Solving the dark energy mystery: A new assignment for a 45-year-old telescope

Solving the dark energy mystery: A new assignment for a 45-year-old telescope
A view inside the dome at the Mayall Telescope near Tucson,
Arizona. The 2-meter corrector barrel atop the telescope
will be removed and replaced with a new corrector barrel
for the Dark Energy Spectroscopic Instrument. DESI’s
installation will begin soon. Credit: P. Marenfeld and

Forty-five years ago this month, a telescope tucked inside a
14-story, 500-ton dome atop a mile-high peak in Arizona took
in the night sky for the first time and recorded its
observations in glass photographic plates.

Today, the dome closes on the previous science chapters of the
4-meter Nicholas U. Mayall Telescope so that it can prepare for
its new role in creating the largest 3-D map of the universe.
This map could help to solve the mystery of , which is driving the accelerating
expansion of the universe.

The temporary closure sets in motion the largest overhaul in
the telescope’s history and sets the stage for the installation
of the Dark Energy Spectroscopic Instrument (DESI), which will
begin a five-year observing run next year at the National
Science Foundation’s Kitt Peak National Observatory (KPNO) –
part of the National Optical Astronomy Observatory (NOAO).

“This day marks an enormous milestone for us,” said DESI
Director Michael Levi of the Department of Energy’s Lawrence
Berkeley National Laboratory (Berkeley Lab), which is leading
the project’s international collaboration. “Now we remove the
old equipment and start the yearlong process of putting the new
stuff on.” More than 465 researchers from about 71 institutions
are participating in the DESI collaboration.

The entire top end of the telescope, which weighs as much as a
school bus and houses the telescope’s secondary mirror and a
large digital camera, will be removed and replaced with DESI
instruments. A large crane will lift the telescope’s top end
through the observing slit in its dome.

Besides providing new insights about the universe’s expansion
and large-scale structure, DESI will also help to set limits on
theories related to gravity and the formative stages of the
universe, and could even provide new mass measurements for a
variety of elusive yet abundant subatomic particles called

“One of the primary ways that we learn about the unseen
universe is by its subtle effects on the clustering of
galaxies,” said DESI Collaboration Co-Spokesperson Daniel
Eisenstein of Harvard University. “The new maps from DESI will
provide an exquisite new level of sensitivity in our study of

The Mayall Telescope has played an important role in many
astronomical discoveries, including measurements supporting the
discovery of dark energy and establishing the role of dark
matter in the universe from measurements of galaxy rotation.
Its observations have also been used in determining the scale
and structure of the universe. Dark matter and dark energy
together are believed to make up about 95 percent of all of the
universe’s mass and energy.

It was one of the world’s largest optical telescopes at the
time it was built, and because of its sturdy construction it is
perfectly suited to carry the new 9-ton instrument.

“We started this project by surveying large telescopes to find
one that had a suitable mirror and wouldn’t collapse under the
weight of such a massive instrument,” said Berkeley Lab’s David
Schlegel, a DESI project scientist.

Arjun Dey, the NOAO project scientist for DESI, explained, “The
Mayall was precociously engineered like a battleship and
designed with a wide field of view.”

The expansion of the telescope’s field-of-view will allow DESI
to map out about one-third of the sky.

Brenna Flaugher, a DESI project scientist who leads the
Astrophysics Department at Fermi National Accelerator
Laboratory, said DESI will transform the speed of science at
the Mayall Telescope.

“The telescope was designed to carry a person at the top who
aimed and steered it, but with DESI it’s all automated,” she
said. “Instead of one at a time we can measure the velocities
of 5,000 galaxies at a time – we will measure more than 30
million of them in our five-year survey.”

DESI will use an array of 5,000 swiveling robots, each
carefully choreographed to point a fiber-optic cable at a
preprogrammed sequence of deep-space objects, including
millions of galaxies and quasars, which are galaxies that
harbor massive, actively feeding black holes.

The fiber-optic cables will carry the light from these objects
to 10 spectrographs, which are tools that will measure the
properties of this light and help to pinpoint the objects’
distance and the rate at which they are moving away from us.
DESI’s observations will provide a deep look into the early
universe, up to about 11 billion years ago.

The cylindrical, fiber-toting robots, which will be embedded in
a rounded metal unit called a focal plane, will reposition to
capture a new exposure of the sky roughly every 20 minutes. The
focal plane, which is now being assembled at Berkeley Lab, is
expected to be completed and delivered to Kitt Peak this year.

DESI will scan one-third of the sky and will capture about 10
times more data than a predecessor survey, the Baryon
Oscillation Spectroscopic Survey (BOSS). That project relied on
a manually rotated sequence of metal plates – with fibers
plugged by hand into pre-drilled holes – to target objects.

The first of 10 wedge-shaped petals for the DESI project is
fully stocked with 500 slender robotic positioners. These
positioners will each swivel independently to gather light
from a preprogrammed sequence of space objects, including
galaxies and quasars. The petals will fit snugly together to
form DESI’s focal plane, which will be composed of about
600,000 individual parts. Credit: DESI Collaboration

All of DESI’s six lenses, each about a meter in diameter, are
complete. They will be carefully stacked and aligned in a steel
support structure and will ultimately ride with the focal plane
atop the telescope.

Each of these lenses took shape from large blocks of glass.
They have criss-crossed the globe to receive various
treatments, including grinding, polishing, and coatings. It
took about 3.5 years to produce each of the lenses, which now
reside at University College London in the U.K. and will be
shipped to the DESI site this spring.

The Mayall Telescope has most recently been enlisted in a
DESI-supporting sky survey known as the Mayall z-Band Legacy
Survey (MzLS), which is one of four sky surveys that DESI will
use to preselect its targeted sky objects. That survey wrapped
up just days before the Mayall’s temporary closure, while the
others are ongoing.

Data from these surveys are analyzed at Berkeley Lab’s National
Energy Research Scientific Computing Center (NERSC), a DOE
Office of Science User Facility. Data from these surveys have
been released to the public at

“We can see about a billion galaxies in the survey images,
which is quite a bit of fun to explore,” Schlegel said. “The
DESI instrument will precisely measure millions of those
galaxies to see the effects of dark energy.”

Levi noted that there is already a lot of computing work
underway at NERSC to prepare for the stream of data that will
pour out of DESI once it starts up.

“This project is all about generating huge quantities of data,”
Levi said. “The data will go directly from the telescope to
NERSC for processing. We will create hundreds of universes in
these computers and see which universe best fits our data.”

Installation of DESI’s components is expected to begin soon and
to wrap up in April 2019, with first science observations
planned in September 2019.

“Installing DESI on the Mayall will put the telescope at the
heart of the next decade of discoveries in cosmology,” said
Risa Wechsler, DESI Collaboration Co-Spokesperson and associate
professor of physics and astrophysics at SLAC National
Accelerator Laboratory and Stanford University. “The amazing
3-D map it will measure may solve some of the biggest
outstanding questions in cosmology, or surprise us and bring up
new ones.”

Explore further:
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