Dispersed fringe sensor for the Giant Magellan Telescope.

The Giant Magellan Telescope (GMT) consists of seven 8.365 m segments, with gaps of 0.345 m between adjacent segments. A unique challenge for GMT lies in phasing the segments and, in particular, how to measure segment piston optically while the telescope is in science operation. In this paper, we present a dispersed fringe sensor (DFS) to make these measurements using a novel algorithm. We show that using four off-axis DFSs operating at J-band with 10 ms exposures, we are able to measure segment piston to the required 50 nm accuracy every 30 s with over 90% sky coverage.

A low density of 0.8 g cm(-3) for the Trojan binary asteroid 617 Patroclus.

The Trojan population consists of two swarms of asteroids following the same orbit as Jupiter and located at the L4 and L5 stable Lagrange points of the Jupiter-Sun system (leading and following Jupiter by 60 degrees ). The asteroid 617 Patroclus is the only known binary Trojan. The orbit of this double system was hitherto unknown. Here we report that the components, separated by 680 km, move around the system's centre of mass, describing a roughly circular orbit. Using this orbital information, combined with thermal measurements to estimate the size of the components, we derive a very low density of 0.8(- 0.1)+0.2 g cm(-3). The components of 617 Patroclus are therefore very porous or composed mostly of water ice, suggesting that they could have been formed in the outer part of the Solar System.

Modeling low order aberrations in laser guide star adaptive optics systems.

When using a laser guide star (LGS) adaptive optics (AO) system, quasi-static aberrations are observed between the measured wavefronts from the LGS wavefront sensor (WFS) and the natural guide star (NGS) WFS. These LGS aberrations, which can be as much as 1200 nm RMS on the Keck II LGS AO system, arise due to the finite height and structure of the sodium layer. The LGS aberrations vary significantly between nights due to the difference in sodium structure. In this paper, we successfully model these LGS aberrations for the Keck II LGS AO system. We use this model to characterize the LGS aberrations as a function of pupil angle, elevation, sodium structure, uplink tip/tilt error, detector field of view, the number of detector pixels, and seeing. We also employ the model to estimate the LGS aberrations for the Palomar LGS AO system, the planned Keck I and the Thirty Meter Telescope (TMT) LGS AO systems. The LGS aberrations increase with increasing telescope diameter, but are reduced by central projection of the laser compared to side projection.

Quasi-static aberrations induced by laser guide stars in adaptive optics.

Laser Guide Star Adaptive Optics (LGS AO) systems use the return from an artificial guide star to measure the wavefront aberrations in the direction of the science object. We observe quasi-static differences between the measured wavefront and the wavefront aberration of the science object. This paper quantifies and explains the source of the difference between the wavefronts measured using an LGS and a natural guide star at the W. M. Keck Observatory, which can be as high as 1000 nm RMS.

Geographic control of Titan's mid-latitude clouds.

Observations of Titan's mid-latitude clouds from the W. M. Keck and Gemini Observatories show that they cluster near 350 degrees W longitude, 40 degrees S latitude. These clouds cannot be explained by a seasonal shift in global circulation and thus presumably reflect a mechanism on Titan such as geysering or cryovolcanism in this region. The rate of volatile release necessary to trigger cloud formation could easily supply enough methane to balance the loss to photolysis in the upper atmosphere.

A 5-micron-bright spot on Titan: evidence for surface diversity.

Observations from the Cassini Visual and Infrared Mapping Spectrometer show an anomalously bright spot on Titan located at 80 degrees W and 20 degrees S. This area is bright in reflected light at all observed wavelengths, but is most noticeable at 5 microns. The spot is associated with a surface albedo feature identified in images taken by the Cassini Imaging Science Subsystem. We discuss various hypotheses about the source of the spot, reaching the conclusion that the spot is probably due to variation in surface composition, perhaps associated with recent geophysical phenomena.

Direct detection of variable tropospheric clouds near Titan's south pole.

Atmospheric conditions on Saturn's largest satellite, Titan, allow the possibility that it could possess a methane condensation and precipitation cycle with many similarities to Earth's hydrological cycle. Detailed imaging studies of Titan have hitherto shown no direct evidence for tropospheric condensation clouds, although there has been indirect spectroscopic evidence for transient clouds. Here we report images and spectra of Titan that show clearly transient clouds, concentrated near the south pole, which is currently near the point of maximum solar heating. The discovery of these clouds demonstrates the existence of condensation and localized moist convection in Titan's atmosphere. Their location suggests that methane cloud formation is controlled seasonally by small variations in surface temperature, and that the clouds will move from the south to the north pole on a 15-year timescale.