Sky & Telescope - November 2023 - 17

NSO / AURA / NSF
in 2002, and the 1.5-m Gregor Telescope just east of it on
Tenerife, completed in 2012. DKIST joined the ranks in 2021.
DKIST has a 4-meter parabolic mirror that's more than
double the size of previous solar telescopes' primaries and
comes with cutting-edge stabilization, filtering, and cooling
technologies, making it more powerful and adaptive than any
solar telescope currently in operation. It is meant to be a kind
of " microscope " for the Sun, with an unprecedented ability to
zoom in on the surface, enabling detailed observations of the
magnetic field and other dynamic behaviors. It can resolve
features as small as 16 kilometers (10 miles) across on the
solar surface, somewhat larger than the city of San Francisco;
previous solar telescopes could only resolve features on a scale
of roughly 100 km.
The design and operation of solar telescopes present
unique challenges compared with dark-sky telescopes.
Namely, solar telescopes must be able to withstand large
amounts of heat. We are taught not to look right at the Sun,
because it can hurt our eyes. The same is true for telescopes.
The Sun emits a tremendous amount of energy and radiation,
which can damage detector technology. To prevent this,
DKIST has a specially designed facility and coolant system.
On the outside, DKIST's dome is covered by thin, actively
cooled plates and shutters. Inside, the observatory has more
than 11 kilometers of piping running throughout the walls,
floors, and machinery. During observing hours a special kind
of coolant - dynalene - flows through these tubes alongside
the telescope's heat-sensitive systems and instruments, keeping
them at room temperature.
The primary mirror is carefully chilled on both sides, by
cold air behind and a liquid-cooled ring in front. Focused
light from the primary is then directed towards the secondary
mirror, where the beam would intensify even more. To
mitigate this, a heat stop, a liquid-cooled, metal doughnut,
sits between the mirrors and filters out more than 95% of the
heat before it hits the secondary mirror.
So far, these cooling technologies have worked, and none
of the telescope elements has suffered heat damage. But no
system is foolproof. This is why sensors constantly monitor
conditions in the observatory. If the temperatures within
climb too high, safety covers deploy to protect the primary
and secondary mirrors, and the dome automatically closes.
Here Comes the Sun
Kilometer by square kilometer, advancing technologies have
zoomed in on the Sun. The hope is that a better understanding
of what's happening on the smallest scales will help us
make sense of how the Sun acts as a whole. DKIST, a project
30 years in the making, takes this approach to the next level.
The DKIST instrument suite is at the heart of the science
mission. It includes an imager and four spectropolarimeters,
which measure the Sun's magnetic properties. Each instrument
plays a specific role in measuring the ever-changing
conditions in the solar atmosphere, from the visible surface
(the photosphere) to the outermost atmosphere (the corona).
EYE OPEN DKIST stands with open aperture beneath a blue sky near
the summit. A series of windows called vent gates enables operators to
control the temperature and airflow inside the enclosure, keeping equipment
stable in a variety of conditions.
sk yand tele scope .o r g * NOVEMBER 2023 17
They are housed in a 16-square-meter (170-square-foot),
rotating clean room located directly below the observing platform,
called the Coudé Laboratory.
Light from the telescope is directed downwards from the
secondary mirror and through a complex system of mirrors.
Then it passes through an air knife - a barrier created
by a steady, rushing stream of air - before entering the lab
through a hole in the ceiling. The air knife acts to separate
the observatory environment from the lab below. This is
necessary because the observatory is open to the outside,
and conditions on the platform can vary by season and time
of day - whereas the instrument lab requires a dust-free
environment with a stable room temperature of 20°C (68°F)
to ensure everything works properly. Typical cleanrooms use
glass or plastic barriers, but these would absorb infrared light
from the Sun, which scientists want to study. Air knives are
most commonly used in manufacturing; this is the first time
one has been used in a solar telescope.
As the intensified light from the Sun shines down through
the ceiling aperture into the Coudé, it looks futuristic, like a
transporter beam from Star Trek. Before the sunlight hits the
instrument table, it encounters a deformable mirror. The purpose
of this technology is to cancel out the effects of Earth's
atmosphere on the light. A series of tiny pistons continuously
skews the mirror's surface according to a complex computer
algorithm, changing the mirror's shape up to 2,000 times per
second. Imagine wearing a pair of swimming goggles that corrects
for underwater light distortion, allowing you to see as
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Sky & Telescope - November 2023

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