Differential phase technique with the Keck Interferometer

We present the motivation and development of the novel `differential phase' technique being developed for the Keck Interferometer with the goal of detecting faint companions near a bright source. The differential phase technique uses simultaneous phase measurements at several infrared wavelengths to detect the astrophysical signature produced by a chromatic, asymmetric brightness distribution. We discuss the origin of the differential phase signature and present results of test observations taken at the Palomar Testbed Interferometer. One important test result is the larger than expected effect of water vapor turbulence on these multi-wavelength observations due to the infrared dispersion of water. In order to reach the design goal of 0.1 milliradians, the phase noise caused by both temperature and water vapor fluctuations in the atmosphere must be corrected, and we discuss several ways to achieve this

Binary star astronomy with optical interferometry

The Mark III Interferometer on Mt. Wilson, a long-baseline optical interferometer, was in daily operation for more that seven years. During that time it achieved milliarcsecond angular resolution for binary star astronomy, with submilliarcsecond accuracy. For the first time many spectroscopic binaries have been resolved, including binaries in which the companion cannot be detected with spectroscopy. The high angular resolution means that the traditional gap between visual and spectroscopic binaries has been decreased by more than an order of magnitude. In order to confirm the performance of the Mark III Interferometer, this paper uses the results of astronomical observations, and compares the Mark III Interferometer with other high-resolution techniques, including astrometry, lunar occultation, photometry, speckle, and spectroscopy. Comparisons for a variety of binary stars among these techniques indicate that long baseline optical interferometry proves a reliable, fully automatic, daily accessible astronomical capability for achieving high resolution, high accuracy, high dynamic range, and high photometric measurement precision for the study of binary stars

Narrow-Angle and Wide-Angle Astrometry via Long Baseline Optical/Infrared Interferometers

Long baseline optical/infrared interferometers, such as the Mark III Stellar Interferometer on Mt. Wilson and the ASEPS-0 Testbed Interferometer on Palomar Mountain, California, have good capabilities for narrow-angle and wide-angle astrometry with very high precision. Using the Mark III Interferometer many spectroscopic binaries became “visual” for the first time. The measurement accuracy of angular separation is 0.2 mas, the smallest separation measured between two components is 2 mas, the maximum magnitude difference is 4 mag, and the smallest semimajor axis is 4 mas. Such high angular resolution and dynamic range have been used to determine stellar masses with precision of 2% and differential stellar luminosities to better than 0.05 mag for separations of less than 0.″2. For some binary stars, not only have the systems been resolved, but also the diameter of the primary component has been determined, yielding direct measurements of stellar effective temperature with high accuracy. For parallax determination, the precision is 1 mas or better and is unaffected by interstellar extinction. For wide-angle astrometry with the Mark III interferometer, the observation results yielded average formal 1σ errors for FK5 stars of about 10 mas. Presently a new infrared interferometer, the ASEPS-0 Testbed Interferometer on Palomar Mountain is under construction, and is being optimized to perform high accuracy narrow-angle astrometry using long baseline observations at 2.2 μm, with phase referencing for increased sensitivity. The goal is to demonstrate differential astrometric accuracies of 0.06–0.1 mas in order to allow for detection of extra-solar planets in the near future

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. It is being developed by the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. 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 will use a 2-5 μm fringe detector in combination with a precision path length modulator and a vacuum delay line for dispersion control

Primary mirror dynamic disturbance models for TMT: vibration and wind

The principal dynamic disturbances acting on a telescope segmented primary mirror are unsteady wind pressure (turbulence) and narrowband vibration from rotating equipment. Understanding these disturbances is essential for the design of the segment support assembly (SSA), segment actuators, and primary mirror control system (M1CS). The wind disturbance is relatively low frequency, and is partially compensated by M1CS; the response depends on the control bandwidth and the quasi-static stiffness of the actuator and SSA. Equipment vibration is at frequencies higher than the M1CS bandwidth; the response depends on segment damping, and the proximity of segment support resonances to dominant vibration tones. We present here both disturbance models and parametric response. Wind modeling is informed by CFD and based on propagation of a von Karman pressure screen. The vibration model is informed by analysis of accelerometer and adaptive optics data from Keck. This information is extrapolated to TMT and applied to the telescope structural model to understand the response dependence on actuator design parameters in particular. Whether the vibration response or the wind response is larger depends on these design choices; "soft" (e.g. voice-coil) actuators provide better vibration reduction but require high servo bandwidth for wind rejection, while "hard" (e.g. piezo-electric) actuators provide good wind rejection but require damping to avoid excessive vibration transmission to the primary mirror segments. The results for both nominal and worst-case disturbances and design parameters are incorporated into the TMT actuator performance assessment

