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Liquid-Mirror Telescopes

An old idea for astronomical imaging is undergoing a technology-driven renaissance

Paul Hickson

The Modern Revival

In 1982, Canadian physicist Ermanno F. Borra, working at Laval University in Quebec, decided to revisit the approach Wood had pioneered. Borra and his colleagues realized that modern technology could readily solve the difficulties that Wood had had no way to address during the Edwardian era.

In particular, Borra realized that the problem of the image drifting across the field of view as the Earth turned could be solved by replacing traditional film with a modern detector, a solid-state sensor known as a charge-coupled device (CCD), which is a particular kind of silicon integrated circuit. Photons impinging on the silicon deposit enough energy to boost electrons into the conduction band. These electrons are stored in potential wells created by voltages that are applied to an array of electrodes. Thus, an optical image is converted to an electronic image within the silicon. At the end of an exposure, the shutter is closed and applied voltages are manipulated in such a way as to move the collections of electrons across the face of the device to one side and from there bucket-brigade style to an output amplifier that produces a series of voltage signals, each proportional to the number of electrons that were collected in each potential well. In this way, the image is read out one pixel at a time to a computer where it can be stored, processed and displayed.

Most of today's digital cameras use CCDs to capture images in this way. When the illumination is low, one simply needs to allow the CCD to gather light for a longer interval before the shutter is closed. Unfortunately, this tactic will not work with a liquid-mirror telescope, which cannot track a celestial object as it moves through the sky. If the CCD was used in the usual way, star images would appear as streaks rather than as sharp points.

Charge-coupled devices can, however, be operated in a different manner, by manipulating the applied voltages so as to shift the electronic image to the side of the chip during the exposure. By aligning the direction of shift with the direction in which the projected starlight is moving and by applying voltages to the CCD electrodes at the appropriate rate, one can coax electrons along at the same speed as the drifting image. If that shifting is done properly, there is no blurring, because the electrons keep pace with the photons that are producing them.

When a star image reaches the edge of the CCD, so do the corresponding electrons, which are then transferred to the amplifier that measures them. No shutter is needed because the readout takes place continuously, typically at the rate of some tens of lines per second. The effective exposure duration is the time that it takes for a star image to drift across the face of the CCD, typically one or two minutes. Astronomers often use this tactic, called "drift scanning," when they make observations with conventional telescopes, because it provides a very efficient way to image a large area of sky.

Figure 3. Large liquid mirrors are prone to distortionsClick to Enlarge Image

Although the prospect of drift scanning inspired Borra to revive the idea of liquid-mirror telescopes, he never progressed to the point of employing this technique. He did, however, introduce other innovations. To overcome vibration and wobble, Borra employed an air bearing, which has precisely ground surfaces separated by a thin film of pressurized air. Such bearings are virtually frictionless and can thus provide very smooth rotation. Using a synchronous motor driven by a crystal-controlled oscillator, Borra also eliminated the speed variations that had plagued Wood's instrument.

 In less than a decade, Borra and his colleagues had built mercury mirrors as large as 1.5 meters in diameter; they would later go on to build one that was 3.7 meters across. These mirrors had surfaces of very high optical quality. Indeed, Borra and his team were able to obtain diffraction-limited images from these mirrors in the laboratory—that is, ones that were as sharp as is theoretically possible for an optical element of that size. And Borra used some of his mirrors to make astronomical observations in combination with a 35-millimeter film camera.

These efforts attracted the attention of other astronomers, including me. After much work, my colleagues and I demonstrated the first truly practical liquid-mirror telescope, one that took advantage of drift scanning, in 1994. That instrument employed a 2.7-meter-diameter mercury mirror, a four-element lens for correcting off-axis distortion and a CCD camera. With this telescope, we obtained digital images of stars and galaxies at a resolution that was limited only by what astronomers call "seeing" (blurring from atmospheric turbulence)—not by imperfections in the liquid mirror. This telescope was soon followed by three others, two being used for laser investigations of Earth's atmosphere and the third, the 3-meter NASA Orbital Debris Observatory (which operated in New Mexico through 2002), for studies of "space junk."

Building on this experience (and on some skills I had picked up in my spare time constructing experimental aircraft), in 2005 my coworkers and I fabricated a 6-meter mirror for an instrument we dubbed the Large Zenith Telescope (LZT). The aim of this project was to develop and perfect liquid-mirror technology on a scale competitive with the largest conventional astronomical telescopes. Of course, the inability to look in any direction other than straight up (the zenith) means that such telescopes will never replace their conventional counterparts, which can point to just about any part of the sky.

But this lack of flexibility is not quite so limiting as you might think. For a large number of scientific studies, fixed pointing is not a hindrance. If the aim is to determine the statistical properties of a large number of objects, say distant galaxies, it often does not matter what part of the sky you probe. One finds distant galaxies everywhere, and the zenith is as good a place as anywhere else to look. Indeed, it is the best place to direct your telescope because it offers the least amount of air in the light path and hence the least amount of atmospheric absorption, scattering and image distortion.

For such research programs, liquid-mirror telescopes serve just as well as conventional telescopes but don't cost nearly as much. The LZT was built for less than $1 million—an order of magnitude lower than what one would spend for a conventional telescope of comparable size. So if some of the funds slated for the construction of telescopes were channeled into instruments that use liquid mirrors, astronomers doing this kind of research could get considerably more observing time.

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