sun. The spectroscope and the related instrument of the spectrometer became standard tools in the new field of spectroscopy, allowing astronomers to measure the chemical composition of the Sun and the stars. While the components and design have changed over time (modern day spectrometers are digital), it has remained one of the most commonly used tools in astronomy.

By the 1860s, the field of spectroscopy was in its prime. Chemists had determined that heated chemical elements corresponded to emission lines in spectra, and that was now being applied to the chemical composition of the stars. However, the Sun still caused a problem for detailed observation. The only way to study the Sun’s outer atmosphere, the corona, was during total solar eclipses, which were few and far between. During the total solar eclipse of 1868, Norman Lockyer and Pierre Janssen both independently used spectroscopy to observe and discover a new line in the yellow part of the spectrum of the Sun, only visible in its outer limbs, which did not match any known element yet known on Earth. This element was named helium. For more on the importance of the 1868 total solar eclipse for modern astrophysics, see this recent review article from the history newsletter, and this Ex Libris Universum post on solar eclipse expeditions.

The invention of photography also revolutionized the study of the Sun. The photoheliograph, a telescope that allowed scientists to take the first photographs of the Sun, was designed for the Royal Society of Great Britain by Warren de la Rue and instrument maker Andrew Ross. In 1857, the first photoheliograph was installed at the Kew Observatory in Greenwich, England, which became the model for subsequent solar photography instruments. The photoheliograph, however, ran into the same problem as observations: the Sun’s luminosity was too powerful for use outside total solar eclipses.

George Hale’s Spectroheliograph

Fascinated with the Sun and spectroscopy from childhood, astronomer George Ellery Hale sought to solve this problem while an undergraduate student at MIT in the 1880s. He realized that the solution to studying the Sun’s atmosphere outside of eclipses was to limit the portion of the Sun that was being observed. Inspired by the slit on his spectroscope, Hale discovered that if the Sun was filtered through a diffraction grating and a slit, he would be able to view the Sun in detail at certain wavelengths so that phenomena normally overwhelmed by the Sun’s light would be visible and able to be photographed. Hale’s device, which combined a spectroscope and a helioscope, was called the spectrohelioscope.

This sketch by Russell W. Porter shows the design for a spectrohelioscope designed by George Ellery Hale. Porter was a telescope architect and artist, who later worked with Hale on the design of the Palomar Telescope in 1927. In this spectrohelioscope design, sunlight hits a set of coelostat mirrors which focus the light through a lens into a hole in a darkened room. That light is then reflected off a mirror into a viewing box with a prismatic grating that separates certain wavelengths, visible through an eyepiece. A spectroheliograph works similarly replacing the view piece with a photographic plate allowing one to take photographs of the Sun. (For a schematic of how Hale’s spectroheliograph works see Hale George H1 ).

To see a spectroheliograph in action on a telescope here is a photo of the Rumford spectroheliograph attached to the 40-inch telescope at Yerkes Observatory, which was used by Hale to first observe vortical structure around sunspots. This image is from the 1903 paper The Rumford spectroheliograph of the Yerkes Observatory by George E. Hale & Ferdinand Ellerman.

This spectroheliograph instrument allowed for astronomers to take spectroheliograms of the Sun at particular bands of light and observe what Hale termed flocculi (from Latin for “little tufts of wool”) to describe the wispy solar prominences only seen in particular wavelengths (such as the Calcium K line (396.8 nm)).

The Dawn of Solar Observatories

Spectroheliographs and the funding for giant telescopes ushered in a new era of ground-based solar observatories.

The Snow Horizontal Telescope at Mount Wilson Observatory in California was the first such solar observatory built in 1904. It used a coelostat, or a set of rotating mirrors, to track the Sun and direct it horizontally into a spectroheliograph. The system was developed by Hale to be used at Yerkes Observatory but was moved to Mount Wilson in 1905. This telescope aided Hale in his discovery that sunspots are associated with a magnetic field.

In 1924 The Einstein Tower (Einsteinturm) Solar Observatory was built in Potsdam, Germany to study redshift and test Einstein’s theory of relativity. It was one of the first European solar observatories ever built and is now run by the Leibniz Institute for Astrophysics. This tower telescope is a classic example of a distinctive style of many early solar observatories, where a ceolostat mirror directs sunlight down a tower to a variety of detectors and telescopes within.

