The McDonald Observatory

A View into the Sky

Fort Davis, TX

July 7th, 2005

Just about everyone has, at one time or another, passed by one of those massive domes, perched high on some distant hill. When we would come across one, I would marvel at their massive construction and unusual shape.  I knew that they all held a giant telescope of some kind, but I had no idea how such a thing worked.  So, when we were staying in Fort Davis we decided to go see what we could find out about the McDonald Observatory. The Observatory did indeed contain a telescope. A 107 inch telescope, completed in 1968. It is one of the world's largest telescopes, with a record of many firsts in astronomical research.  The building is almost as impressive as the telescope it houses. The steel dome is 110 feet high.  The upper half, which rotates on solid steel wheels weighs more than 200 tons. Because the telescope and other light gathering instruments require absolute stability, the telescope is anchored to the solid bedrock below by a joined pair of independently standing columns.  No part of the telescope actually touches the building that houses it. It is for all intents and purposes, a building within a building. From the many displays we learned that the telescope's light-gathering ability is enhanced by the clear, dry air of the Davis Mountains, where skies are clear two-thirds of the year. Smog and light pollution problems encountered by many other observatories are not evident here on Mount Locke. We had arranged for a tour and at the proper time, were ushered into a freight type elevator. There were maybe 10 of us in it when the door closed. We were instantly plunged into total darkness. The elevator had no light. That in itself was kind of an unusual experience. We rarely ever see total darkness, and it surprised me when it happened. Upon arriving on the observatory floor, my first impression was BIG!  This thing is gigantic. Our guide who had arrived by some circuitous route, set about the task of explaining the various functions of the telescope. In his right hand he held a controller, attached to the telescope by a thin black wire. With a push of a button, the top of the building began to groan. The two sides of the dome slowly slid away, exposing the sky above.  Simultaneously, like a sleeping giant, the telescope began to rise, and turn at the same time.  It was all quite impressive. The initial incentive for the 107-inch telescope was lunar and planetary research.  Its observations of the physical and atmospheric conditions of the planets contribute significantly to the success of NASA's later planetary probes. Another notable project was the firing nearly every day for more then a decade of Laser pulses at reflectors left on the moon by Apollo astronauts.  By timing the flight of the small amount of light reflected back to earth, McDonald astronomers charted the moon's distance from the earth with an accuracy of a few inches.  This experiment known as lunar laser ranging, began with the goal of testing Einstein's general relativity theory through its predictions of the effect of gravity on the shape of the moon's orbit. The results agreed perfectly with Einstein's theory.  The ongoing experiment has developed a number of other applications, including the measurement of continental drift which is the cause of earthquakes. However, the bulk of the lunar laser ranging work is now conducted by a smaller mobile telescope and laser station, freeing the 107-inch for more use in other areas of research. Current research topics include precise and detailed analysis of the chemical composition of stars.  These studies tell the astronomer about the nuclear reactions that power the stars and even by studying many stars of different ages, about the history of star formation in our galaxy. The 107-inch telescope's building is divided into four floors plus the observing floor. The first floor contains several research rooms, a large storage area and a machine shop. Modifications, repairs and some modest construction of observatory equipment are done here. The second floor houses additional offices and research rooms plus the all important electronics and computer center where the observatory's computer system helps to control the telescope and to process astronomical data. The third and fourth floors contain one of the most important tools of the observational astronomy, the Coude spectrograph. This giant instrument spreads out the light from stars into a thin-strip rainbow of colors.  The rainbow is punctuated by a complex pattern of dark and bright lines.  By measuring these lines astronomers obtain an inventory of chemical elements and molecules in the star.  Careful study of the lines reveals additional information such as the surface temperature, how fast the star is moving and spinning, whether it has a magnetic field, how old it is and much more.  There is also a part of the third floor reserved for recoating the mirrors with highly reflective aluminum.  The fifth and final floor is devoted to the telescope itself.  The visitors' gallery is enclosed by glass to help keep the temperature on the observing floor the same as the temperature outside the building.  Differences in temperatures destabilize the air in the dome and in the telescope tube, causing distorted star images. From here we walked the half mile or so up the hill to the Hobby-Eberly telescope to see what an electron telescope looked like.  Of particular interest to me was the mirror which is the driving force of any telescope.  Astronomers use telescopes to gather light from stars, and other celestial objects.  The bigger the mirror, the more light it gathers and the more astronomers can see.  Really big mirrors are not made in one piece. It takes 91 segments to create the 432 in mirror for this telescope.   Each segment weighs 320 lbs. The concave mirrors are polished to perfect spherical surfaces.  After fabrication, the glass is coated with silver and a protective layer is applied to resist tarnishing.  Each mirror segment is independently supported by a structure that allows it to float, stress free.  Small motors position the segments so that all 91 mirrors work together as a giant eye to gather starlight. The mirror segments are attached to a steel truss.  The 25 thousand pound truss forms an extremely stiff backbone that prevents the mirror from sagging and distorting the light.

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