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By Linda Copman, based on informal interviews with astronomer Dr. Mike Liu,
Senior Electronics Engineer Jason Chin, Laser Operations Engineer
Kenny Grace, and Adaptive Optics Operation Manager and Support
Astronomer Randy Campbell
Images and photos courtesy of Keck Observatory except where noted otherwise.
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| Images: (Left) The planet Uranus without
adaptive optics. Image courtesy of Keck Observatory.
(Right) Both hemispheres of the planet Uranus with adaptive
optics. By L. Sromovski. |
The evolution of adaptive optics (AO) technology spans the
past several decades. AO was first proposed by astronomer Horace
Babcock in 1953, in a paper titled “The Possibility of Compensating
Astronomical Seeing.” It took a few decades longer to develop
the technological precision necessary to manufacture a successful
AO system.
In 1972, in the thick of Cold War politics, the U.S. Advanced
Research Projects Agency was wrestling with the problem of
identifying Soviet satellites. State-of-the-art images of Soviet
satellites were too “fuzzy” to provide conclusive information.
The Itek Corporation came up with a technique for correcting
for atmospheric turbulence by making such corrections before
the image was recorded. This led to the development of deformable
mirrors, wavefront sensors, and the other technology needed
for adaptive optics. The Itek Corporation patented their Compensated
Imaging System and used this system to look at astronomical
objects, as well as at Soviet satellites.
“It is very true that astronomical AO has benefited
from military technology. However, one of the reasons,
besides the Berlin Wall coming down, that this work was
declassified was that astronomers were already implementing
AO systems at observatories. The Europeans were first to
install an AO system on their 3.6-meter telescope on La
Silla in the late 1980s, and other groups were experimenting
with prototype systems on their telescopes. As a personal
example, I was involved in doing this at the University
of Arizona in 1990”
— Peter Wizinowich, Optical Systems Manager at Keck
Observatory
In 1992, much of the adaptive optics research and development
performed by the U.S. Government was made public in the refereed
literature.
Here is how adaptive optics works. First, a sensor measures
the incoming light waves from a star or science object.
The adaptive optics control system reconstructs the wavefront
of the incoming light from the sensor input, and then sends
corrections to a deformable mirror that changes its shape to
flatten out the wavefront. This closed loop system corrects
for atmospheric turbulence in incoming light waves thousands of times per
second, thus eliminating blurriness and producing high-resolution
images. View
an excellent animation of the adaptive optics process,
courtesy of Gemini Observatory. (Requires Quicktime player.)
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| Image: Milestones in the development of
LGSAO at Keck Observatory. Image by Randy Campbell. |
Adaptive optics was planned for Keck Observatory from the start.
In December 1993, the W. M. Keck Foundation awarded the Observatory
a grant for the initial funding of the Keck adaptive optics
facility on the Keck II Telescope. Serious design efforts ramped
up in 1994 when the Keck II telescope was being built, under
the leadership of Dr. Peter Wizinowich, who continues to lead
the Observatory’s AO program today.
The technical components of Keck’s Adaptive Optics (AO) system
were the result of a collaboration between staff at the Observatory
and staff at Lawrence Livermore
National Laboratory (LLNL), with both parties providing
funding. LLNL built the original laser, along with the wavefront
sensor and wavefront controller, which were subsequently replaced
with the Next
Generation Wavefront Controller in 2007. Keck Observatory
provided the optics, motion control, control software, and
system integration.
The major challenge for the application of adaptive optics
to astronomy is that astronomers need a bright star or object
to measure the incoming wavefront. Since the number of naturally
occurring bright guide stars is limited, adaptive optics was
useful only within a small fraction of the sky. A potential
solution to this problem was the creation of an artificially
bright object or “star” utilizing a laser beam. The idea of
using a “laser guide star” was first conceived by the U.S.
military, while undertaking classified research in the 1970s.
In 1985 two French astronomers, Foy and Labeyrie, conceived
of utilizing a laser guide star for collecting wavefront data.
Foy and Labeyrie’s research inspired the first
sodium laser guide star experiment on Mauna Kea in 1987,
and ultimately helped lead to the declassification of the military
technology.
Since lasers can be pointed anywhere in the sky, laser guide
stars can be created and used to collect wavefront measurements
virtually anywhere astronomers wish to point their telescopes.
Thus laser guide stars vastly increase the coverage and applicability
of adaptive optics technology.
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| Image: Wide-angle view taken from underneath
the Keck II laser. Photo by Laurie Hatch. |
“The laser opens up the sky to AO research.
Without the laser, only about one to two percent of the sky is available,
but with the laser, nearly the entire sky can be observed
with the benefit of AO correction.” — Randy Campbell,
Keck Observatory’s Adaptive Optics Operations Manager
One particular area of research that has benefited greatly
from the laser is extra-galactic research. To study the cosmos
outside of our galaxy, one must necessarily look away from
the Milky Way plane so that the foreground stars, gas, and
dust don’t obstruct the view. This means that there are fewer
natural guide stars available to use. With the advent of Keck’s
Laser Guide Star Adaptive Optics (LGSAO) system in 2005, there
was a dramatic increase in the volume of extra-galactic research
at Keck Observatory being done with adaptive optics.
“LGSAO is a complicated technology, so one of the biggest challenges
was transitioning from development to nightly operations,” reports
Jason Chin, a Keck senior electronics engineer who led the
integration effort when the laser was first installed on the
Keck II Telescope. “In the early days of LGSAO, rooms full
of engineers, technicians, and scientists were needed at the
summit and at headquarters to operate the systems,” recalls
Chin.
