Sky & Telescope - November 2021 - 23

The Origami Observatory
The sting of Hubble's spherical aberration was fresh in
people's minds during Webb's early conceptual development,
and naturally when thinking about a new space telescope
one thinks of optics and how to make and test them. But
there's more to Webb than its optics. Webb is a giant - and
frigid - origami observatory, and the difficulties presented
by a folding space observatory that will deploy in space by
remote command and operate at cryogenic temperatures add
to the challenge.
Virtually every modern spacecraft unfurls, deploys, or
releases in some fashion, such as extending a solar array for
power generation. By necessity, Webb takes on-orbit releases
and deployments to the extreme:
* The telescope primary mirror backplane structure has to
fold up for launch and then deploy precisely.
* Primary mirror segments have to be movable in any direction
so that they can correctly align to a few millionths of
a millimeter and act as one.
* The secondary mirror must deploy on a hinged tripod
(seen folded on page 21) and also move in any direction.
* The telescope structure is attached to the spacecraft bus
when stowed for launch, but a telescoping tower must
extend to separate the structure and bus, so that the telescope
is isolated from any mechanical vibrations or heat
from the spacecraft and sunshield.
* The star tracker, used to monitor the telescope's position
in space, is attached to this extendable tower and must
release from launch locks on the bus once in space.
* Various radiators must also release from launch locks and
deploy to provide thermal and mechanical isolation.
GO FOR GOLD An optical engineer examines two test mirror segments,
one coated in gold. The frame is a composite material designed
to handle Webb's space environment.
The sunshield posed the grandest stowage and deployment
engineering challenge of them all. The sunshield's job
is to be an umbrella, shielding the telescope from the heat of
the Sun as well as stray light from Earth and the Moon. This
protection allows the telescope and instruments to radiate
their own heat away and stay cool in the 7K deep space at L2.
The shield needs to let through a mere millionth of the total
heat hitting it, attenuating more than 200 kilowatts of solar
insolation down to a fraction of a watt. It also needs to span
a much larger area than the telescope itself - roughly that of
a tennis court - so that it provides adequate shadow for the
telescope to access as much of the sky as possible. Lastly, it
must weigh extremely little, stow compactly for launch, and
INFRARED EYE Unlike Hubble's instruments, which focus primarily on ultraviolet and visible wavelengths, Webb's observe the near-infrared and
mid-infrared parts of the spectrum. Each instrument is a specific combination of observing modes, wavelength range, field of view, and resolution,
combining multiple instruments within themselves. NIRCam, NIRISS, and MIRI can do both imaging and spectroscopy, but the NIRCam and NIRISS
spectrographs cover a smaller wavelength range than their cameras.
Imaging
NIRCam
(Near-Infrared Camera)
NIRISS
(Near-Infrared Imager and Slitless Spectrograph)
Hubble
Spectroscopy
NIRSpec
(Near-Infrared Spectrograph)
NIRISS
Hubble
0.1
Ultraviolet
0.2
Visible
0.3
0.4
0.6 0.8 1
Near-infrared
2
3
Wavelength (microns)
sk yand tele scope .o r g * NOVEMBER 2021 23
4
NIRCam
Mid-infrared
5 6 7 8 9 10
20
30
MIRI
(Mid-Infrared Instrument)
MIRI
MIRRORS: CHRIS GUNN / NASA; INFRARED: LEAH TISCIONE / S&T, SOURCE: STSCI

http://skyandtelescope.org

Sky & Telescope - November 2021

Table of Contents for the Digital Edition of Sky & Telescope - November 2021