Introduction
The McMath-Pierce Solar Telescope was designed and built between 1958-59 by architect Myron Goldsmith of Skidmore , Owings & Merrill, taking into account the previous designs made in 1957 by the civil engineer William F. Zabrinskie. At the time of its completion it was the largest built instrument dedicated to seeing and studying the sun. Its inverted V shape can be seen miles high on the National Kitt Peak Observatory.
During his inauguration in 1962, President John F. Kennedy defined the McMath-Pierce Telescope as “… bold in concept and magnificent in its execution …”. In addition to allowing progress in studies on the “king star”, the telescope has become one of the most photographed outdoor monuments of the place.
But over the years and technological advances, the National Solar Observatory has moved its headquarters from Tucson to Boulder, Colorado. The organization abandoned its solar telescopes here and in New Mexico for a larger instrument in Hawaii, the Daniel K. Inouye Solar Telescope on the island of Maui, which began operating in 1919.
Location
The telescope is located at the Kitt Peak National Observatory, 2,096 meters high in the Quinlan Mountains, in the middle of the Sonoran Desert, Arizona, United States, west of the city of Tucson.
Choice of place
In 1955, possible locations began to be found to build the “World’s Largest Solar Astronomical Telescope,” limited to the southwestern states of the United States: California, Arizona, New Mexico, Nevada, Utah and Texas. The grounds were studied by land and air, reducing the list of possible candidates to 150. Again they were explored from the ground by using jeeps and on foot at the end of this phase of the site survey, only five candidates remained.
Test towers were erected at the five sites with equipment to measure the transparency of the sky, humidity, winds and temperature fluctuations. Two places in Arizona emerged as the most suitable: Hualapai Mountain in southeastern Kingman and Kitt Pieak 70km southwest of Tucson. The final report on site selection was published in March 1958. The report showed that Kitt Peak was clearly preferred over Hualapai. Both sites were similar in rain accumulation, transparency and light pollution. Hualapai seemed to have a slight advantage in the number of clear nights, but Kitt Peak was found to have superior vision, lower winds, better temperature stability and fewer traces of vapor from aircraft overflights.
Concept
The main instrument of the building is a heliostat, which tracks the sun through the sky and focuses its light down through the diagonal axis. This axis continues about 50 vertical meters underground towards a primary mirror of 1.6 meters, forming the largest unobstructed opening of any optical telescope system. From here the light travels back a part of the axis, to a flat mirror, which then reflects an image 85 centimeters wide from the sun down to an underground laboratory.
In addition to being the world’s largest solar telescope, the McMath-Pierce is also unique because it is sensitive enough to observe bright stars at night. The telescope also has a low cost adaptive optics system. This configuration uses a rapidly deformable mirror to correct distortions introduced by the turbulent atmosphere. Using sensors to measure the degree of image distortion, the adaptive optics system adjusts the shape of the mirror accordingly and converts a blurred image into a clear one.
A main area of study in the observatory is the structure of sunspots, which are relatively cold, dark spots on the surface of the Sun created by intense magnetic activity. Some of the most important discoveries made at McMath-Pierce include the detection of water vapor in the Sun, the measurement of kilogauss magnetic fields (thousands of times stronger than those on Earth) outside sunspots and the detection of a natural maser (like a laser, but with a microwave instead of visible light) in the Martian atmosphere.
The engineer Zabrinskie, in charge of his original design, thought that the triangular shape of the telescope’s structure made it look like the index of a gigantic sundial. Because of this, he referred to the telescope building as the “Index Building.”
Building design
Functioning
In 1957, civil engineer William F. Zabrinskie was commissioned to prepare three preliminary designs for the solar telescope building. The three designs are conceptually quite similar. They all appear as very large right triangular structures with the heliostat mounted on top of the vertical section. The heliostat beam shines down the hypotenuse of the triangle, which is aligned along the equatorial axis of the telescope. The image, mirror n ° 2, is located in the lower part of the hypotenuse, which reflects a backup towards the flat folding mirror, n ° 3 is located just below the heliostat. This was placed out of the path of the beam coming down from the heliostat, forming an unobstructed optical path. Mirror 3 deflects the beam down to an air-conditioned observation room located just below ground level. Spectrographs and other instruments were mounted horizontally in the observation area along the base of the triangle.
