Engineering topics

Akeson, Rachel L.

Publications and source records attributed to Akeson, Rachel L..

5 records

NASA NTRS2006

Nulling at the Keck Interferometer

The nulling mode of the Keck Interferometer is being commissioned at the Mauna Kea summit. The nuller combines the two Keck telescope apertures in a split-pupil mode to both cancel the on-axis starlight and to coherently detect the residual signal. The nuller, working at 10 um, is tightly integrated with the other interferometer subsystems including the fringe and angle trackers, the delay lines and laser metrology, and the real-time control system. Since first 10 um light in August 2004, the system integration is proceeding with increasing functionality and performance, leading to demonstration of a 100:1 on-sky null in 2005. That level of performance has now been extended to observations with longer coherent integration times. An overview of the overall system is presented, with emphasis on the observing sequence, phasing system, and differences with respect to the V2 system, along with a presentation of some recent engineering data.

interferometry↗

NASA NTRS2004

Keck Interferometer Science: Present and Future

The Keck Interferometer is a NASA funded project developed by the Jet Propulsion Laboratory, the William M. Keck Observatory and the Michelson Science Center at the California Institute of Technology. A technical description of the interferometer is given elsewhere in this volume. This paper will discuss the science topics and goals of the Keck Interferometer project, including a brief description of the Key Science projects, the science projects executed to date and the current availability of the interferometer for new projects. The Keck Interferometer Project consists of the Keck-Keck Interferometer, which combines the two Keck lo-meter telescopes on an 85-meter baseline, and the Outrigger Telescopes Project, a proposal to add four to six 1.8-meter telescopes that would work in conjunction with the two Kecks.

infrared interferometry↗

NASA NTRS2004

Keck Interferometer status and plans

Keck Interferometer is a NASA-funded project to combine the two 10 m Keck telescopes for high sensitivity near-infrared fringe visibility measurements, nulling interferometry at 10 µm to measure the quantity of exozodiacal emission around nearby stars, and differential-phase measurements to detect "hot-Jupiters" by their direct emission. Recent activity has included formal visibility mode commissioning, as well as science observations, and we briefly review some of the significant technical aspects and updates to the system. We have also completed laboratory development of the nuller. The nuller uses two modified Mach-Zehnder input nullers, a Michelson cross combiner, and a 10 µm array camera to produce background-limited null measurements. To provide required temporal stability for the nuller, the system incorporates end-to-end laser metrology with phase referencing from two 2.2 µm fringe trackers. The nuller recently completed its pre-ship review and is being installed on the summit. After nuller integration and test, the differential phase mode will be deployed, which uses a 2-5 µm fringe detector in combination with a precision path length modulator and a vacuum delay line for dispersion control.

Akeson, Rachel L.↗

NASA NTRS2001

Polarizing Grids, their Assemblies and Beams of Radiation

This article gives an analysis of the behavior of polarizing grids and reflecting polarizers by solving Maxwell's equations, for arbitrary angles of incidence and grid rotation, for cases where the excitation is provided by an incident plane wave or a beam of radiation. The scattering and impedance matrix representations are derived and used to solve more complicated configurations of grid assemblies. The results are also compared with data obtained in the calibration of reflecting polarizers at the Owens Valley Radio Observatory (OVRO). From these analysis, we propose a method for choosing the optimum grid parameters (wire radius and spacing). We also provide a study of the effects of two types of errors (in wire separation and radius size) that can be introduced in the fabrication of a grid.

Houde, Martin↗

NASA NTRS1995

CO and OH absorption toward the low-latitude radio source 2023+336

We report interferometric observations of (12)CO (0 to 1) 115 GHz and OH 1667 MHz absorption toward the extragalactic radio continuum source 2023+336. Our data show two partially blended absorption features near +2 km/s LSR which probably originate in local molecular clouds in the Cygnus rift, approximately 0.7 kpc away. The CO lines are optically thin, with tau(sub CO) = 0.5 +/- 0.1, while tau(sub OH) = 0.030 +/- 0.004. The detected absorption features have a combined velocity width of 3.0 km/s FWHM, greater than the width of the corresponding (12)CO emission features, Full Width of Half Maximum (FWHM) = 2.3 km/s, indicating that the absorbing gas in a approximately 1 AU diameter column shows a greater velocity range than the gas in the 0.2 pc diameter emission beam. CO excitation temperatures T(sub ex), range from 2.7 to 7.0 K across the line profile, implying that the gas along this line of sight is rotationally quite cold. By measuring the (12)CO excitation temperature and optical depth, we derive an abundance ratio, (CO)/(H2), of 6 x 10(exp -6), which is more than an order of magnitude lower than is seen elsewhere. Our low observed CO optical depth is inconsistent with the ratio of (12)CO to (13)CO emission line integrals, 16:1, which implies that the (12)CO is optically thick. As a solution to these anomalies, we propose that the CO absorption and emission arise from preferentially different parts of the molecular medium. Other solutions would require an anomalous (12)CO/(13)CO abundance ratio and an anomalous CO/H2 abundance ratio, or much lower excitation temperatures. Since we find no absorption at negative velocities with tau greater than 0.23 (2.5 sigma), there is no compelling evidence for the existence of an extensive population of cold molecular gas in the outer Galaxy along this line of sight.

Kobulnicky, Henry A.↗