During the past few years the Observatory has made a concerted
effort to streamline operations, and a Laser Operations Transition
(LOT) team was formed led by David Le Mignant, Keck Adaptive
Optics Instrument Scientist. The LOT team developed tools,
procedures, policies, and practices that led to more efficient
operations - without sacrificing performance. “The team successfully
turned a system that used to be operated by five AO experts
and a laser expert into one that is routinely operated by just
a telescope operator and a laser operator, neither of whom
are specialists,” explains team member and Adaptive Optics
Scientist Marcos van Dam.
Recent improvements to the Keck LGSAO system include increased
laser power and greatly improved laser reliability; an upgrade
to the wavefront sensor and the wavefront controller; more
automated user tools and performance monitoring; simpler user
interfaces to reduce the possibility of human error; and more
reliable, efficient, and precise calibration techniques. As
a result of these upgrades, Keck Observatory is significantly
ahead of other observatories in the field of LGSAO in terms
of performance, reliability, operational efficiency, and, most
significantly, scientific output as measured by the volume
of publications.
“Keck Observatory surpassed a major milestone
in FY2007, achieving a total publications count of 279 papers
in professional astronomy journals. This is a record for
Keck, and it significantly exceeds the number of papers
published per telescope of any ground-based observatory
worldwide. More sophisticated metrics that track how impactful
each paper is to influence a scientific area or spawn a
new field of research confirm that Keck is the most scientifically
productive observatory ever built.” — Taft Armandroff,
Director of Keck Observatory
The Keck I laser is part of a joint collaborative effort with
Gemini Observatory to procure two new lasers, one to be deployed
at each observatory. The National Science Foundation (NSF)
funded the design and construction of the two lasers, and Keck
Observatory is expecting the delivery of its new laser in fall
2008.
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| Photo: The Keck I LGSAO team (clockwise
from bottom left): Kenny Grace, Ed Wetherell, Doug Summers,
Chris Neyman, Craig Nance, Peter Wizinowich, Randy Mogensen,
Roger Sumner, and Jason Chin. Not pictured are Paul Stomski,
David Le Mignant, Drew Medeiros, Kim Sweeney, and Junichi
Meguro. |
The new laser will be a fraction of the size of the existing
laser and will therefore require much less accessory infrastructure.
Laser Operations Engineer Kenny Grace is pleased to report
that the new laser will also require only about 25 percent of the
power currently used to operate the Keck II laser. But the
new laser’s power output will be approximately 25 percent greater
than the Keck II laser. Watt for Watt, there will be a higher
return from the Keck I laser versus the Keck II laser. “This
is because the new laser will be roughly twice as efficient
at exciting the sodium atoms in the upper atmosphere,” explains
Grace. “Higher laser power along with superior pulse format
and a narrower wavelength equates to higher returns from the
sodium atoms in the mesosphere, thus providing a brighter guide
star,” says Grace. “The brighter the guide star, the higher
the speed and accuracy of the AO corrections.”
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| Photo: Testing the fibers that will transport
the laser light from the laser to the launch telescope
is a collaborative effort between Gemini Observatory,
Subaru Observatory, and Keck Observatory. This collaborative
effort is significant since all the observatories will
benefit from the results, which, in this case, include
a better understanding of the fibers which will transport
the laser beam. Gemini North Observatory’s laser was
utilized for this particular test conducted by Kenny
Grace, Keck Laser Operations Engineer. |
On the Keck II Telescope, the laser beam emanates from a launch
telescope on the side of the Keck II Telescope. “Due to the
structure of the sodium atoms as a layer in the mesosphere,
the return from these atoms does not act as a ‘point source.’ Imagine
holding a flashlight directly in front of you, versus holding
it with your right hand and your arm extended. The beam you
see when the light is shining directly in front of you will
look circular. But when the light is shining from the side,
you will see a column of light which creates an oblong pattern.
This same phenomenon occurs 90 kilometers up in the mesosphere,” explains
Jason Chin, project manager for the Keck I LGSAO system. The
wavefront sensors in the AO system must recognize this elongated
laser spot and correct for the shape of the laser guide star
at the same time as atmospheric corrections are being made.
The bigger the telescope, the more oblong the laser spot is,
since the laser is projected from the side of the telescope.
The Keck I laser will be propagated from the center of the
telescope at a location behind the secondary mirror. Therefore,
the Keck I laser spot will be smaller and less elongated. This
will enable the Keck I laser to provide a higher return per
sensing area.
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| Photo: The Keck I laser launch telescope
being built by Italian company Galileo Avionica. |
“From my point of view, the new laser on Keck
I will increase the amount of LGS time available, improve
performance, provide redundancy in case one system fails,
balance the instrument loads between the two telescopes,
and allow for the operation of two lasers simultaneously
for interferometry, where starlight is combined between
the two telescopes. The new laser will allow Keck Observatory
to perform interferometry on objects that are currently
too faint for the existing AO systems.” — Jason
Chin, Senior Electronics Engineer
Keck Observatory’s Laser Guide Star Adaptive Optics system
is the envy of the world. Using LGSAO, the Keck Telescopes
can provide high-resolution images of almost anything out there
in the sky. “Almost every time I observe here, I discover something
new and very interesting,” proclaims Mike Liu, Professor of
Astronomy at the University of Hawai`i.
Read
more about Dr. Liu’s recent discovery of several ultra-cool
brown dwarf binary systems, which Liu’s team found using Keck
Observatory’s Laser Guide Star Adaptive Optics system. Learn
more about the history of adaptive optics from Claire Max,
Director of the Center for Adaptive Optics and Professor of
Astronomy & Astrophysics at UC Santa Cruz. 
Click here to return to the main
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