A Breakthrough in Telescope Making
As the final decade of the 20th century dawned, the 200 inch (5.08 meters) Hale telescope at the Mt. Palomar observatory remained the largest viable telescope in the world. But the huge mirror of the Hale was at the limit of the mirror maker’s craft. A new approach had to be found if larger telescopes were to be made.
The thinking actually began as far back as 1977. The University of California formed a committee to develop a 10 meter telescope, twice the size of the Hale. Their thinking at first was a single mirror like the Hale’s. But it quickly became obvious the size and weight of such a mirror was untenable.
Then one member of the group, an astrophysicist named Jerry Nelson, presented an astounding idea. Don’t make a monolithic huge mirror. Use many small thin mirror segments together to create the large parabolic reflecting surface.
The group first balked at the idea, but as Nelson worked on his idea, and refined it they came around. The final design was for a mosaic of 36 hexagonal mirrors, each 1.8 meters wide and 7.5 centimeters thick (about three inches).
Nelson’s next quandary was how to grind and polish the segments. He turned to UC Berkeley Professor of engineering Jacob Lubliner for help. Lubliner harkened back to Bernard Schmidt who developed the Schmidt camera telescope used at Palomar to map the sky for the Hale’s explorations. Lublinar adapted Schmidt’s technique of selectively applying forces and torques to the edges of a mirror blank so it is warped to a desired degree of distortion. A simple sphere is then polished into the blank, and the forces and torques removed. The elasticity of the blank causes it to relax into the desired shape.
The next problem Nelson faced was keeping all 36 segments aligned. Even a single monolithic mirror undergoes stresses that cause it to change shape slightly. With 36 individual mirrors, keeping them aligned was critical for the telescope to be usable.
Clearly, only a computer could make adjustments fast enough. Nelson asked for help from two other colleagues, physicist Terry Mast and engineer George Gabor. The triumvirate developed a system of 168 sensors mounted on the edges of the mirror segments. These were connected to 108 ‘position actuators,’ three of which are connected to the back of each mirror segment.
The sensors continually compare the difference in height between each mirror segment and its neighbors. If there is even a difference of one millionth of an inch, the computer calculates the necessary adjustments and they are made. The adjustments are made twice every second.
With the mirror system and support and structure finalized, astronomers from the California Institute of Technology ( Caltech) agreed to join the program. Now funds had to be found to pay for this amazing telescope.
Toward First Light
Caltech found a donor in the W.M. Keck Foundation, named for William Keck, founder of the Superior Oil Company. The Foundation provided $70 million to build the telescope.
The association of UC and Caltech was known as CARA–California Association for Astronomical Research. The group had already decided on the location for their amazing telescope. It would be on what astronomers consider the finest observing location on Earth—Mauna Kea in Hawaii.
Mauna Kea is an extinct volcano almost 14,000 feet high, above 40 percent of the Earth’s atmosphere and away from any pollution and urban lights. The air is dry, and cloudless most of the time, ideal for astronomical observing.
Construction began in 1987 and by 1990 the fist mirror segment was installed. Two months later nine segments had been installed.
Astronomers call the first time a telescope captures the image of a celestial object “first light.” With just nine segments, the Keck captured first light of NGC 1232, a pinwheel galaxy 65 million light years away. The image was spectacular. With only nine segments, the Keck matched the performance of the Hale!
The Keck became fully operational in 1993. It has a uniquely short Newtonian focal length of 17.5 meters, making it a f/1.75 very wide-angle instrument. However, it uses a Ritchey Chretien system for a Cassegrain focus of f/15. The Nasmyth focus mentioned in the drawing is similar to the Coude focus.
Expanding the Vision
But even as the Keck was capturing first light, CARA had even more innovative plans in mind. They would build a twin of the Keck and then be able to combine the images from both instruments. This would give them a telescope not just twice as big, but due to a unique characteristic of optics, one as big as the distance between the two instruments. In the case of the twin Kecks, that would be 85 meters (280 feet). A mirror that size would resolve things never seen before. It would resolve planets around other stars.
Keck II was completed in 1996. Since then, the primary mission of the twin Kecks has been to search for extrasolar planets. And it has found them—at least evidence they exist. No images have been seen, but their affect on their star has been measured, and the twins have imaged a dust cloud around a star that astronomers are certain is the birthplace of planets. It may even contain at least protoplanets now.
The Kecks however have imaged a brown dwarf orbiting a small, dim red star. The dwarf is at about the distance of Neptune. They have also imaged the indications of the massive black hole at the center of our galaxy. Of course, you can't 'see' the black hole, as no light can escape from it. What the photo shows is the swirl of stars and gas around it.
The twins work together using a technique called interferometry, in which two light beams are combined. Astronomers do not actually produce an image in this way, but create interference rings as the two beams go in and out of sync. In this way they can measure the size of stars, their rotation, and other characteristics.
This method was invented by Thomas Young in 1805, in an attempt to prove whether light was a wave or a particle. He used two slits in cardboard to produce the interference rings. In the late 1800s, Abraham Michelson improved on the technique with his interferometer and used it in the famous Michelson-Morley experiment in 1887 that proved that the speed of light was constant. It was on that experiment that Albert Einstein built his Special Theory of Relativity.
The Keck interferometer is far more complex, with the light beams traveling between the two instruments in a circuitous path through the complex.
The Keck twins continue their work today, now in concert with the Gemini telescope, an 8 meter monolithic mirror on the same complex as Keck. Yet, even this monolithic mirror was constructed using Jerry Nelson’s segmented technique. Many octagonal segments were fused together to make a single blank. In the photo below, the Kecks are on the far end of the cinder cone. The Gemini is the silver dome. The small dome is a 3.6 meter scope jointly operated by Canada, France and Hawaii.
Today, the 21st century giants such as the Giant Magellan and the Thirty Meter Telescope, and even others to come, use some form of Nelson’s astounding idea of individual segments and position actuators to create telescopes larger than George Hale and George Ritchey could ever have imagined.
All Images: Keck Observatory https://keckobservatory.org/gallery