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By Linda Copman, based on an informal interview with Dr. Jerry Nelson
Images and photos courtesy of Dr. Jerry Nelson
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| Image: Schematic of the Keck I Telescope
facility. |
“I am endlessly curious and at the same time
very skeptical. I tend to only believe something if I actually
understand it at an intuitive level that satisfies me.
My father taught me to question everything and to find
and challenge unstated assumptions. As a scientist, these
traits tend to make me very careful in what I do. They
also make me look for new solutions to problems.” — Dr.
Jerry Nelson
Jerry Nelson was the first kid in his rural California hometown
of Kagel Canyon to go to college. His father was a tool planner
at Lockheed, whose job was to figure out how to manufacture
parts and then record this process in the form of a blueprint.
Nelson’s father worked a lot with metals, massaging them thermally
in particular ways in order to retain their malleability. “I
subliminally picked this up,” says Nelson, “and I have always
been reasonably grounded in the real world of manufacturing
processes and in the properties of materials.”
At Caltech, Nelson worked part-time in the machine shop, where
he learned more about what could and could not be done with
various machines. His skills in building apparatus were significantly
strengthened when, while still an undergraduate, he helped
to design and build a 1.5-meter telescope to conduct an all-sky
survey to find the brightest infrared sources in the sky.
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| Image: The 200-inch telescope at Palomar
Observatory in California, built in 1948. |
In mid-1977, after closely studying the designs of most existing
large telescopes, Nelson began to appreciate how difficult
it would be to make a very large telescope in the same fashion.
The Russians were building a 6-meter telescope and people at
UC (University of California) were talking about a 7-meter
telescope. Casting aside conventional technological approaches,
Nelson suggested that a segmented mirror approach was the only
sensible approach for increasing the size of the primary mirror.
With this approach UC could make a truly significant step,
by doubling the diameter of the 200” telescope at Palomar to
10 meters.
Early on, Nelson started working closely with Terry Mast of
Lawrence Berkeley National Laboratory (LBNL). Mast lived next door to
Nelson in the dorm when they were undergraduates at Caltech,
and both were confirmed experimentalists, or physicists who
prefer to devise experiments to measure physical properties
- as opposed to theoreticians, who prefer to write equations
to describe physical properties. Nelson and Mast had been good
friends since the age of 18 and they were used to brainstorming
solutions to fundamental problems together. “The two of us
spent essentially all of our time thinking about the challenges
of making a very large telescope,” says Nelson. Nelson and
Mast’s philosophical decision to pursue a segmented design
was a step that led to the next generation of large telescopes.
The two realized that there were theoretical limits to making
large monolithic mirrors, which they referred to as the “last
of the dinosaurs,” and that segmented mirrors would become
the basis for bigger, better, and more economical telescopes
of the future.
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| Image: Nelson and Mast made a key philosophical
decision to pursue a design path that would lead to an
entirely new generation of telescopes, not just a single
giant telescope. |
Traditional telescopes utilize large primary mirrors which
are bigger, heavier, more expensive, more fragile, harder to
support properly against gravity, and harder to coat since
they require a big vacuum chamber in which to do this job.
Segmented mirrors allow the optical element size to be limited
to the segment size, which makes most of these problems become
no harder than the optics for a 1.8m diameter telescope. With
segments, one can imagine building telescope optics with basically
no size limit.
Segmenting surfaces is a very old idea, and radio telescopes
are frequently constructed of panels or segments. Early on
(1977-1979) there was a lot of skepticism about the segmented
design approach, and Nelson had to convince skeptics both within
the project and people in the astronomical community at large
of the viability of his idea. “Please do your best to prove
me wrong” was his motto during this time. An alternate design
to build a 10-meter monolithic mirror was proposed, and Nelson
had to find a way to convince the UC leadership that this was
the wrong way to proceed.
To convince his colleagues and the UC leadership to pursue
a segmented design, Nelson and his team developed and built
working prototypes of the key components of their design. The
team developed algorithms for the active control system. They
also designed and built everything from edge sensors (glass
pieces that sense where the segment is with respect to its
neighboring segments), to actuators (the motorized screws on
the back of each mirror segment that cause them to move), to
the passive support apparatus for supporting segments against
gravity. They even polished a subscale off-axis mirror utilizing
an entirely new polishing technique. This polishing technique
involved extensive mathematics derived from the theory of elasticity,
as well as very thoughtful apparatus. The polishing technique
proved successful, and, together, these efforts were sufficient
to persuade UC administrators that the segmented concept worked.
