Adaptive optics enhances power of astronomical telescopes

Images produced by astronomical telescopes operating from the surface of the Earth are degraded by atmospheric turbulence much like reflections shimmering above warm pavement on a summer day. The term ‘adaptive optics’ (AO) encompasses a wide range of techniques and technologies for removing time-variable aberrations in optical systems. In astronomical applications, AO is used to remove wavefront distortions induced by turbulent structures in the atmosphere between the source (a celestial object outside the Earth's atmosphere) and the primary aperture of the telescope. For most of its development, AO efforts focused on achieving diffraction-limited images, with a resolving power defined as 1.2λ/D (where D is the diameter of the entrance aperture and λ the operating wavelength). For the largest telescopes currently in operation (e.g., the Keck Observatory's twin 10m telescopes, the Gemini North and South telescopes, the Very Large Telescope, the Large Binocular Telescope, and other 8m-class telescopes), this corresponds to image sharpnesses of ~30–50 milliarcseconds. For the next generation of 25–40m-diameter telescopes (the Giant Magellan Telescope, the Thirty Meter Telescope, and the European Extremely Large Telescope), AO will produce images of 6–10 milliarcseconds at a wavelength of 1μm. For comparison, the Hubble Space Telescope produces images that are ~120 milliarcseconds in diameter, while uncorrected images from the ground are typically 600–900 milliarcseconds for state-of-the-art telescopes at good sites.

In astronomical AO systems, errors in the wavefront are canceled out by one or more deformable mirrors whose shapes change on a timescale comparable to the coherence time of the wavefront in response to commands from a wavefront sensor and reconstructor. The short coherence time and scalelength of atmospheric turbulence has generally confined AO work to IR wavelengths (the characteristic scale grows as λ^5/3 and is on the order of 1m at a wavelength of 2μm on a good night). The effectiveness of a diffraction-limited AO system is defined by the ratio of the peak image intensity achieved to that expected from a perfect optical system. This ‘Strehl ratio’ is typically 20–50% in near-IR AO systems. The field of view over which the wavefront can be fully corrected is limited. For single deformable-mirror systems, it is on the order of 30 arcseconds for an area covering 0.1% of the full moon.

Many astronomical investigations require fields of view that are larger than those that can be achieved with diffraction-limited AO. Moreover, some experiments do not require (or even benefit from) the full power of diffraction-limited images but would gain substantially from image quality between that produced by the uncorrected atmosphere and the diffraction limit.

The atmospheric turbulence that is responsible for image blurring is strongest at a small number of discrete altitudes where turbulence is generated by boundary-layer interactions or two-stream instabilities. For most astronomical sites, at elevations of 2000–4000m, roughly half of the image degradation is caused by turbulence at high altitudes (e.g., 5km) while the remainder is generated by boundary-layer interactions in the first few hundred meters or less above the site.

Ground-layer AO (GLAO) corrects aberrations originating in this lower layer of turbulence. Because its characteristic length scale subtends a large angle at the telescope, correcting the layer sharpens images over fields of view much larger than those offered by diffraction-limited AO systems. In addition, because surface winds are generally lower than those at high altitude, GLAO can operate at somewhat lower correction rates than conventional AO systems. However, because high-altitude turbulence remains uncorrected, GLAO systems do not reach the diffraction limit. Instead, the images remain ‘seeing’ limited, but with improvements by factors of 3–4 in sharpness and ~10 in peak intensity as compared to uncorrected observations. These large gains can be realized over fields of view 250 times larger than the field area offered by standard AO systems.

In practice, GLAO works by observing multiple guide stars along different lines of sight (see Figure 1). The aberration arising from the boundary layer is estimated as the mean wavefront from all guide stars, which averages out the effects of high-altitude turbulence. The deformable mirror is most effective at correcting large fields of view if it is an optical conjugate to a location near the turbulence layer. Gregorian optical systems using an adaptive secondary mirror are ideally suited to this type of AO work. (The Giant Magellan Telescope is being designed with such a secondary mirror as part of its AO system.)

Figure 1. Ground-layer adaptive optics (GLAO) uses multiple laser beacons (green) to correct for turbulence on the Earth's surface. Light from science targets (red and blue) is affected by the same aberration across a wide field of view.

Until fairly recently, GLAO operated primarily in the realm of theory and laboratory demonstration. A number of systems are now coming online, in both the US and Europe. Recently, we field tested a GLAO system at the MMT observatory (Arizona). GLAO can dramatically improve image quality and sensitivity over large fields of view (see Figure 2).¹

Figure 2. The core of the globular star cluster Messier 3 seen at 2μm wavelength in two 60s exposures with a GLAO demonstration system at the MMT observatory. (a) Full 110 arcsecond (″) field of the IR camera in the prevailing atmospheric limit of 0.7 arcseconds on a logarithmic intensity scale. Two smaller 27 arcsecond regions of the same image, indicated by the boxes in (a), are shown on a truncated linear scale in which bright stars appear saturated but that reaches the noise floor and brings out the faintest observable stars: (b), centered on the tip-tilt star, indicated by the arrow, and (d), positioned to show the edge of the field. In the same two regions—(c) and (e), shown on the same linear scale as (b) and (d) from a separate 60s exposure taken with GLAO running—the stellar image width is reduced to 0.3 arcseconds with very similar point-spread-function morphology across the entire field of view. Stars can be seen in the corrected image that are six times fainter than the dimmest stars visible in the uncorrected image.

One of the primary drivers for GLAO is to use it as input for multi-object spectrographs operating in the near-IR regime. The density of targets on the sky is rarely high enough for conventional AO systems to provide multiple targets for spectroscopy. The wide fields of view offered by GLAO systems enable simultaneous observations of tens to hundreds of targets. Improvements in image concentration offered by GLAO translate directly to gains in signal-to-noise ratios and speed as the contrast between the signal from the astronomical source and the terrestrial sky emission is improved. Over the next few years, a number of such systems will be deployed on telescopes around the world. This will increase their sensitivity at a cost that is a small fraction of that needed to produce larger apertures or launch telescopes in orbit above the Earth's atmosphere. GLAO is one of the latest applications of optical science that promise to improve the performance of existing and future astronomical telescopes by removing obstacles that seemed insurmountable only a short while ago.