Broader view for adaptive optics

Adaptive optics has become standard on large ground-based telescopes because it offers far sharper images than otherwise obtainable. However, standard adaptive optics can compensate atmospheric turbulence only over small areas, so they don't let ground-based telescopes match the celestial panoramas imaged by the Hubble Space Telescope. Now a new generation of adaptive optics has demonstrated high-resolution imaging over a larger field of view with the Gemini South telescope in Chile.

Proposed more than a decade ago by François Rigaut, now at Australian National University (Canberra, Australia), the Gemini Multi-conjugate adaptive optics System (GEMS) uses five laser guide stars and three deformable mirror to measure atmospheric distortion and compensate for its affects. Sampling at 500 to 1000 Hz, GEMS can compensate for turbulence over an area of sky 16 times larger than previously possible.

The picture below tells the story, alternating images of the "Orion Bullets" region in the Orion Nebula taken with GEMS in December 28, 2012 and of the same region taken in 2007 with the previous-generation ALTAIR adaptive-optics system, which uses a single laser guide star. The larger field of view is 85 arcsec across. Without the adaptive optics, the telescope's resolution at the observation time was 0.8 to 1.1 arcsec. Adding GEMS improved resolution by a factor of ten to 0.084 to 0.103 arcsec.  The bright spots are "bullets" of gas ejected from the core of the nebula that are ripping through molecular hydrogen at speeds to 400 km/s, leaving behind wakes of hot hydrogen.

GEMS also benefits from processing enhancements, which use tomographic techniques to map air turbulence in three dimensions, and correct uniformly across the entire field of view. "This is huge when it's time for astronomers to reduce their data," says Adam Ginsburg, a graduate student at the University of Colorado (Boulder, CO), because observers often need to compare objects in the same field.

Field size has long been a crucial limitation on adaptive optics. The 85-arcsec width of the GEMS image still falls well short of the more than nearly 200-arcsec width of the Ultra Deep Field image taken by the Hubble Space Telescope, but it's an important step. With Hubble now well into its third decade in orbit, astronomers need new ways to study the depths of the sky from the ground.

Comparison of images of the same field in the Orion nebula recorded with GEMS and ALTAIR. The white "Orion Bullets" are fast-moving gas clouds leaving hot hydrogen in their wake. Their motion is fast enough to detect in the five years between the 2007 ALTAIR and the 2012 GEMS images.