The differences between the three designs were mainly in the framing style that was to be used. One design required a structure made of H-beams and columns, another used large steel pipes, while the third was designed to be constructed of reinforced concrete.
It is interesting to note how many of Zabrinskie’s preliminary design concepts survived to be incorporated into the final structure of the telescope. The final structure retains the general triangular shape. All optical media were placed on railroad-like tracks to allow them to move to focus and facilitate service. The outside of the structure should act as a shield against the wind and cooled with water. All these ideas of Zabrinskie’s designs can be seen in the final structure of the telescope.
Final design
Following the preliminary design work of Zabrinskie, AURA (Association of Universities for Radio Astronomy) hired the Chicago firm Skidmore, Owings and Merrill to study all possible structural designs for the solar telescope. His studies should include, but not be limited to, the designs made a year earlier by Zabrinskie.
Skidmore, Owings and Merrill prepared ten designs for the construction of the telescope. Of these ten, they recommended two for final consideration by AURA. One consisted of a conical cantilever tower that extended from the ground at the local latitude angle of 32 °. This design has been described as an “extreme case of the leaning tower of Pisa” (Kloeppel 1983). The design may have been aesthetically beautiful, but more expensive and less stable than the simplest tower structure that was adopted by AURA in June 1959.
Structure
Tower
The heliostat tower is a 30.5 m high structure, a vertical concrete cylinder, 7.9 meters in diameter and a wall thickness of 1.2 meters, from which an axis of 60.96m tilts towards the ground. The axis continues in the mountain, forming an underground tunnel where the sun is seen in the main focus.
The tower is protected from the wind by external cooling panels that are mounted on the frame of the structure. The framing of the external structure is acoustically isolated from the tower. It was calculated that the deflection of the tower would be less than 0.4 mm in a wind of 40 kilometers per hour, which would move the main image of the telescope in less than 1/3 of an arc second.
An aerial shot of the top of the McMath-Pierce telescope reveals the 3-mirror heliostat that picks up the light and directs it through the tunnel. Unlike other solar telescopes, the McMath-Pierce is sensitive enough to observe bright stars at night. At the base of the tunnel, a 86.36cm parabolic mirror captures the images that are then used to study the sun. The structure was designed to defeat the forces of heat and wind. Both the tower and the light tunnel are freely placed inside concrete decks protected with a liquid that resists freezing. A steel cover, independent of the concrete structure, which covers it, protects the telescope from the winds of the mountain top as it emerges, using a square shape inclined at an angle of 45 degrees.
Permanent instruments include a double-grid spectrograph capable of obtaining extended wavelength coverage (0.3-12 microns), a transformed Fourier spectrometer of 1 meter for solar and laboratory analysis, and a stellar high dispersion spectrometer.
Optical configuration
The first step in designing the new solar telescope was to determine the optimal image scale. Working on the spectra of the solar granules, on the physical structure of the sunspots and their associated magnetic fields, requires a considerable image size. Past experience has shown that the optimal image of the sun should be approximately 0.91 m “(McMath and Pierce 1960). Several optical designs were considered for the design of the solar telescope.
Heliostat
The heliostat is the configuration that was finally chosen for the solar telescope. The heliostat uses only a mirror to track the sun. The mirror is mounted equatorially and rotates with the celestial sphere, once a day. The heliostat can also move in decline. This mirror configuration reflects the solar beam down the equatorial axis of the telescope to the telescope’s image optics that is stationary and therefore requires fairly simple assemblies.
The disadvantage of the heliostat is that its image rotates once a day. To compensate for rotation, instruments must be constructed in such a way that they can rotate at the same speed to compensate. This complication was considered to be a small price to pay for the other virtues of the heliostat: a single mobile mirror system that did not darken on the day of observation.
