ADAPTIVE OPTICS: Tomography promises better stargazing

May 1, 2000
Thanks to adaptive-optics systems installed on large ground-based telescopes, astronomers now have instruments capable of approaching the resolution of the Hubble Space Telescope while at the same time collecting far more light
Two atmospheric layers aberrate light incoming from three seperate astronimical objects. Because the pupil of the telescope is at ground level, a low perturbing layer aberrates the wavefronts similarly, while a high-altitude layer adds individual aberrations to each wavefront. Although in reality the perturbations occur over a three-dimensional continuum, tomographic modeling of a few limited layers results in an adequate model. (Image orginally appeared in Nature 403, 54, 6 Jan. 2000. Used with permission.)
Two atmospheric layers aberrate light incoming from three seperate astronimical objects. Because the pupil of the telescope is at ground level, a low perturbing layer aberrates the wavefronts similarly, while a high-altitude layer adds individual aberrations to each wavefront. Although in reality the perturbations occur over a three-dimensional continuum, tomographic modeling of a few limited layers results in an adequate model. (Image orginally appeared in Nature 403, 54, 6 Jan. 2000. Used with permission.)

Thanks to adaptive-optics systems installed on large ground-based telescopes, astronomers now have instruments capable of approaching the resolution of the Hubble Space Telescope while at the same time collecting far more light. For example, the Keck 10-m telescopewhich collects 16 times the light of the 2.4-m Hubblecan now resolve to 0.034 arc sec using adaptive optics (see Laser Focus World, May 1999, p. 26). These systems rely on the presence of a bright guide star near or within the region of sky to be studied; such a star allows wavefront error due to atmospheric turbulence to be measured and corrected. (Occasionally an astronomical object of interest is both bright enough and small enough in angular size to be used as its own guide star.)

Conventional adaptive-optics systems, however, suffer from two problems. First, they correct well only over a tiny patch of skythe isoplanatic patchimmediately surrounding the guide star. Second, because sufficiently bright natural guide stars are few and far between, astronomical studies over most of the sky must rely on a laser-produced artificial guide star 90 km or so up in the atmosphere. Not only is such a star difficult to produce, but its path also encompasses a conical section of the atmosphere, rather than the cylindrical section required for ideal correction of an astronomical object's wavefront.

Multiconjugate adaptive optics (MCAO) under development promise to ease the first problem, although not the second. Rather than measuring wavefront error at a single point, a MCAO system will measure many points in the telescope's field and use the data to calculate corrections for multiple deformable mirrors placed in series. The system will require the production of several artificial mesospheric guide stars in a constellation. Each star will be monitored by a separate wavefront sensor; the resulting data must undergo massive computer processing.

Tomography demonstrated

A group of researchers at the Astronomical Observatory of Padova (Padova, Italy) has now demonstrated that the calculations needed to derive a tomographic representation of the atmospheric path between a telescope and multiple guide starsand thus the information needed to correct a MCAO systemcan result in a large reduction in wavefront variations across a patch of sky much larger than the isoplanatic patch. Using the 3.6-m Telescopio Nazionale Galileo (La Palma, Canary Islands, Spain) and imaging at a center wavelength of 700 nm, the researchers used natural guide stars for their observations.

They chose a triangular formation of three stars ranging in magnitude from 9 to 11 with a fourth star of magnitude 12 at its center. By defocusing, they produced four pupil images, each carrying information about the atmospheric path affecting the wavefront of its respective star. Analysis of the data produced wavefront maps of the four stars, which were then fitted to the first 27 Zernike polynomial orders.

After capturing many individual sets of data, the researchers computed the variance of the turbulence effects on the central star. Such information was calculated for the raw data and for two additional cases as well. In the first, the average of the three surrounding stars was subtracted from the central stara crude approach to compensating for turbulence. In the second, the data from the neighboring stars were processed through a modal tomography matrix; this is the true tomographic approach.

Because of the unusual close triangular configuration of stars, the averaging approach worked surprisingly well, reducing the variance by 77.4% compared to the raw data; however, other star configurations would likely not work as well. The true tomographic approach reduced the wavefront variance by 92.3% and has the potential to do even better and to work over larger patches of sky.

These results bode well for the MCAO project at the Gemini Observatory (Hilo, HI). The sole complete MCAO system currently under design and funded, the Gemini project aims to deliver 0.1-arc sec or better image quality at near-infrared wavelengths and ultimately in the visible. If things go as planned, the two 8.1-m Gemini telescopesone now in operation atop Mauna Kea, HI, and the other under construction atop Cerro Pachón (Chile)will both be fitted with MCAO systems. Each system will include three deformable mirrors and five artificial guide starsas well as a powerful computer to keep up with the flow of data.

Closed versus open loop

The calculations done by the Padova astronomers solved the tomographic problem in an open-loop fashion. According to Roberto Ragazonni, one of the astronomers, everything can be made much easier in a real MCAO system by closing the loop. Information on star images is then used to continuously update the tomographic model, which thus quickly and stably converge on a nearly ideal solution.

Ragazonni's group envisions not just closing the loop, but also creating a "layer-oriented" multiconjugate adaptive optics system that is tightly designed around the atmospheric layers causing the most perturbation (see figure). Such a system would contain one deformable mirror that is conjugate to the ground atmospheric layer and another that is conjugate to a high-altitude layer (a third deformable mirror could be incorporated into the design if necessary). The advantage of this approach is in the computing; because the mirrors sit at the conjugates of the perturbing layers, compensations for one perturbing layer do not much affect those for another. This would simplify and speed the computation. In addition, fewer laser guide stars would be needed, according to the researchers' proposal (see the Web site lenin.pd.astro.it/adopt/ for more details).

To aid their approach, Ragazonni and his colleagues have developed a pyramidic wavefront sensor to replace the Shack-Hartmann wavefront sensor that is standard in adaptive-optics systems. The sensor contains a transmissive optical element in the shape of a shallow sharp-edged pyramid having four sides, with a detector placed beyond the pyramid. In operation, a star is imaged at the apex of the pyramid while the pyramid is oscillated to generate an error signal; very small changes in the star image cause large changes at the detector. The researchers in Padova have built an adaptive-optics module that includes pyramidic wavefront sensors and have installed it on the Telescopio Nazionale Galileo. In contrast to Shack-Hartmann sensors, the pyramidic sensors demonstrate a large increase in signal to noise once the loop is closed.

Whole-sky viewing with natural guide stars

Concepts exist for ground-based telescopes containing primary mirrors of up to 100 m in diameter.1,2 Because the light-collecting ability of such telescopes would allow vast numbers of stars to be imaged, the Padova astronomers believe that enough natural guide stars would then be available to a MCAO system to permit viewing of the whole sky, eliminating the trouble of creating constellations of artificial guide stars. They also believe that combining a layer-oriented closed-loop MCAO approach with their pyramidic sensors may bring the necessary primary mirror diameter for whole-sky viewing down below 100 mperhaps even down to the 8-m size that is becoming more and more common in today's large telescopes.

REFERENCES

  1. R. Gilmozzi et al., Proc. SPIE 3352, 778 (1998).
  2. C. M. Mountain, Proc. SPIE 2871, 579 (1997).
About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

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