Active and Adaptive Optics

Large objects are inherently structurally unstable. Skyscrapers, for example, wobble, but their design ensures that the building will hold together under all normal weather conditions, and a certain swaying is tolerated since even steel flexes when a force is applied. Large telescope mirrors are very heavy. Although they are made of glass, a thermal insulator, they still expand and contract slightly as the temperature changes. They also flex, depending on their angle relative to the ground. This flexing leads to distortions in the surface of the mirror. A further complication is that the Earth's atmosphere continually fluctuates in temperature and density, causing light traveling through it to be distorted from its original form. Historically, astronomers have had no fix for these problems beyond passive cooling of the mirror, the use of structurally rigid mirror supports, and the placement of telescopes on high mountains to decrease the influence of the atmosphere on the light they received. The techniques of active and adaptive optics now allow astronomers to overcome these limitations far more effectively.

Active optics provides a way of deforming a mirror to compensate for its inherent lack of structural rigidity. In adaptive optics, the optical elements of the telescope are instantaneously and continually adjusted to compensate for—in effect, to cancel out—the blurring effect of the Earth's atmosphere. Both active optics and adaptive optics use actuators (tiny pistons) and computers in a constantly monitored electronic feedback system to make minute adjustments to the shape of the primary and secondary reflective surfaces (Figure 2.4).

The main difference between the two techniques is that active optics systems make changes that are relatively slow. Active optics is intended to correct for the sagging of the telescope as it tracks an object across the sky or for the low-frequency components of the vibrations caused by the wind buffeting the telescope. In contrast, adaptive optics is intended to remove the effect of turbulence in the Earth's atmosphere and therefore makes much more rapid adjustments. The slower active

Segment Mirror Active Optics

Figure 2.4. This image of Pluto and its companion Charon taken with the Subaru 8.3-m telescope (left) resolves the two bodies into distinct points of light. The accompanying spectra (right) show the importance of visually separating the two bodies, showing that the two objects have very different surface compositions, with Pluto showing evidence for solid ethane on its surface. Pluto and Charon have an apparent separation of only 0.9 arcseconds as seen from the Earth. Courtesy of the National Astronomical Observatory of Japan.

Figure 2.4. This image of Pluto and its companion Charon taken with the Subaru 8.3-m telescope (left) resolves the two bodies into distinct points of light. The accompanying spectra (right) show the importance of visually separating the two bodies, showing that the two objects have very different surface compositions, with Pluto showing evidence for solid ethane on its surface. Pluto and Charon have an apparent separation of only 0.9 arcseconds as seen from the Earth. Courtesy of the National Astronomical Observatory of Japan.

optics systems generally distort the primary mirror surface, while the more rapid adaptive optics systems typically make adjustments to smaller mirrors in the optical path of the telescope.

In addition to distortions of the primary mirror, most active optics systems also depend on movements of the secondary mirror, allowing the mirror to tilt and rotate interactively to adjust for errors in the primary surface. Active surfaces have been successfully tested on smaller telescopes and are now being installed on the next wave of large optical telescopes. The Keck active optics system adjusts the relative positions of the mirror segments and their shapes twice a second. The presence of an active optics system means that the mirror need not be as rigid as would have been otherwise necessary. An active optics system acts as a kind of "girdle," maintaining the ideal curvature of the mirror despite the tug of gravity. The mirrors on the Gemini telescope have 120 of these actuators behind them, capable of minute adjustments.

Active optics systems, sometimes known as "active surfaces," are not limited to use in optical telescopes. The Green Bank Telescope, the 100-m by 110-m radio telescope dedicated in August 2000, has an active surface controlled by over 2,000 actuators located at the junctures of the surface panels. As we build ever larger, ever thinner telescopes, we are becoming more and more dependent on active optical surfaces.

