Building a Better Guide Star, Times Five
Wavefront reconstruction of multiple beams improves telescope performance.
The nursery rhyme is half correct: Stars are not little, but they do twinkle — thanks to fluctuations in the atmosphere. Consequently, when astronomers peer upward, they get a less-than-perfect image of stars and planets. So they have turned to guide stars and adaptive optics to improve their ability to see. If no suitable natural guide star is present, they generate one using a laser.
In this picture, a guide star is created with a laser beam fired upward from the MMT Observatory’s 6.5-m telescope atop Mount Hopkins in Tucson, Ariz. Images courtesy of Gabor Furesz.
Now a group from the Center for Astronomical Adaptive Optics at the University of Arizona in Tucson has demonstrated a five-laser guide star system that performs a tomographic reconstruction of the incoming wavefront to maximize telescope performance.
Assistant astronomer Michael Lloyd-Hart noted that the scheme, although not cheap, was necessary, especially for future telescopes that scientists project will be constructed around a mirror 25 m or more in diameter. “Beyond the 10-m size of today’s largest telescopes, tomographic wavefront sensing with lasers becomes essential,” he said. “It is not worth the cost of building such expensive instruments if we do not design them for the absolute maximum scientific return.”
By using a guide star, whether natural or artificial, astronomers try to correct for atmospheric instability and try to enable imaging performance near the diffraction limit. Unfortunately, natural guide stars allow such corrections over only a small percentage of the sky; thus, astronomers make their own, typically by firing a laser into the sky. About 90 km up, the beam strikes a layer of sodium atoms, creating a single beacon.
In their work, the researchers created five beacons using two pulsed Nd:YAG lasers that were frequency-doubled to emit at 532 nm. They created the guide stars via Rayleigh scattering at an average 24-km height. One benefit to this approach, Lloyd-Hart noted, is that the lasers they used are much cheaper and more robust than those used to create sodium guide stars.
Each beam from the five-laser system devised by the researchers provided a Shack-Hartmann wavefront pattern. Combined, the patterns were used for adaptive optics correction of atmospheric aberrations in the telescopic images.
In a demonstration, the investigators used the technique with the 6.5-m telescope at the MMT Observatory on Mount Hopkins in Arizona. They measured the wavefront with a Shack-Hartmann analyzer and ran the system without feedback between the system and the telescope, or in an open-loop configuration. To improve results, they employed the sensor system’s optics to focus the telescope dynamically 5000 times per second on each laser pulse as it rose from 20 to 29 km. Because the guide stars were located within the atmosphere, a single beacon could not provide all of the parameters required to correct for perturbations in the entire atmospheric column. However, five beacons and a tomographic reconstruction overcame that problem.
The results showed a significant improvement in image quality at the 2.2-μm band, with the peak intensity increasing fourteenfold and the width of an image dropping from an uncorrected 0.543 to 0.088 arc sec — close to the diffraction limit of 0.070 arc sec.
As for those elements that may be limiting system performance, Lloyd-Hart said, “We believe that [they are] things we can do something about — such as noise in our wavefront sensor detector or light losses in the receiving optics — rather than [the] fundamental physics of the atmosphere or the lasers.”
Besides improving performance, the researchers plan to create a closed control loop. According to Lloyd-Hart, they should test one by the end of the year. If the results are good, the technique will be even closer to being available for the next generation of large telescopes.
Optics Express, Aug. 21, 2006, pp. 7541-7551.
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