Adaptive Optics Research at Yerkes Observatory

Perhaps the greatest limitation that exists with all modern astronomical observatories is the fact that they are located on the surface of the Earth, and therefore have to contend with looking at stars, planets, nebulae and galaxies through the atmosphere. The atmosphere is constantly in a state of turmoil, or turbulence, because of uneven temperature and pressure variations which results in mixing motions. The air may seem clear, but if you have ever seen a mirage over a highway, similar phenomena work to distort the images in all telescopes rendering them with the limitation to resolve features on objects no better than a Celestron 8" telescope! Astronomers want larger telescopes to see fainter objects, but they would also like to resolve finer features with them. Putting large telescopes into space is one albeit very costly solution. An alternative technique is known as adaptive optics where a rubber, or "deformable," mirror is used to undo what the atmosphere is doing. These systems are being designed and built for many observatories, though they are very complex systems and there still remains much to be learned on how to best build and use these systems. Work at Yerkes Observatory is conducted in part to better understand the function of these systems and to use that knowledge to build better and less costly devices to serve those who desire such systems.

Numerous investigations are presently underway at Yerkes Observatory utilizing the 41" reflecting telescope. Located in the ground floor coude room 40' below the telescope is the Wavefront Control Experiment (WCE) which was built at a cost of $85,000,000 for an experiment to be conducted on the Space Shuttle. That SDIO experiment was canceled in 1990, and the University of Chicago was awarded the WCE for fundamental astronomical investigations in 1992; it has resided here at Yerkes since mid-1994. The WCE corrects for atmospheric image distortions by measuring the wavefront and then applying signals to the actuators in a deformable mirror at a rate of 4000 times a second. It is one of the world's fastest adaptive optics systems. We are presently using it to study the harshest form of atmospheric seeing, which is when one looks at stars just over the horizon (horizontal, or "volume" turbulence); besides horrible image degradations there is also significant brightness fluctuations, or scintillation, in the source. We have set up a green laser source three miles distant on the other side of Lake Geneva to serve as a bright star source for these tests. We are testing numerous mathematical algorithms and instrumental techniques, and taking fundamental measurements, to quantify image quality seen through horizontal paths and to figure out ways to improve these images. One astronomical application is to image the planet Mercury while it is just above the horizon and still in a reasonable dark sky.

Other experiments include monitoring Jupiter for the Galileo mission team at JPL using a simple, but powerful, off-the-shelf item, called the AO-2, that corrects for image motion due to either atmospheric turbulence effects or to vibrations in the telescope system. Another are tests using a "low-order" adaptive optics unit, called the AO-5, funded by the Meyer Foundation for the Meyer-Womble Observatory on Mt. Evans in Colorado, which is on loan to Yerkes and installed in the WCE laboratory. Using that apparatus we hope to monitor the atmospheres of the planets Uranus and Neptune, and to do other interesting astronomical investigations. We have an ideal facility for testing and experimenting with novel instrumentation because we are guaranteed ample telescope time for this research, and we have an accessible and stable laboratory environment for these tests. Though the weather is not always ideal, artificial sources in the laboratory, behind the telescope, or across the lake give us varying degrees of turbulence and can be used even in cloudy weather! Considerable efforts over the previous three years have made the Yerkes 41" telescope a powerful tool for conducting leading edge adaptive optics research. We look forward to expanding our program and new collaborative arrangements with scientists throughout the world.

For further information or for answers to any questions that you may have about our work, please contact Walter Wild at (773) 702-8747, or use email at walter@troy.uchicago.edu.

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Content originally generated in March 1995.
This file was last modified on 20 April 1999.