Unorthodox Telescopy

LiquidAstronomy
Eagle Gamma

The Large Zenith Telescope’s mercury mirror
provides critical insights for next-gen instruments.

Paul Hickson / University of British Columbia

Deep within the forests of British Columbia, on a granite outcrop atop a hill, sits a hidden gem — a shining poolof mercury. At 6 meters in diameter, the Large Zenith
Telescope (LZT) ranks among the world’s largest scopes.
It’s also the largest of a unique type of instrument: theliquid-mirror telescope.
After a long trek through tranquil forests — surrounded by scents of western red cedar and sights ofsprawling woodland growth — it feels surreal to standnext to the cool, clear primary “mirror,” like stumbling
on a spaceship in a fairy tale. A thin layer of mercury mere
millimeters deep rests on a dish designed to minimize
the volume of liquid. This dish spins the mercury at aconstant speed of 8.5 revolutions per minute, creating asmooth, highly reflective parabolic surface. (Stir a cup ofcoffee with circular motions, and you’ll see a similar valley

26

The Large Zenith Telescope sits in the University of British
Columbia’s Malcolm Knapp Research Forest, about 50 kilometers (30 miles) east of Vancouver.

Apr il 2013 sky & telescope

form in the center.) When united with corrector lenses andother telescope hardware, the immense mirror works likea conventional scope.
This unique instrument is clearing the way for otherincredible technology. Researchers are using studies ofEarth’s atmosphere undertaken with the LZT to design
the next generation of superscopes, particularly the proposed Thirty Meter Telescope and the 39-meter European
Extremely Large Telescope. Liquid-mirror telescopesmight also be the inexpensive solution astronomers needin order to pursue survey work that eats up valuable timeat conventional observatories.

Simple Has its PerksPaul Hickson (University of British Columbia, Canada)
built and directs the Liquid-Mirror Observatory, the homeof the LZT. He leads the world in designing these exotic
telescopes. A clever, acute man with a sense of adventure,
Hickson also flies and builds experimental aircraft, andhis telescope’s design has influences from aerospace.
When it came time to build the Liquid-Mirror Observatory, his team surveyed several candidate sites withinreach of the university — looking at weather statistics andmaps, flying overhead, and driving around by Jeep. They
selected this forest sanctuary, far away from city lights,
over several other locations.
The LZT has a relatively tiny price tag. Large conventional telescopes cost many millions of dollars to build,and tens of thousands of dollars per night to operate.
That’s orders of magnitude more than the Liquid-Mirror
Observatory, which was built for half a million dollars.
Canadian physicist Ermanno Borra (Laval University),

How Liquid-Mirror Telescopes Work
Isaac Newton first described the rotating-fluid conceptthat makes the liquid-mirror telescope possible. Whena liquid spins at a constant speed, the combinationof gravity and rotational acceleration finds a dynamic
equilibrium that makes the top surface of the fluid
form a smooth parabolic shape. The paraboloid arises
because a liquid surface always forms its local surface
perpendicular to the net acceleration it experiences.
In the case of a spinning mirror, the net accelerationbecomes stronger and more inclined with distancefrom the spin axis at the mirror’s center.
Put a camera at the focal point of the paraboloid,
where the reflecting surface focuses light into a single
point, and you have yourself a liquid-mirror telescope.
Mercury vapors from the Large Zenith Telescope can be dangerous
during the first hours after its setup. But oxidation soon prevents vapor
emission, and after a day or so gas masks are unnecessary.

Paul Hickson / University of British Columbia

pioneer of liquid-mirror telescopy, thinks that cost is theessential advantage of these instruments.
“It’s something like a factor of 100, the difference inthe cost of the mirrors,” he says. “It’s really very, very
inexpensive.”
The same simplicity that makes liquid-mirror telescopes so affordable also results in excellent optical quality. The LZT produces astronomical observations with resolutions comparable to a conventional telescope of similarsize, and it observes stars and distant spiral galaxies ataround the atmospheric resolution limit. Because a fluid
naturally flows to a smooth shape, liquid mirrors achieve
impeccable optical quality far more easily than polishedglass, with the potential to produce a perfect mirror.
That perfection depends on finely tuned hardware.
A display in the control room shows variations in therotational speed of the spinning mirror: nine parts oferror per million. When Hickson first built the mirror, hemeasured the rotational error at 1,000 parts per million.
The ensuing jitters set mercury sloshing, destroying thereflected images.

