Sky & Telescope - February 2024 - 22

Dark Energy Experiment
Reaching out into Space -
and Back in Time
Although many newer telescopes are
larger than the Mayall - the Keck telescopes
in Hawai'i, for example, have
mirrors more than twice its diameter
- the half-century-old machine turns
out to be perfectly suited for DESI.
For starters, it has a wide field of view,
which has been made even wider with
a specially designed set of corrector
lenses. The 9-ton assembly holds six
corrector lenses, each roughly a meter
across, and rests on a metal frame
some 15 meters (50 feet) above the
observatory floor. Luckily, the Mayall was " built like a battleship, "
says Arjun Dey (NOIRLab). " It was one of the few telescopes
that had enough structural integrity to be able to hold
what is essentially a giant school bus up above its mirror and
not have it all collapse. " The corrector lenses give the telescope
an effective field of view of just over 3° - equivalent to
six times the apparent size of a full Moon. The wide field of
view is essential for efficiently covering large swaths of sky.
DESI will ultimately map one-third of the celestial sphere -
some 14,000 square degrees.
With its wide field of view, DESI can record thousands
of galaxies at a time; it's currently collecting some
100,000 spectra per night. That's where DESI's most remarkable
innovation comes in: its " robot army " of 5,000 miniature,
automated positioners that feed the incoming light
from each galaxy into one of 10 identical spectrographs via
OVERHAULED In order to install the DESI
instrument, the entire top half of the Mayall
telescope (pictured here) was replaced.
fiber-optic cables, one fiber per galaxy.
After obtaining spectra from all the
galaxies in a particular patch of sky,
the telescope swivels over to the next
patch, and the little robots spring into
action, reconfiguring the positioners so
that the fiber-optic cables line up with
new targets. The whole repositioning
procedure - a precisely choreographed
dance of 5,000 little robots - takes
only a couple of minutes.
DESI's technological innovation is
even more apparent when contrasted with its predecessor,
the Sloan Digital Sky Survey, which began mapping the sky
in 2000 using a telescope in New Mexico (S&T: Jan. 2023,
p. 20). Although the SDSS telescope also now employs robots
to position fiber-optic cables, for many years those fibers
had to be placed by hand. To that end, engineers would drill
about 1,000 tiny holes in an aluminum plate that was put at
the telescope's focal plane. A human would then connect the
cables manually to each hole. Whenever the telescope was
aimed at a new part of the sky, a new aluminum plate (with
a completely different set of drilled holes) had to be inserted
and the fibers re-attached. With DESI's robot army, no plates
and no drilling are required; the robots simply move the fibers
to where they need to be. (Juneau keeps one of the old Sloan
plates in her office, propped up against a wall - a relic of how
astronomical surveys were done not all that long ago.)
ROBOT HELPERS DESI's focal plane, which sits high atop the Mayall telescope, carries 5,000 robotic positioners, distributed among 10 wedges
with 500 positioners each. (Part of a single wedge is shown here.) Each robot holds a fiber-optic cable, which it positions for a single target for every
observation. The movements are carefully choreographed to avoid collisions. Visit https://is.gd/DESIrobots to see the fiber-positioners in action.
22 FEBRUARY 2024 * SKY & TELESCOPE
MAYALL TELESCOPE: MARILYN CHUNG / LAWRENCE BERKELEY NATIONAL LAB / KPNO /
NOIRLAB / NSF / AURA; LITTLE ROBOTS: MARILYN SARGENT / © 2017 THE REGENTS OF THE
UNIVERSITY OF CALIFORNIA, LAWRENCE BERKELEY NATIONAL LABORATORY

https://www.is.gd/DESIrobots

Sky & Telescope - February 2024

Table of Contents for the Digital Edition of Sky & Telescope - February 2024