Adaptive optics systems must adjust the received light in such a way as to compensate for the distorting effect of the atmosphere. The light from distant objects arrives at the top of the Earth's atmosphere in a planar wavefront. The atmosphere distorts this wavefront through variations in the index of refraction. These variations, in a sense, slow down the light randomly across the wavefront. Adaptive optics systems compensate for this "rough" wavefront by adjusting the focal plane to compensate. Since the atmosphere varies quite quickly, the corrections to the optical wavefront occur on much shorter timescales than is the case with active optics; typically, thousands of adjustments are made each second. In addition to being faster, adaptive optics systems must also be able to control the surface of the primary or secondary mirror more finely. The incoming wavefront must be "sampled," or "sensed," with change-coupled devices (CCDs), and then corrections must be calculated and fed to a corrective mirror system. This sampling and correction of the wavefront should be done over approximately 20-cm subapertures on the mirror surface, since this scale corresponds to the typical size of the fluctuating regions in the atmosphere. For telescopes with 4-m size mirrors, adjustments to the primary mirror on this scale would require several hundred actuators. For larger telescopes like the Very Large Telescope and Gemini telescopes, thousands of actuators on the primary surface would be required (Figure 2.5). To overcome the need for a very large number of actuators, adaptive systems make adjustments not to the primary mirror but to the surface of the smaller secondary mirror.

The light from a single star in the field can be used to determine the distortions caused by atmospheric turbulence. This light is sent to a "wavefront sensor" that determines, on very short timescales, where and by how much the primary (or secondary) mirror must be distorted to produce a nearly pointlike image of the star.

Adaptive optics systems produce both a sharper and a brighter image. The blurring of the atmosphere spreads out the light of a star over (at best) 1 arcsecond or so. An operational adaptive optics system can concentrate that light into a point whose size is limited only by the diameter of the telescope. Alternatively, lasers can be used to create artificial beacons in the sky via scattered light. A laser aligned with the telescope effectively "samples" the atmosphere, and the mirror surface is adjusted to correct the detected scattered light into a pointlike source.

All of the large optical telescopes in existence or now under construction are candidates for adaptive optics systems. The Keck II telescope has recently provided practical demonstrations of its adaptive optics system.

References

Active Optics. http://www.physics.usyd.edu.au/physopt/ao/ss.html. Active Optics. http://www.us-gemini.noao.edu/public/active.html (July 25, 2000). Active Optics. http://www.eso.org/projects/vlt/unit-tel/actopt.html (July 25, 2000). Adaptive Optics. http://op.ph.ic.ac.uk/research/index.html (July 27, 2000). Adaptive Optics. http://www.us-gemini.noao.edu/public/adaptive.html (July 25, 2000).

Adaptive Optics. http://athene.as.arizona.edu:8000/caao/ (July 25, 2000). Ageorges, N., and Hubin, N. "Atmospheric Sodium Monitor for Laser Guide Star Adaptive Optics." Astronomy and Astrophysics Supplements, 144 (2000): 533. Gebhardt, Karl; Pryor, Carlton; O'Connell, R.D.; Williams, T.B.; and Hesser, James E. "Canada-France-Hawaii Telescope Adaptive Optics Observations of the Central Kinematics in M15." Astronomical Journal, 119 (2000): 1268. Lloyd-Hart, Michael. "Thermal Performance Enhancement of Adaptive Optics by Use of a Deformable Secondary Mirror." Publications of the Astronomical Society of the Pacific, 112 (2000): 264-272. Ragazzoni, Roberto; Marchetti, Enrico; and Valente, Gianpaolo. "Adaptive-Optics Corrections Available for the Whole Sky." Nature, 403 (2000): 54.

Figure 2.5. The completed and operational 8.1-m-diameter Gemini North telescope is shown here. Notice the minimal skeletal supporting structure for the primary and secondary mirrors. The primary mirror has a diameter of 8.1 m, and the secondary mirror can be seen in reflection. Courtesy of National Optical Astronomy Observatory.

Figure 2.5. The completed and operational 8.1-m-diameter Gemini North telescope is shown here. Notice the minimal skeletal supporting structure for the primary and secondary mirrors. The primary mirror has a diameter of 8.1 m, and the secondary mirror can be seen in reflection. Courtesy of National Optical Astronomy Observatory.

Wizinowich, P.; Acton, D.S.; Shelton, C.; Stomski, P.; Gathright, J.; Ho, K.; Lupton, W.; Tsubota, K.; Lai, O.; Max, C.; Brase, J.; An, J.; Avicola, K.; Olivier, S.; Gavel, D.; Macintosh, B.; Ghez, A.; and Larkin, J. "First Light Adaptive Optics Images from the Keck II Telescope: A New Era of High Angular Resolution Imagery." Publications of the Astronomical Society of the Pacific, 112 (2000): 315.

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