Sky & Telescope - April 2020 - 27

Of course, building a large, unconventional telescope
presented many technological hurdles, too. Right from the
start, it was clear that the Belgian company AMOS (Advanced
Mechanical & Optical Systems), also based in Liège, would be
the main contractor for the ILMT. AMOS knew all about constructing professional telescopes (it built the four 1.8-meter
auxiliary telescopes for the Very Large Telescope Interferometer
in Chile), but the company had no experience with liquid mirror technology. Luckily, while visiting the NODO facility in
New Mexico in 2000, Surdej ran into Hickson and convinced
him to join the project. "The man is a genius," Surdej says.
"Without him, the ILMT would never have been possible."
One complicated component of the future telescope was
the corrector in front of the CCD camera. Because a quicksilver telescope has a ﬁ xed orientation, stellar images move
across the CCD as a result of Earth's rotation. They do so in
slightly curved paths, dependent on the telescope's geographic
latitude. Apart from compensating for off-axis optical aberrations, the corrector - an intricate set of special-purpose
lenses - also needs to rectify these curved tracks into straight
lines. As a result, each stellar image moves across a straight
row of pixels at a constant pace, enabling longer exposures
by reading out the CCD in a "co-moving" way - a process
known as time-delayed integration.
Initially, Surdej and his team were eying the European La
Silla Observatory in Chile as the site for the new telescope.
After all, back in the 1960s Belgium had been one of the
founding members of the European Southern Observatory
(ESO). Moreover, Riccardo Giacconi, ESO's director general
from 1993 to 1999, was enthusiastic about the project. By
the late 1990s, following a design proposed by Hickson and
optical engineer Harvey Richardson, a British company was
already constructing the ILMT corrector for La Silla's latitude
of 29° south. But it never panned out, says Surdej. "We didn't
develop a good personal relationship with La Silla's director,
Jorge Melnick, and he wasn't very supportive. We would have
had to pay €70,000 [about US$78,000] per year, just for the
site, water, and electricity."
Another setback came in 2002, when the United Kingdom

-PAUL HICKSON

joined ESO. Just a few years earlier, the British research council PPARC had ranked UK participation in the ILMT project
as one of their top priorities, but now they decided that all
their astronomy funding should go to ESO. Around the same
time, the French funding agency CNRS gave priority to the
378-megapixel MegaCam instrument on the Canada-FranceHawai'i Telescope on Mauna Kea. Suddenly, the ILMT's future
started to look bleak.

Himalayan Home
Meanwhile, back in Vancouver, Hickson had secured funds
for the construction and operation of a larger, 6-meter liquid
mirror telescope. Building on the experience with the earlier
2.7-meter prototype and using parts of the decommissioned
NODO instrument, this Large Zenith Telescope (LZT) was
erected between 2003 and 2005 for less than $1 million
(S&T: Apr. 2013, p. 26).
"It took us about a year to get it working properly," says
Hickson. "The main problem was that the fast rotation of the
mirror - one meter per second at the rim - was causing air
ﬂows that resulted in tiny ripples on the mercury surface."
Eventually, the researchers solved the problem by suspending
an ultra-thin, transparent sheet of Mylar just above the mirror's surface to damp any unwanted turbulence.
However, the Maple Ridge site suffered from light pollution and bad weather. "It proved to be a good place to develop
the technology, but not such a good location for astronomy,"
says Hickson. Instead, starting in 2008 the LZT was mainly
used in combination with a lidar facility to precisely study the
variable properties of the sodium layer in Earth's mesosphere.
Large telescopes ﬁre laser beams into the sky to excite sodium
atoms in this 90-kilometer-high layer; the resulting laser
guide stars enable the use of adaptive optics to compensate for
the effects of atmospheric turbulence.

q SPIN CASTING (From left to right) Team
members Denis Defrère, Paul Hickson, Arnaud
Magette, and Stefan Denis pour polyurethane
over the spinning mirror basin, which is made
of sheets of carbon fiber.

q SMOOTHER THAN SILK Surdej checks
the smoothness of the mirror basin's
polyurethane surface during the first spin
casting in 2010.

ANNA AND JE AN SURDEJ (3)

q TEAM EFFORT (From left to right) Jean
Surdej, Paul Hickson, Stefan Denis, and
Tatyana Sadibekowa prepare for the basin's
spin casting.

"We had the most powerful lidar
facility in the world."

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Sky & Telescope - April 2020

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