Sky & Telescope - November 2021 - 26

80 Years: Where We Are
of careful planning meant that even when Hurricane Harvey
hit right in the middle of this 100-day-long cryo-vacuum test
in 2017, we completed it without interruption.
Meanwhile, to prove that the observatory could properly
manage heat not only from the Sun but also from its own
electronics, we had to combine results from multiple tests.
To check the sunshield design, we tested a 1
/3-scale model of
CHAMBER A Webb emerges from months of cryogenic vacuum testing
in 2017.
into the world's finest large cryo-vacuum optical test facility
for the work. This chamber is about nine stories tall, taller
than the Lincoln Memorial Building - so large that the air
inside it weighs 12 tons (before all but 2 grams gets pumped
out for testing). Engineers placed the deployed flight hardware
on a truss structure platform and rolled it into the bottom
of the chamber on rails, then connected it to long steel rods
suspended from the ceiling. Using a combination of mirrors,
cameras, and other carefully tested instruments positioned
inside the chamber, we successfully aligned all 18 segments
to act as one. We also verified the primary mirror's shape and
tested everything from the secondary mirror's positioning to
the ability of the telescope to acquire and track targets. Years
A LONG JOURNEY Webb will spend about 29 days traveling to L2
before it enters orbit there. Along the way, it will deploy its sunshield and
power up instruments. Around day 96, it will have cooled to the temperatures
necessary to enable mid-infrared observations. Telescope alignment
will complete around day 118. If all goes as planned, the telescope
will be ready for business 180 days (6 months) after launch.
the deployed sunshield in a temperature-controlled vacuum
chamber. To verify that the telescope and instruments would
stay cool despite the heat the electronics give off, we built
and tested a full-scale version of Webb's core section - the
thermal Grand Central Station of the observatory - to
confirm that heat moves around the way it needs to. Such
tests required milliwatt-level precision. We then combined
these results with those from the thermal-vacuum test of the
bus-plus-sunshield assembly and the cryo-vacuum test of the
assembled telescope and instrument package.
On top of all that, we've done exhaustive, iterative checks
of the unfolding processes. All flight-deployable items have
been unfolded multiple times; the flight sunshield, for
example, has been stowed four times and deployed three
times before flight. So much of the observatory can fold out
that every stow operation was like reassembly, with a regimen
of extra checks and precautions.
And after more than a decade of testing flight hardware,
we'll soon be ready for the next step: launch.
800,000 kilometers
600,000
400,000
200,000
1,600,000
Earth
1,400,000 1,200,000
Launch
200,000
Enter L2 orbit
L+29 days
400,000
Course correction
0.5 to +1 day
Sunshield and
observatory
deployments
L+3.0 to L+20 days
Cools to
temperature
sufficient for
near-infrared
observations,
instruments finish
powering on
L+20 to L+40 days
26 NOVEMBER 2021 * SKY & TELESCOPE
Commissioning
complete
L+6 months
600,000
1,000,000 800,000
600,000
400,000
200,000 kilometers 200,000 400,000
L2 point
800,000 kilometers
Tr
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CHAMBER A TESTING: CHRIS GUNN / NASA; TRAJECTORY
AND COMMISSIONING TIMELINE: GREGG DINDERMAN / S&T,
SOURCE: NASA
Sky & Telescope - November 2021

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