James Webb Telescope Issue - 26

Feature
Understanding Our Universe Through Imagery and Data:
An Interview with HKN Eminent Member Dr. Asad Madni
Q: Can you begin by giving us some context
of Hubble in comparison to Webb?
The Hubble and Webb are two different types of
Telescopes. Hubble is pretty close to us in low Earth
orbit (150 miles), while the James Webb Space
Telescope will actually orbit the Sun 1.5 million
kilometers (1 million miles) away from the Earth
at what is called the second Lagrange point or L2.
Lagrange points are positions in space where objects
sent there tend to stay put. At Lagrange points, the
gravitational pull of two large masses precisely equals
the centripetal force required for a small object to
move with them. These points in space can be used
by spacecraft to reduce fuel consumption needed to
remain in position. What is special about this orbit is
that it lets the telescope stay in line with the Earth as
it moves around the Sun. This allows the satellite's
large sunshield to protect the telescope from the light
and heat of the Sun and Earth (and Moon).
Additionally, while both Hubble and Webb are large
space telescopes (though Webb is considerably
bigger), the two actually " see " the universe very
differently. While Hubble observes light at primarily
optical and ultraviolet wavelengths, Webb is designed
to detect primarily infrared light (IR) and will be able
to see things that Hubble cannot. It is so powerful
that it can detect heat signature of a bumble bee
from the moon.
Hubble Space Telescope as seen from
Space Shuttle Atlantis after Servicing
Mission 4 in 2009. Image credit: NASA
The pointing
systems are very
different. James
Webb isn't using
mechanical
gyroscopes like
Hubble to keep
it on target.
Instead, it's using a different technology called
hemispherical resonator gyros, or HRGs (The HRG
makes use of a small, thin solid-state hemispherical
shell, anchored by a thick stem. This shell is driven
to a flexural resonance by dedicated electrostatic
forces generated by electrodes which are deposited
directly onto separate fused quartz structures
that surround the shell). Similarities exist in signal
processing and several electronics boards of the
THE BRIDGE
Hubble, which have been reduced to ASICs in the
Webb. These will be used to actually upgrade Hubble
during the next servicing mission.
Q: Can you tell us about the servo control
system you developed for Hubble?
At our BEI Precision Space and Systems Division
(PSSD), we developed an extremely slow-motion,
dual-axis servo control system for Hubble Space
Telescope's (HST) star selector that provided the
HST with unprecedented pointing accuracy and
stability, resulting in truly remarkable images that have
enhanced our understanding of the universe.
Hubble Star Selector Servo System. Image credit: A. Madni
The system utilizes proprietary high-resolution optical
encoders in conjunction with a patented Ultra Loc®
technique to provide unprecedented accuracy &
resolution. Together with integral DC torque motors,
the system provides precise, digital control over a
range of 0.5 arcsecond/ second (0.0333 revolution
per day) to 16,384 arc second/second (0.759
revolutions per minute) and 21- bit absolute position
words to an accuracy of 2.0 arc seconds peak-to-peak.
In the fine tracking mode, the system provides position
tracking to the order of 0.1 micro radian.
This system, which is part of the HST Fine Guidance
System, allows a fine lock to the guidance system
and plays a pivotal role in providing a highly stable
reference required for pointing the HST and
maintaining a pointing accuracy equivalent to pointing
at the face of a U.S. quarter dollar as seen from 200
miles away and the pointing stability less than the
width of the quarter dollar over a 24-hour period. This
required development of optical encoding technology
with accuracies previously unachieved (21 up to 23
James Webb Telescope Issue

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