The McMath-Pierce solar telescope actually comprises three telescopes in one, so that three independent research projects can be run at the same time. In addition to studying the Sun, the main mirror also can be used for night-time observing of bright stars. Of the three primary heliostat mirrors, one has a diameter of 2.1 meters and two have diameters of 0.9 meters. The heliostat is the moving part of the telescope that follows the Sun through the sky (or stars if it is used for night-time observing). The original solar telescope (which has since been replaced with the current mirrors) opened in 1962, at which time it was named for Dr. Robert McMath, who was instrumental in getting it built. Sadly, he died
The McMath-Pierce solar telescope: in the foreground, with the tower of the vacuum telescope behind it and the dome of the Razdow telescope in the lower right. Photo courtesy of National Optical Astronomy Observatory/ Association of Universities for Research in Astronomy/National Science Foundation.
several months before the telescope was dedicated. It was later renamed the McMath-Pierce telescope to honor the contributions of Dr. Keith Pierce to our understanding of the Sun.
The vertical tower is almost 100 feet tall, while the slanted portion of the telescope extends for about 200 feet above ground, and another 300 feet underground. At the top of the vertical tower is the main heliostat mirror, which is flanked by two secondary flat The 2.1-meter heliostat, which tracks mirrors. The heliostat reflects the Sun through the sky. Photo by LJS.
the sunlight 500 feet down the tunnel, where it hits the curved primary mirror (1.6-meter diameter) that forms the image. The light is then reflected partway up the tunnel to one of two flat mirrors on a set of rails, which directs the image to the scientific observing rooms. The position of the flat mirror on the rails determines to which instrument room the light is sent. This system produces an image of the Sun nearly 3 feet across, or a spectrum almost 70 feet long. It is the largest solar telescope in operation in the world.
A schematic drawing of the McMath-Pierce solar telescope, which extends much further underground than it does above the ground. The public viewing room is around the back of the structure, where the angled part of the building enters the ground. Image courtesy of National Optical Astronomy Observatory/Association of Universities for Research in Astronomy/National Science Foundation.
One interesting feature of the McMath-Pierce solar telescope is that the shell of the building surrounding the light path contains about 25 000 feet of pipes through which a mixture of chilled water and antifreeze is run to keep the air inside the telescope from getting too hot and turbulent. The skin of the building is made of over 14000 copper panels, which are cooled by the pipes. Those of you who have burned holes in paper with a small magnifying glass know what the focused light of the Sun can do. The energy in that sunlight heats the surrounding air in the tunnel, so the air has to be cooled.
Astronomers study the Sun for a variety of reasons. It is the closest example of an average star, and therefore provides a model for our understanding of stars in general. The main aspects of the Sun that are studied by the McMath-Pierce telescope are sunspots and solar variability. Sunspots are dark regions on the surface that are slightly cooler than the surrounding gas, which makes them appear dark. They mark regions of high magnetic fields, and the number and position of the sunspots varies systematically over a 22-year cycle whose origin still is not understood. In addition, the telescope is used to study the composition of the Sun, using spectroscopy. The McMath-Pierce telescope discovered the presence of water molecules on the Sun!
The outer layer of the Sun (called the corona), which is visible to the naked eye only during a total eclipse, is much hotter than the lower "surface" of the Sun. It's as if a pot got hotter as we lifted it higher off the stove! The McMath-Pierce telescope has contributed substantially to the view that magnetic fields play a prominent role in heating the corona.
It is the only solar telescope in the world sensitive enough to be used at night. During one set of observations, it was used to discover that there was little or no water in Venus' atmosphere, contrary to the view at the time that the clouds contained substantial amounts of water (like Earth's clouds).
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