Materials
In the construction of the structure mainly reinforced concrete and steel have been used.
In the final design of the optical part, the heliostat is located on top of a 7.92m diameter solid concrete cylinder with reinforced steel walls that are 1.22m thick.
The strategy to deal with the telescope’s excessive heat load was separated into two independent cooling systems, one for the underground optical tunnel and the other for the part of the structure that is above the ground.
Optical tunnel cooling
About two fifths of the telescope’s trajectory was excavated underground. The optical tunnel was cut through the south side of the mountain down at an angle of 32 degrees (the local latitude) so that the tunnel points towards the celestial north pole. The bottom, south end, opens to the southern slope of the mountain. Fans were installed in the lower opening to allow outside air to enter the telescope through the upper part and be ejected through the opening at the south end.
The rock surrounding the optical tunnel is maintained at a fairly constant temperature of around 13 ° C throughout the year. It was found that this temperature was low enough that no additional cooling was needed to stabilize vision in the hot Arizona summers. Ambient temperatures in winter can fall much lower than the freezing point for prolonged periods, so a cooling system was designed and installed.
The walls of the underground optical tunnel are lined with aluminum panels, approximately 18,000 m2, which are connected to a network of more than 7500 meters of 2.5 cm diameter water pipe. A 7.6 cm thick insulation was installed between the cooling pipes and the rock wall of the tunnel. Water pipes circulate a refrigerated solution of 42% glycol and 58% water cooled by a 3-cylinder compressor. The glycol / water solution enters the pipe grid at the bottom of the optical tunnel and exits from the top. The temperature of the liquid will be constantly heated as it rises along the optical path of the telescope, absorbing heat from the surrounding rock. This constant increase in temperature is exactly what is desired to create stable air mass in the optical path. The cooling system is left off during the summer months. During winter, there are internal and external temperature indicators that regulate the compressor to keep the tunnel walls at the proper temperature.
Cooling structure on the ground
The structure on the ground is covered by approximately 2,787 m2 of panels. Various materials and paints were evaluated to test their heat absorption from sunlight. It was discovered that a surface painted with white titanium dioxide pigmented paint was heated only between 5.6 to 8.3 ° C in full sun (McMath and Pierce 1960). Copper was chosen as a material for the panels due to its very high thermal conductivity. Even so, it was calculated that during the midday, the Sun would heat the skin panels with approximately 23.45 kilowatt hours in the winter and would approach 41 kilowatt hours during the summer months (Donovan & Bliss 1962).
The copper panels used to cover the telescope include built-in 1.27 cm diameter water lines known as “Tube-in-Strip.” The “Tube-in-Strip” exterior panels were covered with white titanium dioxide pigment paint to minimize solar heating absorption. A mixture of glycol and water circulates continuously through the Tube-in-Strip panels. The glycol cooling mixture is pumped into a 60,566 liter retention tank. This large retention tank acts as a mass of heat for the telescope. The warm liquid returning from the telescope’s skin mixes with the water and glycol in the tank and slowly heats the liquid in the tank throughout the day. The temperature of the glycol coolant flowing from the tank is delayed a few degrees for most of the day. The cold glycol mixture enters the telescope at ground level and heats up as it moves up through the skin and returns to the tank a few degrees above room temperature. This continues the constant rise in temperature inside the telescope all the way to the top where the telescope air mixes with the outside air with a very small temperature differential. The speed at which the coolant flows through the Tube-in-Strip panels is regulated by temperature meters that read the temperature difference between the liquid flowing to and from the telescope.
The circulation system continues to operate overnight. This allows the coolant that heats during the day to radiate its excess heat back into the cold night sky. In the morning, the temperature in the tank returns to room temperature and the process is ready to begin once again. If the outside temperature rises unexpectedly the fans at the bottom of the tunnel can be turned on to attract outside air to the telescope through the opening at the top.
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