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Thirty Meter Telescope

The Project Manager's Corner: Whole Lotta Shakin' Goin' On
Gary Sanders, TMT Project Manager

October 2006

When Jerry Lee Lewis sang those immortal words, he was probably not thinking of a giant telescope. But many of the world’s giant telescopes got a whole lotta shakin’ on the morning of October 15, when a magnitude 6.7 earthquake disturbed the Big Island of Hawaii. The good news is that all of the observatories, as well as the population of the island, fared pretty well in this exceptional event.

As someone involved in designing the next-generation telescope, an event like the Big Island earthquake is a reminder to be very careful in our design. Of course, earthquake resistance and safety are a standard part of the requirements that all telescope designers use. But an actual event, especially such a big one, is a chance to see if the design solutions taken by past designers worked, what did not work, and what new insights can be gained.

The figure shows the basic elements of TMT. The example is situated on one of our candidate mountains, Armazones in Chile. All 5 candidate mountains (Chile, Mexico, Hawaii) are in seismically active zones. Therefore, our discussion applies to all locations.

The summit facility buildings, like any office and technical support building, can be designed to well-defined seismic construction requirements. The hemispherical dome enclosure must be stiff and seismically resistant. The basic structure has to be stiff against high mountain winds and snow loads, and this assures that it is not a delicate structure. But like any house that may slide off of its foundation when the ground moves in an earthquake, the enclosure, situated on wheels (called bogies) and tracks, can slide off of its round track system. Or the wheels can be stressed to failure. Earthquake resistance is assured by taking the expected accelerations from an earthquake into account and by designing the strength and restraints appropriately. Safe failure modes are designed in by including earthquake stops on the track systems.

The telescope itself, like the enclosure, is a structure that must be stiff enough to resist wind loads and not disturb the required precision image quality. This very demanding requirement assures that it is not flimsy. It also must rotate around on the circular azimuth track at its base so as to point anywhere about the 360 degrees of a compass bearing. And it must be able to point straight up or be lowered to point as far down as directly at the horizon. This motion is called elevation motion, and the upper half of the structure rotates about a massive elevation bearing. The telescope cannot sag, even minutely, during any of these motions. This means it is very stiff. Once again, the tracks and bogie wheels and bearings and the drives and encoders that control these motions are the critical and potentially failing elements. They are designed to be robust and restrained against foreseeable earthquake disturbances. And failure modes are anticipated with earthquake stops.

The first time I saw one of the giant 10-meter Keck telescopes move, I was most impressed by how smooth and quiet and swift the motion was—almost elegant. Careful design, ultrasmooth machined bearing surfaces, high-quality drive mechanisms, precision controls…all orchestrating a stately and hushed move by the towering telescope.

When the crust of the Earth tears and cracks, and a mighty telescope is shaken, what is the aftermath? Let’s review how the giants on Mauna Kea prevailed.

Here’s a report from the W. M. Keck Observatory:

"The primary mirrors of both Keck telescopes are intact and there is no earthquake damage to mirror segments. There does not appear to be any damage to Observatory instruments. There is no damage to observatory computers or servers and all data has been backed up and is safe. Power has been restored to all headquarter and summit facilities and the headquarter facilities are open.

"Keck Observatory technical staff are working long hours at the summit to repair the systems damaged in the earthquake and to verify the proper functioning of the many precision systems required to operate a state-of-the-art telescope. I am struck by the dedication and resourcefulness of the staff performing these activities. We are all eager for the Keck telescopes to resume observing and astronomical discovery," said Observatory Director Dr. Taft Armandroff.

Damage occurred in glass and ceiling fixtures in office areas, and in a number of control, drive and earthquake restraint assemblies, but all are being repaired and reintroduced to service by the Keck staff. Observing is expected to resume within a week or two. All in all, the system was robust and once everything is back in operation, “lessons-learned” will be collected and TMT will incorporate the insights.

The Gemini North Observatory reports no serious damage, with each of its systems being checked out or debugged in turn. Frequent updates are posted. The 8-meter Subaru Telescope has frequent updates as it works through repairs and verification. Finally, the venerable Canada France Hawaii Telescope returned to operation after only a remarkable four-day interval!

Once all of the observatories are operating, a lessons-learned exercise will sharpen preparations for the inevitable next time. And TMT will extract valuable guidance for our design.

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Drew Medeiros (left), Chris Hunt (right), and Jim Bell (bottom) adjust the Keck I telescope dome restraints to return the wheels to proper position.

Credit: S. Anderson

 
 
The TMT project is a collaboration of Caltech, University of California (UC) and the Association of Canadian Universities for Research in Astronomy (ACURA). © Thirty Meter Telescope
 

 

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