Some unique issues posed by a segmented mirror design were
the need to actively control the mirror positions in real time,
as the telescope temperature and zenith angle changed. Telescope
mirrors must be accurate to within a small fraction of the
wavelength of light, and they must be rigid enough not to deform
as a result of gravity, wind, or temperature variations from
night to night. To give some perspective, human hair has a
typical diameter of 50 microns or about 100 times the wavelength
of visible light. Telescope optics must be no worse than about
10% of the wavelength of light, or accurate to within 1/1000
of the diameter of a human hair. There was also the extremely
challenging problem of polishing the optical surfaces of the
off-axis segments which would be needed to form a gigantic
hyperboloid-shaped primary mirror. These challenges were recognized
fairly early on in the design process, but Nelson’s team spent
several years developing solutions.
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| Photo: Nelson poses with one of the Keck
I Telescope mirror segments during construction of the
telescope. |
By 1979 they had selected the actual pattern of segmentation
that appeared best, consisting of 36 hexagonal segments. They
considered a petal design, but this resulted in a less efficient
use of materials than hexagonal segments, which have three-fold
symmetry. Hexagonal segments wasted the least amount of very
expensive materials required to make the mirror. The design
required 6 different kinds of segments to fit together to form
a hyperboloid. When manufacturing the segments for the Keck
I Telescope, they ordered 7 of each kind of segment, so that
there is one spare segment of each kind. The optics in the
Keck II Telescope are identical to those in Keck I, so that
the mirror segments in the two telescopes are interchangeable. “This
is a real boon to the functional economy of the two telescopes,” explains
Nelson.
Nelson’s team then engaged in building and testing prototype
components for the active control of the segment positions.
Coming up with a method of controlling the mirror segments
so they could work as though they were a single mirror was
perhaps the biggest overall challenge they faced. The solution
involved utilizing a mixture of geometry, suitable control
approach, precision actuators, and edge sensors. All of this
was groundbreaking work and required ideas, development, exploration
of various options, iteration on designs, and prototypes and
revised prototypes. The end products were an unprecedented
success: the Keck Telescope primary mirrors act like a continuous
surface, and the gaps between segments lose less than 1% of
the total collecting area and image quality.
“A very straightforward mathematical algorithm
is used to operate the mirror at Keck Observatory. There
are 168 edge sensors, 168 linear equations to solve, and
108 actuators to control. Computers do the math and the
system works just like it’s supposed to. We used principles
from high school mathematics to figure this out.” — Dr.
Jerry Nelson
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| Image: Diagram of the whiffletree support
system for the mirror segments, with 36 points of attachment
and multiple pivot points. |
The team also had to develop and test a unique method for polishing
the segments, called stressed mirror polishing. This technique
applies external forces to the mirror blank to deform it into
a spherical shape. “We figured out how to push the blank in
just the right places to make it into a sphere,” explains Nelson.
It is much easier to polish spheres than hexagons, so the circular
segments were first polished and then cut into hexagons. The
final step in the process involved ion figuring, or bombarding
the mirror surface with high-energy ions to remove any remaining
inaccuracies — one atom at a time. They had to convince optical
companies to use the stressed mirror polishing method and then
help these companies to properly apply the method. This process
was expensive, very exacting, and fraught with technical and
interpersonal challenges.
Concurrently, they were faced with the task of developing and
prototyping an economical support system to hold the segments
stable against gravity. This system, called a whiffletree,
was loosely based on a whipple-tree
horse harness system, which distributes the weight evenly
across a team of horses and increases maneuverability. Similarly,
the Keck Telescope’s whiffletree support system is designed
to distribute the weight of the mirrors across a series of
points, so that each point pushes on the mirror segments properly
to minimized the segment deformations. The whiffletrees work
on the principle of a see-saw, rotating freely about a pivot
point. This allows the ratios of the loads at the support points
to be maintained, even when the total load varies. The distributed
load of the whiffletrees is concentrated down to 3 points of
support for each segment. This is the number required for stability. “Consider
how much more stable a 3-legged table is than a 4-legged one,” suggests
Nelson.
“Jerry Nelson had been thinking about the segmented
mirror concept for a while when he started on this project.