In the 1930s, French scientist Bernard Lyot invented an instrument called the coronagraph, which finally allowed astronomers to photograph the solar corona outside of total solar eclipses. The device mimicked an eclipse by placing an occulting disk as an artificial moon over the telescope lens so that the solar corona could be seen. This instrument allowed for comprehensive study and photography of the solar corona, the hottest and least understood part of the Sun. Since the solar corona is so faint compared with the rest of the Sun, the coronagraph works best at high altitudes with less atmospheric scattering and clearer skies.

The coronagraph was first used in the United States at the High Altitude Observatory (HAO) in Climax, Colorado . Founded as a branch of the Harvard College Observatory by Donald Menzel and Walter Orr Roberts in 1940, it was built solely to study the Sun and the corona. The observatory provided valuable early coronal research and data during the war which helped monitor radio communications. The original Climax Observatory and coronoagraph ceased operation in 1972, when it was no longer able to compete with newer solar ground and space based observatories, but the HAO still remains active as a world class center for solar research at the National Center for Atmospheric Research in Boulder, Colorado.

In 1962, the McMath Solar Telescope at Kitt Peak National Observatory in Tuscon, Arizona, was built with funds by the National Science Foundation (NSF). With an aperture of 161 cm, it was the largest solar telescope in the world, at the time. It held this title for nearly 60 years until 2019, when the Daniel K. Inouye Solar Telescope (DKIST) in Hawaii reached first light with a 400 cm aperture.

This photograph’s description sums up the impressiveness of this instrument:

In order for the McMath telescope to work, a massive heliostat mirror tracked the Sun and funneled it into the telescope so that the light could be directed to the appropriate detectors. (See this diagram of McMath Solar Telescope: Kitt Peak National Observatory H18 ) A heliostat is similar to the coelostat mirrors used in tower telescopes, although heliostats have just one mirror and are manually moved to track the Sun, while coelostats have multiple mirrors allowing for rotation.

Heliostats also have uses outside solar telescopes, namely to concentrate light into solar energy! In the late 1970s, the US Department of Energy worked with Californian energy companies to build a pilot solar-thermal power plant called Solar One in the Mojave Desert. It used hundreds of heliostats to track and reflect the Sun’s energy to a concentrated point on a tower so that the absorbed heat could be converted into energy. Below is an image of Solar One with the heliostats in action, pictured on a day when atmospheric conditions showed off their concentrated effect.

Solar observation instruments of today and tomorrow

Since the mid-twentieth century there have been many advances in solar telescopes and instruments for observing the Sun, both on the ground and in space. The areas of solar science are numerous and varied and still pose questions today.

To learn more about some recent advances in solar telescopes and instruments see the August 2023 Physics Today article “Advances in solar telescopes” by Holly Gilbert, the director of the High Altitude Observatory and previous heliophysics science director at NASA. The article gives an overview of the space missions such as Skylab and major problems in solar science from the 1970s onwards.

If you are interested in modern solar instruments and science projects for studying the Sun, there are some great resources online, such as NASA’s website on the Sun and Parker Solar Probe .

We hope you enjoyed learning about these instruments and are inspired to learn more about the Sun!

Glossary

• Helioscope: A telescope for observing the Sun that protects the viewer’s eye from damage. Traditionally this involved projecting the image of the Sun on another surface, or using a filter such as colored glass or darkened mirrors that reduce the light coming through the eyepiece.
• Spectroscope: Optical device for producing and observing the spectrum of light from a source. Often consists of a slit to reduce the portion of the light source that is being examined, either a prism or diffraction grating to separate the light into discrete lines, and collimating lens to direct the light into the eyepiece.
  
  • Spectrometer: A device for measuring properties of light, such as its wavelength or intensity, through a prism. It consists of a slit through which light enters the device, a collimator such as a prism or diffraction grating that separates the light into a spectrum, and a telescope through which the resultant spectrum is able to be examined.
  • Spectrograph: A type of spectroscope that is capable of photographing a spectrum
  • Spectroscopy: The field of science that studies spectra and using the spectroscope.
• Photoheliograph: An instrument for photographing the Sun, consisting of a camera and a customized telescope.
  
