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Mirror Installation Begins on 4-Kilometer Interferometer at Hanford!

Mirror Installation Begins on 4-Kilometer Interferometer at Hanford!

- Contributed by Fred Raab and Doug Cook

[Editor's Note: This story was written mere days before the powerful 6.8 earthquake that rocked much of Washington state. To read how the quake affected the meticulous work going on at Hanford, click here.]

Doug Cook does last touch-up cleaning. Even as LIGO science teams push forward the commissioning of a 2-km interferometer at Hanford, engineering teams started the process of mirror installation for its big brother, the 4-km interferometer known as WA4K. This is not the beginning of WA4K installation. Most of the interferometer--many tons of it, in fact--consists of vibration isolation structures and optical tables. These were installed during 2000. Installation of the pre-stabilized laser system began in late autumn of 2000 and is ongoing. But the suspended mirrors are the most delicate and the highest precision mechanical parts of the interferometer.

These mirrors arrive at Hanford after a world-wide journey with different stops depending on the precise specifications and end uses for each mirror. They are born as large synthetic fused-silica blanks in New York state or in Germany. They are precision polished by labs in California or Australia. They receive the world's finest mirror coatings in Colorado. And final metrology, of how smoothly and uniformly they reflect light, is done in a specially developed infrared metrology lab at the California Institute of Technology. Then they are loaded into special shipping containers for the ride to the Hanford Observatory. Once arrived, the process of cleaning, outfitting and suspending the mirrors begins. Special fixturing is used to attach magnets for suspension actuators and guides for the suspension wires. The mirrors are balanced on a loop of wire 1/100th of an inch thick to a specified angle with an accuracy of a few minutes of arc. After gluing the suspension guides, the mirrors are vacuum baked and certified pure by residual gas analysis. A final wet cleaning is then followed by insertion into the vacuum-prepped suspension cage. Even in the clean air of our optics lab--where dust levels are typically kept below 1 particle/cup of air--an occasional particle will land on a mirror. In the photo at right above, optics engineer Doug Cook does a last touch-up "dry" cleaning before the optic leaves the lab, using a blast of carbon dioxide "snow" to remove particles, followed by an ionization treatment to neutralize static charge on the mirror and keep particles off the surface. The mirror module is quickly wrapped for transport to the vacuum chamber area.

Mark Lubinski and Doug Cook with mirror module. Hugh Radkins maintains the alignment station.

Above: The photo at left shows the mirror module at its final destination, vacuum chamber BSC1. Engineers Mark Lubinski (left) and Doug Cook are sliding the mirror module along a "Teflon Highway," which lessens the friction allowing for a smoother ride. The structure is placed onto temporary fixturing, which allows us to position it precisely under the optics table, getting the translational, rotational and axial coordinates just right. Once the module is secured to the optics table the mirror is released from its safety stops and then precision aligned by adjusting a series of permanent magnets to achieve the final "pitch" and "yaw" angles. Engineer Hugh Radkins, shown in the right-side photo, carefully sets up and maintains the alignment station during this process. A laser autocollimator is "piggybacked" onto the "total station," giving us a visual signal to steer by, using the oscilloscope or digital display. Since we are pushing the accuracy of the total station to its limits to achieve the optic alignment, it is crucial to constantly verify the total station position. We attempt to set the final "pitch" and "yaw" to an accuracy of less than 10 micro-radians. This is like trying to shoot a penny from a mile away.

Doug Cook makes final adjustments to the optic. At left, in the last of our photos, Doug Cook makes the final adjustments to the "pitch" and "yaw" angles of the optic. This is done while observing an oscilloscope screen which monitors the optic position from the laser autocollimator as changes are made. Hugh has been careful to work out all the alignment data from inside the lab and how it connects up to the surveying data for the various LIGO buildings which house the mirrors. If the alignment of all the mirrors is done correctly and if all the calculations are properly worked out, then when we open the vacuum valves to our beam tubes and fire the corner station laser through the WA4K interferometer for the first time, the laser beam will strike within about a foot of its target on the end mirrors four kilometers (or 2-1/2 miles) away in each of the end stations. Then our intrepid installation crew can meet up with the scientists at the control room to tweak up the alignment.