He always relied on what he called ‘first principles,’ or
the basics of freshman-level physics, to test his design
ideas. He recruited a couple of hundred very smart people
to assist him. We needed to use computer modeling to simulate
and refine our designs. The fact is that the Keck Telescopes
really could not have been built before they were, simply
because computer technology was not advanced enough to
do the kind of modeling we needed until that time.” — Barbara
Schaefer, Observing Support Coordinator at Keck Observatory
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| Image: Installation of one segment of the
Keck I Telescope’s primary mirror shows the complex underbelly
required to actively control and support each segment. |
To perfect the segmented mirror design, Nelson’s team drew
upon their expertise in many different scientific fields, including
structural engineering, mechanics, theory of elasticity, electrical
engineering, control theory, lots of applied mathematics and
matrix theory, physical optics, and physics at every turn. “Of
course we had lots of problems and surprises,” recalls Nelson. “We
were paving new ground so it was essential that we had a very
deep and fundamental understanding of our design. This mastery
of the underlying principles allowed us to efficiently develop
the design and the hardware, and when there were surprises,
to solve them,” he says.
Nelson and Mast worked with Gary Chanan of UC Irvine and with
Jacob Lubliner of UC Berkeley. Lubliner helped to perfect the
stressed mirror polishing technique which was used to create
off-axis parabolic segments. Steve Medwadowski, an engineering
professor at UC Berkeley, was responsible for the structural
design of the telescope. The segment passive support system
was aided by the contributions of Bob Weitzman of the UC space
sciences lab. George Gabor and Bob Minor (both of LBNL) played
key roles in the development of the segment position actuators,
the edge sensors, and for many other mechanical issues.
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| Image: An alignment camera is used to adjust
the segments properly. Gary Chanan was largely responsible
for development of the Keck Observatory alignment camera. |
“Ideas were central to the development of the
Keck Telescopes. Our ideas were new and exciting. We did
lots of prototyping to successfully show that our ideas
worked — and that we actually knew how to build the unusual
and hard parts to make the telescopes practical and affordable.” — Dr.
Jerry Nelson
The strong support of the leadership at LBNL and at UC was essential
in providing early funding needed for development of Nelson’s
ideas. “Thanks to these institutions, we actually had all the
money we asked for, so work progressed limited only by our
ability to recognize and solve technical problems,” says Nelson.
They could not have succeeded without the support and enthusiasm
of Caltech, which convinced Howard
Keck, a Trustee, to support the project.
They also had the strong support of UC and Caltech astronomers,
who were responsible for the construction of the superb science
instruments at Keck Observatory that would consistently deliver
outstanding science. Nelson also relied on an excellent team
of engineers at Keck Observatory. “These engineers were dedicated
and inspired by building the world’s largest telescope, and
this was and still is a great motivating force to get the best
out of everyone,” explains Nelson.
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| Photo: The finished primary mirror on the
Keck I Telescope. |
Today Nelson continues to lend his expertise to Keck Observatory. “The
first thing I do when I come to work in the morning is read
the night logs from Keck,” says Nelson. “If a problem arises
during the night, I help to identify solutions which I share
with the staff at the Observatory.” Nelson is the first to admit
that he does this out of love for Keck Observatory, “his baby.”
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| Image of the Egg Nebula taken with Keck
Observatory’s AO system. |
Nelson is now working on a 30-meter telescope (TMT) which will
become the world’s largest when it is finished in 2016 (he
hopes). The Europeans, not wanting to be outdone as they were
with Keck Observatory, are now planning to build a 42-meter
telescope with twice the area of TMT, and they are only a couple
of years behind the TMT team. “Building a telescope like this
is loads of fun, and loads of staying up late at night,” chuckles
Nelson.
Beyond this size, Nelson thinks it will be some time before
even larger telescopes are built. Several reasons suggest this:
cost is critical of course; wind shake appears to be a significant
problem that will not go away with larger telescopes, particularly
spatially variable wind forces on the primary mirror; finally,
to achieve the full value of giant telescopes one wants effective
adaptive optics and this is harder and harder with larger telescopes.
Keck Observatory is well positioned to continue to lead the
field with its innovative instrumentation
and adaptive optics (AO) systems. “Improvements in performance are manageable at Keck
with today’s and tomorrow’s technology,” says Nelson. Such
improvements will increase the Observatory’s scientific productivity
for years to come. Nelson sees Keck as being able to lead in
AO, progressing to shorter wavelengths — opening the door to
a whole new universe of research.
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| Image of the galactic center taken with
Keck Observatory’s AO system. |
“The Keck Telescopes are the most powerful
and productive astronomical instruments on Earth, and we
are the envy of the world’s astronomical community. AO
is clearly the arena where further development is likely
to produce the greatest future discoveries. This is because
the improvements in angular resolution allowed by AO will
give us much greater sensitivity as well as clarity, and
as Keck uses AO at shorter wavelengths, there will be many
great astronomical discoveries. The most distant galaxies,
formed at the earliest times in the universe, should be
very interesting targets for Keck Observatory’s enhanced
AO system.”
— Dr. Jerry Nelson 
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