Servo design and analysis for the Thirty Meter Telescope primary mirror actuators

The Thirty Meter Telescope has 492 primary mirror segments, each incorporated into a Primary Segment Assembly (PSA), each of which in turn has three actuators that control piston, tip, and tilt, for a total of 1476 actuators. Each actuator has a servo loop that controls small motions (nanometers) and large motions (millimeters). Candidate actuators were designed and tested that fall into the categories of "hard" and "soft," depending on the offload spring stiffness relative to the PSA structural stiffness. Dynamics models for each type of actuator are presented, which respectively use piezo-electric transducers and voice coils. Servo design and analysis are presented that include assessments of stability, performance, robustness, and control structure interaction. The analysis is presented for a single PSA on a rigid base, and then using Zernike approximations the analysis is repeated for 492 mirror segments on a flexible mirror cell. Servo requirements include low-frequency stiffness, needed for wind rejection; reduced control structure interaction, specified by a bound on the sensitivity function; and mid-frequency damping, needed to reduce vibration transmission. The last of these requirements, vibration reduction, was found to be an important distinguishing characteristic for actuator selection. Hard actuators have little inherent damping, which is improved using PZT shunt circuits and force feedback, but still these improvements were found to result in less damping than is provided by the soft actuator. Results of the servo analysis were used for an actuator down-select study

Binary star astronomy with optical interferometry

The Mark III Interferometer on Mt. Wilson, a long-baseline optical interferometer, was in daily operation for more that seven years. During that time it achieved milliarcsecond angular resolution for binary star astronomy, with submilliarcsecond accuracy. For the first time many spectroscopic binaries have been resolved, including binaries in which the companion cannot be detected with spectroscopy. The high angular resolution means that the traditional gap between visual and spectroscopic binaries has been decreased by more than an order of magnitude. In order to confirm the performance of the Mark III Interferometer, this paper uses the results of astronomical observations, and compares the Mark III Interferometer with other high-resolution techniques, including astrometry, lunar occultation, photometry, speckle, and spectroscopy. Comparisons for a variety of binary stars among these techniques indicate that long baseline optical interferometry proves a reliable, fully automatic, daily accessible astronomical capability for achieving high resolution, high accuracy, high dynamic range, and high photometric measurement precision for the study of binary stars

Differential phase technique with the Keck Interferometer

We present the motivation and development of the novel `differential phase' technique being developed for the Keck Interferometer with the goal of detecting faint companions near a bright source. The differential phase technique uses simultaneous phase measurements at several infrared wavelengths to detect the astrophysical signature produced by a chromatic, asymmetric brightness distribution. We discuss the origin of the differential phase signature and present results of test observations taken at the Palomar Testbed Interferometer. One important test result is the larger than expected effect of water vapor turbulence on these multi-wavelength observations due to the infrared dispersion of water. In order to reach the design goal of 0.1 milliradians, the phase noise caused by both temperature and water vapor fluctuations in the atmosphere must be corrected, and we discuss several ways to achieve this

Astrometry with the Keck-Interferometer: the ASTRA project and its science

The sensitivity and astrometry upgrade ASTRA of the Keck Interferometer is introduced. After a brief overview of the underlying interferometric principles, the technology and concepts of the upgrade are presented. The interferometric dual-field technology of ASTRA will provide the KI with the means to observe two objects simultaneously, and measure the distance between them with a precision eventually better than 100 uas. This astrometric functionality of ASTRA will add a unique observing tool to fields of astrophysical research as diverse as exo-planetary kinematics, binary astrometry, and the investigation of stars accelerated by the massive black hole in the center of the Milky Way as discussed in this contribution.Comment: 22 pages, 10 figures (low resolution), contribution to the summerschool "Astrometry and Imaging with the Very Large Telescope Interferometer", 2 - 13 June, 2008, Keszthely, Hungary, corrected authorlis

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. It is being developed by the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. 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 will use a 2-5 μm fringe detector in combination with a precision path length modulator and a vacuum delay line for dispersion control