I recently had an interesting visit to the Allegheny Observatory, initiated by a request from Pitt Professor David Turnshek for some advice from the School of Engineering regarding mechanical problems with one of the observatory’s large rotating domes. After reviewing the resumes of a number of engineering professors in the department, he concluded that Professor John Sebastian was his best bet for a contact. Professor Sebastian referred the request to me, with the comment “Looks like it’s right up your alley!” And, he was right, or at least would have been fifty years ago when I was heavily involved with the design of large rotating structures. After trading emails with Professor Turnshek, who is Director of the Observatory, we decided that I might be of some help, so we scheduled a visit to observe their situation first hand and to help them formulate a practical plan to improve it.
The history of the Observatory dates back to 1859 when thirty-two local amateur astronomers formed the Allegheny Telescope Association. Two years later they acquired a 13” refracting telescope and installed it in a new building on Pittsburgh’s North Side. They used it for their own entertainment, primarily focusing on the moon and planets. By 1867 the novelty had worn off, while expenses continued. The facility was donated to the Western University of Pennsylvania (predecessor to the University of Pittsburgh). Professor Samuel P. Langley was named its first Director; he promptly initiated a research program focused on the study of sunspots.
The research activities of the Observatory grew continuously; in 1910 a new building was constructed in Riverview Park, and the equipment and offices moved there. The current facility has three rotating domes. The largest dome is home to a 30” Thaw Refractor (third largest in the United States), forty-seven feet long. Its astronomic parallax research program is credited with establishing the standard for measuring the distance to nearly stars. The smallest dome houses a 13” Fitz-Clark Refractor; the intermediate (Newell) dome protects a recently acquired automated 16” Meade 200 Schmidt-Cassegrain telescope.
The Newell Dome turned out to be the one with problems. Despite housing a twenty-first century leading edge telescope, the mechanical system that supports and moves the dome dates back eleven decades. Recently it has encountered difficulties during cold weather; last winter it absolutely refused to move during one particularly cold snap. Fortunately, a very capable mechanic (his official title is Research Machinist) from the School of Arts and Science, Shared Research Support Services group was able to work his magic and get the system back in working order by the time the temperatures moderated. This particular telescope is the workhorse of the Observatory and its primary teaching tool. Professor Turnshek, Observatory Manager Lou Coban, and mechanic Wil Strang are currently developing a plan to upgrade the dome’s drive system and to minimize the probability of problems in the future.
The dome is located on the roof of the Observatory and is supported on thirteen sixteen-inch diameter wheels running on a railroad rail bent into a circle with a diameter of thirty-one feet. Inboard of the rail, also supported on the roof, is a circular gear with vertical teeth and a diameter of twenty-nine feet. To position its slot opening relative to the telescope orientation, the dome rotates about a vertical axis, driven by a pinion engaging the circular gear. The pinion shaft is driven by a pair of bevel gears, part of a gear train that is driven by an antique 3.5 horsepower Westinghouse motor. The motor is old enough that the nameplate reports that the manufacturer is located in Pittsburg (no “h”), Pennsylvania. The dome itself is wooden, supported by a network of ribs made up of light steel shapes; its total weight is probably less than fifteen tons. It is light enough that it actually can be moved manually with a hand-wheel attached to the gear train.
It is highly probable that the major cause of the failure was inadequate lubrication. There is some indication that the lubricant for the wheel and guide roller bearings applied last year was changed from previous years and that it failed to retain adequate properties when the temperature dropped. In addition, Mr. Strang realigned a wheel assembly that apparently was interfering with something. By the time of my visit the dome drive was working perfectly. However, during our inspection, we noticed that one wheel and one roller were misaligned and must be moved back to their desired positions.
Their short-term plans are to perform this realignment, to monitor radial clearance, and to ensure that the proper lubricant is properly applied. I have also recommended that they install an ammeter on the drive motor. This will provide an early warning when a problem arises and function as a useful tool in determining where the problem is located. Also, Mr. Strang has identified a number of wheels that have deteriorated to the point that significant pieces of their rims are missing. I have recommended that he remove one wheel assembly, disassemble it in his shop, reverse-engineer it, and then manufacture replacement components. It is my assumption that the dome is light enough and stiff enough in flexure that it can operate safely with one missing wheel. Once he has produced a rehabilitated assembly, he can then readily replace all the damaged wheels.
The motor is a different story. If they are bringing the drive up to twenty-first century standards, it warrants a state-of-the-art motor and control system. But what a shame it would be to replace an eleven decades old workhorse that still shows up for work every day and performs its tasks capably! The preservationist in me hopes they shelve that idea and retain it.
It was a great treat for me to become involved in an intriguing project; makes we wish I were seventy-five again.