A Huge Leap Forward
Laser Guide Star Adaptive Optics
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.
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.)
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.
“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.
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.”
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.
“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. Hear 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.