Observations

The rings of Uranus. De Pater et al..

On May 28, 2007 I took the first ground-based images of the dark side of the rings of Uranus, using an advanced instrument called adaptive optics at the W.M. Keck Observatory. The images clearly show that the rings have changed compared to when they were viewed by Voyager, a space craft, in 1986. Images were taken using the NIRC2 infrared camera at two different wavelengths: K-band (2.1 microns) and H-band (1.65) microns. The planet is much more easily seen at H-band because the atmosphere of Uranus absorbs most of the light at K-band. The rings, on the other hand, are most easily seen at K-band. Since the initial observations in May, many astronomers, at W.M. Keck Observatory and other facilities, have also pointed telescopes at Uranus. H-band and K-prime composite of Uranus. Image: Patrick Fry and Marcos van Dam

A science article on the dark side of the rings was written by Imke de Pater, Heidi Hammel, Mark Showalter and Marcos van Dam. It was submitted to the Science journal and published online in Science Express on 23 August 2007. The article can be accessed here.

Adaptive optics

Adaptive optics (AO) works by measuring the atmospheric turbulence up to 2000 times a second and applying corrections to a deformable mirror. The mirror deforms so as to cancel the effect of the atmosphere. In this way, the images obtained have the same resolution as if they were taken from space. In February 2007, we performed a substantial upgrade on the adaptive optics system at the W.M. Keck Observatory and used dedicated engineering time to see how well the upgraded system works. Usually, the atmospheric turbulence is measured by looking at a star. One of the tests we performed was to see how well the images are corrected when using a large object, such as Uranus, to measure the turbulence. We found that the images of Uranus were sharper than any that had ever been taken from the ground! Having such clear images allowed us to determine that the rings had changed substantially from the way Voyager saw them.

Linear scale image of a star with AO off and on Logarithmic scale image of a star with AO off and on

The engineering team

My colleagues in implementing the upgrade to the adaptive optics system were Erik Johansson, Paul Stomski, Roger Sumner, Jason Chin and Peter Wizinowich. The upgrade was generously funded by the W.M. Keck Foundation and realized in collaboration with Microgate.

Online magazine articles on this upgrade, called the Next Generation Wavefront Controller, can be found here:

Squashing the bugs

Celebrating the success of the upgrade to the AO system! Front: Paul Stomski, Marcos van Dam and Erik Johansson. Back: Jason Chin and Stephan Kellner. Photo: S. Anderson

The W.M. Keck Observatory AO system has been used to capture a variety of beautiful images, some of which can be viewed in the AO image gallery. More technical information about our AO system can be found in the AO public website.