An old idea for astronomical imaging is undergoing a technology-driven renaissance
Bumps Along the Way
The success of the LZT did not come easily. We faced countless hurdles, most of them unexpected. Early in the design phase, for example, we discovered that no air bearing in existence was suitable for supporting our 6-meter mirror, so we had to have one built expressly for this unconventional project, which turned out to be no easy feat. It required our contracting with a company in Minneapolis, which managed to put together a unit that could support our 3-ton mirror. Other difficulties ranged from the predictable—equipment failures and problems with overheating or freezing in the pneumatic system that supplies air to the bearing—to the ridiculous, as when a mouse jumped onto the rotating mirror and then ran in circles, breaking the delicate film of mercury, because it was afraid to jump off.
Of all the problems we encountered, two were fundamental and required considerable effort to solve. The first was reminiscent of the speed fluctuations that plagued Wood's telescope. Although our mirror is driven by a motor that is synchronized to a highly stable oscillator, there is no rigid coupling between the motor and the mirror. Such a coupling would introduce vibrations. Instead, a ring of permanent magnets is attached to the rotor of the air bearing, the part that supports the giant mirror. These magnets turn within a magnetic field produced by three stationary field windings. By supplying the proper electrical currents to these windings, a rotating magnetic field is generated, which then turns the mirror.
We had thought that the large inertia of our massive mirror would prevent air currents and other disturbances from causing any variation in rotation speed. We were wrong. When we first turned the telescope on, we saw speed variations as large as 0.1 percent when gusts of wind came through the open roof of the observatory building. These variations were 1,000 times larger than the 1-part-per-million stability we were aiming for. Clearly something had to be done.
The solution was to design and build a system that actively stabilizes the rotation speed. As the mirror turns, a device attached to it generates 2,500 precisely spaced pulses for each 360-degree rotation. The control system measures the arrival time of each pulse and compares it with the expected time for a mirror moving at the prescribed speed. If a pulse is late, the mirror must be slowing, so the power supplied to the stationary windings of the motor is increased to compensate. If the pulse comes early, the motor currents are adjusted to slow the motion of the mirror. This approach cured the problem, and the mirror speed is now constant to about one part per million under almost all observing conditions.
We discovered a second, more sinister, problem when we started to record images with the telescope: Every star was surrounded by a diffuse halo of light. The effect had been seen before in the laboratory, but never to such a large extent. Numerous waves propagating on the surface of the mercury were diffracting light and blurring our images. But what was causing the waves?
Some investigation soon revealed the answer. The liquid mirror of the LZT is large and rotates fairly rapidly. The rim of the mirror is moving through the air at a speed of 2.2 meters per second. Thus the mercury on the outside of the mirror sees air moving over it at this rate. As is the case for wind blowing across water, waves are produced.
We tested this theory by probing the air flow close to the surface using a piece of Christmas tree tinsel attached to a wand. Near the center of the mirror, the flow of air was smooth, but beginning at a radius of about one meter, things became turbulent. Spiral vortices were forming less than a centimeter above the mercury, where they were creating the troublesome waves. Careful optical tests confirmed this result.
Our solution was to suspend a very thin film of transparent Mylar plastic a few centimeters above the mercury. This cover rotates with the mirror, trapping the air beneath and forcing it to rotate at the same speed as the mercury. The vortices are still present, but now they are above the plastic and cannot affect the reflective liquid surface. When we added this cover, the waves vanished. The LZT now achieves an image quality comparable with that of the best conventional telescopes.
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