Sk yandTelescope.com April 201327

Unorthodox Telescopy
Hickson added an optical encoder that measures angular movement, allowing better control. He also installed
an optically clear cover of Mylar film, only a few microns
thick, that sits a few centimeters above the mercury androtates with the mirror. This cover protects the liquid fromwind blowing in through the open roof and also preventsthe formation of small vortices in the air above the moving liquid, which create tiny waves that degrade images.
As the name implies, the Large Zenith Telescope only
sees the portion of the sky directly above the observatory:
tipping the mirror would spill the mercury, so the liquid
mirror must face straight up. Researchers get around thelimitation somewhat by “drift scanning,” delaying theCCD’s readout to match the sky’s drift speed and allow for
artificial tracking. But liquid-mirror telescopes still only
serve for astronomical studies that do not require steering. The observatory sits at a latitude of 49°, which meansthat as Earth rotates, the telescope observes a strip of sky
at 49° declination. Such zenith strip surveys are useful fora variety of scientific pursuits, ranging from cosmology tothe detection of supernovae.
The LZT currently pursues none of these science
projects, though. It has a far greater demand on its time:
guiding astronomers in designing the upcoming gargantuan telescopes by studying sodium in the atmosphere.

S&T: Leah Tiscione

Solving the Sodium Problem

Paul Hickson / University of british columbia (2)A liquid-mirror telescope’s defining trait is its rotating primary
mirror. Paired with correcting lenses and a detector, the parabolic
liquid surface becomes an inexpensive large-aperture scope.

28Apr il 2013 sky & telescope

A new generation of so-called “extremely large telescopes,” such as the Thirty Meter Telescope and theEuropean Extremely Large Telescope, have major design
challenges to tackle. The telescopes will house primary
mirrors three to four times the diameter of today’s largest
optical telescopes. Huge mirrors make it possible to getlarge, sharp, intense images — yet that very same sensitivity subjects the telescopes to atmospheric distortions

The liquid mirror’s construction began with its large supporting steel trusses (left). Over these went segments made of acomposite of fiberglass, foam, and epoxy (above).

Thomas Pfrommer et al. / Astrophysical Journalthat smaller instruments don’t notice. Optical sensitivity grows proportionally with the diameter to the fourthpower, which means that increases in size have dramatic
impacts on scopes’ abilities.
One particular challenge facing the upcoming giants
is their adaptive optics. Adaptive optics systems compensate for the atmosphere’s blurring effects by distorting theshape of a telescope’s secondary mirror. At some observatories, astronomers map these atmospheric changes
by beaming a laser into the mesophere’s sodium layer,
effectively creating a bright fluorescent lamp in the skythat acts as a “guide star.” The secondary mirror thenadjusts its shape rapidly and repeatedly — sometimes
more than 1,000 times per second — in order to matchthe distortions in the atmosphere detected from the laser
guide star. Large ground-based telescopes require adaptive optics to perform better than the seeing limitation
imposed by Earth’s atmosphere.
The sodium layer consists of several distinct levels,
varying in density and altitude. Ocean-like waves rollalong the entire layer, and turbulence induces variability.
These irregularities, coupled with changes in the entire
layer’s average altitude, change the structure and distanceof guide stars, confusing the adaptive optics system. Even
five meters of variation in the sodium layer’s altitude canaffect the system.
“If a meteor trail occurs in the middle of an observation, it can change the average range to the sodium layer
by a hundred times that much, or more,” says BrentEllerbroek, department head of instrumentation for theTMT. The consequent error would be bigger for larger
telescopes, increasing with the square of the diameter.
Such errors would wreak havoc on the behemoths’ observations. “So it’s very important to understand how thesodium layer is evolving in time.”
Scientists had come to this conclusion by extrapolatingfrom data taken on timescales six orders of magnitude
longer than those on which adaptive optics operate. To
verify that the sodium layer would behave on small scales
as they thought it would — before building the billiondollar observatories — the scientists needed a way to measure the actual variations of atmospheric sodium density
at a fine enough resolution to correct for any errors. The
best existing data in the world would not do.
It turns out that the LZT can collect precise enoughdata to resolve the problem. During a lull created by abroken camera sensor, Hickson and one of his graduate
students installed a laser at the facility. The laser effectively turned the observatory into the world’s largest test
bed for “lidar,” or light radar, the laser technique used tocreate guide stars.
Using lidar, the LZT picked up never-before-seeneddies and vortices in the sodium layer, as well as detailsof both its structure and dynamics. Observations havealso revealed the first detection of turbulence waves in

The atmosphere’s sodium layer varies over time. Two nights of observation by the Large Zenith Telescope show multiple, transitory layers, withdensities, altitudes, and structures that evolve over time. The brightestlayers are about two to three times denser in sodium than the background. The dark vertical bars are from aircraft interruptions.

