New Adaptive Optics System Brings Globular Cluster Into Focus
John P. Millis, Ph.D. for redOrbit.com — Your Universe Online
One challenge of ground-based optical astronomy is that photons in this regime, and nearby infrared and ultraviolet bands, get refracted in our atmosphere. The consequence is that imaging of astronomical objects can be blurred, making it difficult to identify and characterize individual objects.
Compensating for these effects can be tricky though, as our atmosphere is not a static system. Rather, it is in a state of constant flux. One needs look no further than an airplane ride to experience this first hand. So astronomers have developed systems for dealing with these chaotic atmospheric conditions.
Known as adaptive optics, a laser system affixed to the observatory is directed to a specific point on the sky. This creates an artificial guide star. The telescope then observes the position of the laser and records the position relative to its intended pointing; watching how the position “wobbles” over time.
The data can then be analyzed to determine the prevailing atmospheric conditions and how this will affect the current observing period. Then, when data is taken on an object of interest, the images can be de-wobbled, creating significantly sharper, more accurate images. Having better resolution means that when two objects are very close together they can be identified separately, when they otherwise would have been blurred together, appearing as a single object.
Such adaptive optics systems have been deployed at observatories around the world, but typically use visible laser systems. But now a new instrument has been deployed by researchers at the Southern Observatory for Astrophysical Research (SOAR) and the Cerro Tololo Inter-American Observatory (CTIO). Known as SAM (SOAR Adaptive Module), this adaptive optics system uses a higher frequency ultraviolet laser to create the guide star.
The result is that SAM can compensate for lower atmospheric effects down to about 10 kilometers. By contrast, most other systems using visible lasers and model the atmosphere at about 90 kilometers above the Earth. Having data about the lower atmosphere will allow observations to be taken over a wider field of view, all the while being more cost effective than traditional sodium-laser systems.
Now that the system has been deployed, the team behind the effort has turned their eye toward the globular cluster NGC 6496. This system has been a source of controversy among astronomers. Because of its location on the other side of the galaxy, obscured by the galactic center, it has been notoriously difficult to study. As a result, conflicting measurements have been reported in the literature, including widely varying estimates of its age and distance.
Astronomers Luciano Fraga, Andrea Kunder, and Andrei Tokovinin, using the new SAM system, were able to image about 7000 stars in the cluster; measuring their colors and magnitudes. With their high quality measurement they were able to determine that the system is about 32,600 light-years away and was created about 10.5 billion years ago. Also, the system appears to have a higher metalicity content compared to most of the roughly 150 globular clusters in our galaxy.
Given the early success of this unique system, coupled with its relatively low cost, similar instruments could be on the horizon at other observatories. In the interim, researchers are only starting to scratch the surface of the possibilities that SAM has to offer.
Image 2 (below): Globular cluster NGC 6496 observed with SAM. The image is about 3 arc minutes across. The enlarged sections of the cluster show the image with SOAR adaptive optics (AO) on and off. Credit: NOAO/AURA/NSF