Telescopes equipped with Adaptive Optics (AO) have the remarkable ability to cancel the image degradation caused by turbulence in the Earth's atmosphere. A revolution is occurring in ground-based astronomy today as more and more AO systems are deployed on the world's largest telescopes. These new AO-equipped telescopes (1) provide diffraction-limited angular resolution for imaging, (2) increase the limiting magnitude in all exposures taken with the AO systems, (3) make it possible to do interferometry with large aperture telescopes, and (4) drastically simplify the design of many large instruments on telescopes -- such as spectrographs -- where the size of a stellar image determines the physical size of the optics required to build the instrument.
The AO revolution began in the last decade of the 20th Century, and it is now sweeping through all of ground-based astronomy. While the first simple steps have involved AO systems that use the light from a single bright natural star to monitor and correct atmospheric perturbations, the most amazing and significant scientific gains will come when both laser guide stars and AO systems are integrated into the designs of the Extremely Large Telescopes that are now on the drawing-boards. The experimental laser beacon systems of today -- like my own system called UnISIS -- are the stepping stones to that future.
The first section below (immediately after the WebPabe Outline) contains a detailed description of UnISIS, one of a very small number of fully operational laser beacon AO systems. Section II gives a brief discussion of how the future of laser guided AO might develop. Then two historical topics are presented: a description of ISIS -- my first effort in adaptive optics-- and second, a description of the first sodium-wavelength laser guide star test on Mauna Kea in 1987. As explained in section IV, this laser guide star test on Mauna Kea was one of several key events that prompted the U.S. military to declassify their own work in laser guided adaptive optics, thereby accelerating the acceptance of laser guided AO systems by astronomers. Section V, the last entry in this list, is a link to the NSF-funded Center for Adaptive Optics (CfAO) at U.C. Santa Cruz where links to other AO projects can be found.
Jump ahead to any of the topics listed below.
UnISIS is an acronym that comes from University of Illinois Seeing Improvement System.
Principal Investigator, Laird Thompson
Co-Investigator,
Prof. Scott Teare, Dept. Electrical Engineering, New Mexico Tech
UnISIS has been funded primarily by the National Science Foundation through the Astronomy Program's Advanced Technologies and Instrumentation Division. NSF Program Directors who have been especially influential in its development have included Dr. G. Wayne van Citters, Dr. Benjamin Snavely (now at AIP), and Dr. James Breckinridge. Additional funding has been provided by the University of Illinois College of Liberal Arts and Sciences and the University of Illinois Astronomy Department.
At the end of each summer/fall observing season at Mt. Wilson, key progress in the development of UnISIS and its experimental / operational status are posted on the project website and linked to the University of Illinois Astronomy Department homepage. Click here for that information. The following sections of this website contain a more detailed summary of the mechanical configuration of UnISIS, some of the design considerations upon which the system is based, and the properties of several key subsystems. The division of material between these two sites is somewhat arbitrary, but in general those interested in understanding how UnISIS works should continue at this website, but those who simply wish to see how well the system is doing at the moment might want to go to the alternate UnISIS website.
Click on the first picture below to see an enlarged version of how the main subsystems of UnISIS are situated in the Mt. Wilson Observatory 2.5-m Telescope dome. This overview picture of UnISIS provides an orientation for the four small thumbnail images that follow. Clicking on the thumbnails will bring up a description of each subsystem and many links to close-up photographs of the individual optical components within UnISIS: the deformable mirror, the tip-tilt mirror, transfer optics, etc.
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Existing telescopes will be equipped eventually with multi-conjugate AO systems with multiple laser beacons and multiple deformable mirrors. These multi-conjugate systems will be capable of correcting fields of view as large a 3 to 5 arcmin in diameter. New extremely large telescopes -- with diameters in the 15-m to 100-m range -- will be designed and built with multi-conjugate AO systems and multiple laser beacons. These AO systems will be integral parts of the extremely large telescope designs.
The future work on these more complex systems is underway at the present time. [And this portion of my webpage is still under construction.]
My first steps into adaptive optics were taken in the early 1980's with the design and construction of a microprocessor controlled tip-tilt mirror system that I called ISIS (Image Stabilizing Instrument System). The development of ISIS was done in collaboration with Herb Ryerson who was, at that time, the Head of Electronics at the University of Hawaii's Institute for Astronomy. Once completed, ISIS was used at the Cassegrain foci of both the University of Hawaii 2.2-m and the CFHT 3.6-m telescopes. It was the first such system to be used successfully on Mauna Kea, even though the CFHT organization was simultaneously developing what they called VHRCam and later HRCam. The CFHT work was started in response to -- or to compete with -- the development of ISIS.
The following schematic, which was first reproduced in the ISIS design paper
(L.A. Thompson & H.R. Ryerson 1983, Instrumentation in Astronomy V, Proc.
SPIE, vol. 445, p. 560) shows the simplicity of the design. The Cassegrain
beam for the telescope is deflected before it comes to focus by the main
tip-tilt mirror, and before sending the beam on to the Cassegrain focus the
beam hits a dichroic where light from the guide star is acquired.
In actual operation, ISIS did an excellent job removing both the
atmospheric tip-tilt jitter and small-scale irregularities in
the telescope drive system during an exposure.
However, ISIS was not able to break the 0.5
arcsec (FWHM) barrier, as was originally anticipated when the system was first proposed.
This fault remained an enigma until the CFHT work with VHRCam and HRCam found
that the telescopes themselves had built-in static aberrations produced by (for
example) the primary mirror support structure and poor collimation of the telescope
optics. Once these static aberrations were removed with corrective optics, the
CFHT HRCam pushed below the 0.5 arcsec. barrier. By that time, ISIS
had been de-commissioned, and I had moved on to work with laser guide stars.
Examples of scientific results produced by ISIS can be found at two other places on my
website:
Nucleus of Cygnus A and
Coma Globular Clusters.
Photographs of the ISIS Control panel and the ISIS Instrument Housing are shown below.
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For this information, click on the link Laser Guide Stars on Mauna Kea circa 1987 to transfer to another section of this webpage.