the mesosphere — curling licks of sodium that interact
chaotically with neighboring layers of the atmosphere.
Ellerbroek lauds the unique situation that such a large
telescope can be devoted to lidar studies. The LZT has 100
to 500 times the collecting area of other lidar systems, hesays, and its incredible sensitivity enables measurementswith resolutions in meters and on timescales of much lessthan one second.
“For an 8-meter telescope that’s not important,” hesays. “But for future 30- to 40-meter telescopes, under-Brief History of Liquid-Mirror Telescopes
Despite coming up with the rotating-fluid idea, Newton apparently
did not consider a telescope based on a liquid paraboloid. The
concept went through several periods of development and dormancy. In 1982 Ermanno Borra revived the idea and soon realized,
in his words, “Whoa, wait a second, you can do science with that!”
In 1994 he and Hickson built a successful 2.64-meter telescope.
After that, Hickson created a series of mirrors, enhancing the sizeand performance with each iteration. At first he essentially workedout of his garage, building liquid mirrors for North American universities. Then NASA found out about his work and contacted him.
The agency wanted a large, affordable telescope — both of which
describe liquid-mirror scopes. Hickson built a 3-meter mirror for adevice housed at NASA’s Johnson Space Center in Houston. NASAlater moved the scope to the Orbital Debris Observatory, in the Lincoln National Forest of New Mexico. The telescope collected dataon space debris for many years, earning a NASA Group Achievement Award. Some surplus NASA components made their way into
the Large Zenith Telescope.

Sk yandTelescope.com April 201329

Unorthodox Telescopy
standing how the range to the sodium layer changes onthat timescale becomes important.”
Ellerbroek says that he and his colleagues use theLZT’s lidar data to design the wavefront-sensing equipment they’ll use on the TMT, even to determine whatkind of lasers they need to buy. “We’re actually able toinput this data into simulations of the adaptive optics systems, then predict how well the components we’re designing and buying will work with the sodium layer as weunderstand it,” he explains. A carefully planned adaptive

optics system will give the TMT three or four times better resolution than one of the Keck telescopes on MaunaKea, which are among the world’s premier ground-based
visible and near-infrared telescopes and the standard bywhich others are measured. “It’s going to have a dramatic
impact on the types of observations that can be done.”

Paul Hickson / University of british columbia (2)Futuristic Scopes

The 6-meter f/1.50 Large Zenith Telescope can observe objects fainter than magnitude 25. These images are from its perpetual view of declination 49° 16′.

30

Apr il 2013 sky & telescope

The LZT may herald a new wave of liquid-mirror telescopes, even though this particular scope will not immediately contribute to astronomical research. Hickson saysthat they chose the observatory’s site because it’s a good
location for testing and developing liquid-mirror technology, but the site sees few clear nights. Good weather
comes mostly in the short summer nights.
“Our aim all along was to put one of these telescopes
at a competitive astronomical site once the technology had
been perfected,” he says.
One of these future instruments, the 4-meter International Liquid Mirror Telescope, was developed independently from the LZT but has benefited from knowledge
gleaned from its forest counterpart. The ILMT is being
installed at Devasthal in the Himalayas, a high-altitudesite already home to two observatories.
Bigger dreams include a network of mirrors reminiscent of the Atacama Large Millimeter/submillimeter
Array in Chile. Moreover, a few years ago Hickson andBorra contributed to a proposal to place a telescope ofionic liquid (basically, molten salt) as large as 100 meters
in diameter on the Moon. The innovative technology,including superconducting bearings and a cryogenic
vacuum, would allow astronomers to observe the early
universe at higher resolutions and fainter magnitudes
than the upcoming James Webb Space Telescope (S&T:January 2010, page 24). Even a much smaller Moon-based
liquid mirror would be a useful survey instrument to follow up on JWST’s observations.
Borra also proposed that technological developments
in future decades might allow scientists to launch anorbiting liquid-mirror telescope as large as one kilometer
across. “It was propelled by a solar sail,” he says. “It wasreally a monster.”
The space proposals received serious consideration:at the time (before the cancellation of the Constellation
program) NASA thought that the 100-meter mirror would
be a major reason for going back to the Moon, Borra says.
Despite changes in NASA’s focus, Borra thinks that thegigantic lunar liquid-mirror telescope will one day see
first light.
In the meantime, astronomers’ next-gen scopes will
stand on the shoulders of a shining pool of mercury in
the forests of British Columbia. ✦Freelance writer Eagle Gamma covers adventurous science,
particularly discoveries in astronomy and optics.