Sodium Laser Guide Star on Mauna Kea in 1987

The concept of making a laser guided adaptive optics telescope was first published in the open literature in late 1985 (R. Foy and A. Labeyrie, Astronomy & Astrophysics, vol. 152, pg. L29, November 1985), and by the time the journal reached the University of Hawaii it was already early in 1986. Immediately upon seeing this paper, I decided to begin experimental work with laser guide stars. First I identified who might have a laser that could be tuned to the 589 nm sodium resonance line. This turned out to be any group doing LIDAR work. Then I began to telephone potential collaborators. One LIDAR group stood out above the rest, a group headed by Prof. Chester Gardner in the Department of Electrical and Computer Engineering at the University of Illinois Urbana-Champaign. Gardner's group was well-suited for this work because (1) they had an excellent reputation, (2) they had a working 589 nm sodium wavelength laser that could make the sodium laser guide star, and (3) they were accustomed to mounting campaigns at remote sites.

After a visit to Champaign-Urbana in the spring of 1986 to cement the collaboration, I applied for telescope time through normal channels at the University of Hawaii's Institute for Astronomy to conduct the first laser guide star test. The project was scheduled for early January, 1987. Three people came to Hawaii from central Illinois for this work: Prof. Chester Gardner and two graduate students, Dan Senft and Kevin Kwon. The aim of the experiment was to make images of the sodium return signal. Astronomers had to begin the process of learning how to make images of real laser return signals. (The LIDAR community was accustomed to projecting and detecting the sodium laser beam, but they made little attempt to produce a concentrated beam -- one that would look like a star -- and they only detected the signal photometrically with time-gated single-channel detectors, rather than imaging the return laser guide star signal.)

For this experiment the plan involved installing the University of Illinois' sodium laser in the dome of a small 0.6-m telescope located adjacent to the larger University of Hawaii 2.2-m telescope. The laser would be projected into the sky through the small telescope and then images would be recorded with the 2.2-m telescope. The view of the illuminated sodium layer from the 2.2-m telescope would be a narrow line because the beam was being transmitted from the 0.6-m telescope located approximately 50 meters away. The two telescopes are shown below as they looked during the January 1987 observing run. The 2.2-m telescope (on the left) has the 500x500 CCD camera mounted at the Cassegrain focus. One tiny corner of the sodium laser system can be seen in the lower right corner of the 0.6-m telescope picture.

University of Hawaii 2.2-m Telescope University of Hawaii 0.6-m Telescope

The picture above is an outside shot showing the domes of the two telescopes used in the laser guide star experiment. The small dome on the right housed the 0.6-m telescope and the large dome to the left houses the 2.2-m telescope. January snow can be seen on the ground. The 0.6-m telescope is no longer on Mauna Kea. It was removed in the 1990's to make way for the construction of the Gemini North telescope which, like all of the large telescopes today, has ambitious plans to install a number of advanced AO systems with sodium laser guide stars. It seems particularly appropriate to have projected the first sodium laser guide star from a site where a large telescope is currently pushing forward on AO technology. The nearby Keck 10-m Telescope (not visible in this image) also has its own sodium laser guide star facility.

The image below shows one of several long integrations taken on the night of January 21, 1987 with the 500x500 CCD camera. As expected, the image of the sodium layer shows a narrow central bright line of emission where the laser beam sliced through the 8 km to 10 km thick layer of sodium that is suspended some 95 km above the surface of the Earth. The width of the bright narrow line (at the center of the oval extended light distribution) shows the approximate diameter this laser guide star would have if it had been viewed as an end-on pencil. This processed image was produced from the original data and analyzed by yet another University of Illinois graduate student, Byron Welsh.

Sodium Laser Guide Star at Mauna Kea Observatory

The work described here was published in the journal Nature (L. Thompson and C. Gardner 1987 Nature vol. 328, pg. 229 Experiments on laser Guide Stars at Mauna Kea Observatory for Adaptive Optics in Astronomy). This paper in Nature caused quite a stir in the U.S. Military adaptive optics community which, as it turned out, had conducted several similar experiments of its own prior to 1987 to test the laser guide star concept. These experiments had been classified as Top Secret and were not published until the 1990's. During a visit I paid to Star Fire Optical Range (Kirkland Air Force Base, NM) in the early 1990's, I met a U.S. Air Force officer who was stationed in London at the time the Thompson & Gardner Nature article was published. It was his job to monitor civilian scientific research and report back to the Pentagon. Because I met him after the declassification, he was able to talk openly. He expressed his amazement at the similarity between our published work and the U.S. Air Force work that was, at that time in 1987, just moving from the planning stages into hardware implementation. In fact, the first laser guided AO system at Starfire Optical Range was built for a 1.5-m telescope in the years 1987 to 1990.

The Thompson and Gardner experimental work -- plus engineering design calculations by Thompson, Gardner, and Gardner's students showing the feasibility of building laser guided systems -- provided the basis for several proposals to the National Science Foundation for funding to move ahead with the construction of such a system. The first Thompson & Gardner NSF proposal was submitted in Fall 1987. Reviewers turned it down. The second Thompson & Gardner NSF proposal was submitted in Fall 1988. Again, reviewers turned it down. Both years there were many excuses cited for doing so. The only comment of substance was the fact that the ideal sodium laser was not available. Some reviewers said that the system was so complex that it would never work. Others argued on the basis of physical principle that laser guided AO could never work. Of course, these arguments were spurious. When it came time to submit the proposal a third time, Gardner had moved on to other interests, and I became the sole PI. The third proposal was more conservative. I aimed to experimentally test the production of Rayleigh laser guide stars in the UV. I decided that there could be no technical objection to producing Rayleigh laser guide stars. The decision on the 1989 proposal was put on hold in the winter months of late 1989 and early 1990. Rumors were circulating in Washington D.C. that the U.S. Air Force was soon going to declassify much of their adaptive optics work. By then the Cold War was ending and the group at Starfire Optical Range had finished the very project that Gardner and I had proposed in Fall 1987.

There is no doubt that the 1987-1990 engineering design work in our published papers and the experimental laser guide star work done at Mauna Kea Observatory accelerated the process of getting the extensive body of secret AO work of the U.S. Military declassified. The military researches themselves wanted to show their work in public: they did not want others to repeat it and then be the first to publish. A wide array of military AO work was made public 4 years and 4 months after the Mauna Kea sodium laser guide star experiment described here. When I first visited the 1.5-m laser guided telescope at Starfire Optical Range, it was like deja vu: a Rayleigh laser guided AO system with all the characteristics of the design papers Gardner and I had been publishing.

The conservative reviewers of our proposals in 1987 and 1988 have been proven wrong, and virtually every observatory is making some attempt to take advantage of the new AO technology. In the 1980's the number of astronomers working in this field was perhaps 10 or at most 20. Today the number is a factor of 10 to 50 higher.