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. |