  • Photoheliogram: photograph of the Sun taken with a photoheliograph
• Spectroheliograph: An instrument designed to take photographs of the Sun with monochromatic light to show details of the Sun’s surface as they would appear only in that wavelength. Light enters the instrument through a slit, as with a spectroscope, and then uses a secondary slit in order to take photographs in a very narrow band of light.
  
  • Spectroheliogram: a monochromatic photograph taken with a spectroheliograph.
  • Spectrohelioscope: An instrument designed to take visual observations of the Sun in a single wavelength of light. It works the same as the spectroheliograph but without a photographic plate.
• Coronagraph: An instrument for observing and photographing the Sun’s corona, which consists of a telescope with special lenses and filters that simulate an artificial solar eclipse.
• Tower Telescope: A solar observatory design where stationary telescopes and other instruments are housed in ground or subterranean rooms while light is captured from an opening in a tower that is reflected into the instruments by a set of coelostat mirrors. The telescopes often have a long focal length in order to get detailed images of the Sun, making them impractical to rotate. Thus the coelostat tracks the movement of the Sun and reflects the light to a fixed position.
  
  • Heliostat: A device that reflects sunlight in a fixed direction using a plane mirror on a rotating stand. The stand is mounted parallel to the Earth’s axis and rotated to compensate for the apparent movement of the Sun. An array of heliostats can be used together to concentrate light to a fixed point.
  • Coelostat: A device that reflects sunlight to a fixed point using two mirrors. The first plane mirror is mounted on a movable stand parallel to the Earth’s axis to track the Sun and then is reflected onto a secondary mirror that reflects the beam into a telescope. This set up is advantageous for tower telescopes.
  • Siderostat: A device using one plane mirror to direct starlight into a fixed point. Mounted on a rotating stand that is adjusted to follow the apparent motion of their celestial sphere.

References and Further Reading:

Books on the Sun and Historic Solar Instrumentation at the Niels Bohr Library & Archives:

• Bud, Robert, and Deborah Jean Warner. Instruments of Science: An Historical Encyclopedia. Garland Reference Library of the Social Sciences 936. London New York: Science museum National museum of American history, Smithsonian institution, 1998. Catalog Link
• Hufbauer, Karl. Exploring the Sun: Solar Science since Galileo. New Series in NASA History. Baltimore: Johns Hopkins University Press, 1991. Catalog Link
• King, Henry C. The History of the Telescope. Cambridge, Massachusetts: Sky Publishing Corporation, 1955. Catalog Link
• Mills, John F. Encyclopedia of Antique Scientific Instruments. New York: Facts on File Publications, 1983. Catalog Link
• Weigert, A., and H. Zimmermann. A Concise Encyclopedia of Astronomy. Translated by J. Home Dickson. New York: American Elsevier Publishing Company, 1968. Catalog Link

Books and Resources on George Hale and Russell Porter:

• The legacy of George Ellery Hale; evolution of astronomy and scientific institutions, in pictures and documents. Edited by Helen Wright, Joan N. Warnow, and Charles Weiner. Cambridge, Mass.: MIT Press, [1972] Catalog Link
  
  • This book was written the Niels Bohr Library & Archives Staff as part of a centenary exhibit AIP put on at the 1968 American Astronomical Society (AAS) meeting, from which many of the diagrams in the Hale section are from.

For more on Russell W. Porter, the artist of the spectroheliograph diagram, you can see some of his architectural designs and drawings for the Palomar Observatory from the Caltech Archives. In addition to a telescope maker, Porter was an arctic explorer and the National Archives holds the amazing sketches he did of people and landscapes during his research trips to the Arctic Circle.

More on Solar Observatories and Telescopes:

• Menzel, Donald and Catherine B. Wyatt. “Astrophysicists watch the sun”, Physics Today 3 (3), 8–14 (1950). https://doi.org/10.1063/1.3066838
  
  • Physics Today article by the founders of the High Altitude Observatory reflecting on the early successes and importance of the HAO coronagraph.

  Space-based solar observatories have a fascinating history. You can read about one of first solar space telescopes in our Ex Libris Universum article: Nancy Grace Roman and Early Space Telescopes.