6,009 research outputs found
Robustness of the Thirty Meter Telescope Primary Mirror Control System
The primary mirror control system for the Thirty Meter Telescope (TMT) maintains the alignment of the 492 segments in the presence of both quasi-static (gravity and thermal) and dynamic disturbances due to unsteady wind loads. The latter results in a desired control bandwidth of 1Hz at high spatial frequencies. The achievable bandwidth is limited by robustness to (i) uncertain telescope structural dynamics (control-structure interaction) and (ii) small perturbations in the ill-conditioned influence matrix that relates segment edge sensor response to actuator commands. Both of these effects are considered herein using models of TMT. The former is explored through multivariable sensitivity analysis on a reduced-order Zernike-basis representation of the structural dynamics. The interaction matrix ("A-matrix") uncertainty has been analyzed theoretically elsewhere, and is examined here for realistic amplitude perturbations due to segment and sensor installation errors, and gravity and thermal induced segment motion. The primary influence of A-matrix uncertainty is on the control of "focusmode"; this is the least observable mode, measurable only through the edge-sensor (gap-dependent) sensitivity to the dihedral angle between segments. Accurately estimating focus-mode will require updating the A-matrix as a function of the measured gap. A-matrix uncertainty also results in a higher gain-margin requirement for focus-mode, and hence the A-matrix and CSI robustness need to be understood simultaneously. Based on the robustness analysis, the desired 1 Hz bandwidth is achievable in the presence of uncertainty for all except the lowest spatial-frequency response patterns of the primary mirror
In-plane effects on segmented-mirror control
Extremely large optical telescopes are being designed with primary mirrors composed of hundreds of segments. The “out-of-plane” piston, tip, and tilt degrees of freedom of each segment are actively controlled using feedback from relative height measurements between neighboring segments. The “in-plane” segment translations and clocking (rotation) are not actively controlled; however, in-plane motions affect the active control problem in several important ways, and thus need to be considered. We extend earlier analyses by constructing the “full” interaction matrix that relates the height, gap, and shear motion at sensor locations to all six degrees of freedom of segment motion, and use this to consider three effects. First, in-plane segment clocking results in height discontinuities between neighboring segments that can lead to a global control system response. Second, knowledge of the in-plane motion is required both to compensate for this effect and to compensate for sensor installation errors, and thus, we next consider the estimation of in-plane motion and the associated noise propagation characteristics. In-plane motion can be accurately estimated using measurements of the gap between segments, but with one unobservable mode in which every segment clocks by an equal amount. Finally, we examine whether in-plane measurements (gap and/or shear) can be used to estimate out-of-plane segment motion; these measurements can improve the noise multiplier for the “focus-mode” of the segmented-mirror array, which involves pure dihedral angle changes between segments and is not observable with only height measurements
Observations of Hierarchical Solar-Type Multiple Star Systems
Twenty multiple stellar systems with solar-type primaries were observed at
high angular resolution using the PALM-3000 adaptive optics system at the 5 m
Hale telescope. The goal was to complement the knowledge of hierarchical
multiplicity in the solar neighborhood by confirming recent discoveries by the
visible Robo-AO system with new near-infrared observations with PALM-3000. The
physical status of most, but not all, of the new pairs is confirmed by
photometry in the Ks band and new positional measurements. In addition, we
resolved for the first time five close sub-systems: the known astrometric
binary in HIP 17129AB, companions to the primaries of HIP 33555, and HIP
118213, and the companions to the secondaries in HIP 25300 and HIP 101430. We
place the components on a color-magnitude diagram and discuss each multiple
system individually.Comment: Accepted to Astronomical Journa
Astrometric and Photometric Measurements of Binary Stars with Adaptive Optics: Observations from 2002
The adaptive optics system at the 3.6 m AEOS telescope was used to measure
the astrometry and differential magnitude in I-band of 56 binary stars in 2002.
The astrometric measurements will be of use for future orbital determination,
and the photometric measurements will be of use in estimating the spectral
types of the component stars. Two candidate companions were detected, but
neither is likely to be gravitationally bound. Nine systems had not been
observed in over 40 years. Eight of these are shown to share common proper
motion, while HD 182352 is shown to be a background star. One of the two
components of the HD 114378 (Alpha Com) is shown to be a variable star of
unknown type. In addition, 86 stars were unresolved and the full-width half
maxima of the images are presented.Comment: 9 pages, 1 figure, 3 Table
Know the Star, Know the Planet. III. Discovery of Late-Type Companions to Two Exoplanet Host Stars
We discuss two multiple star systems that host known exoplanets: HD 2638 and
30 Ari B. Adaptive optics imagery revealed an additional stellar companion to
both stars. We collected multi-epoch images of the systems with Robo-AO and the
PALM-3000 adaptive optics systems at Palomar Observatory and provide relative
photometry and astrometry. The astrometry indicates that the companions share
common proper motion with their respective primaries. Both of the new
companions have projected separations less than 30 AU from the exoplanet host
star. Using the projected separations to compute orbital periods of the new
stellar companions, HD 2638 has a period of 130 yrs and 30 Ari B has a period
of 80 years. Previous studies have shown that the true period is most likely
within a factor of three of these estimated values. The additional component to
the 30 Ari makes it the second confirmed quadruple system known to host an
exoplanet. HD 2638 hosts a hot Jupiter and the discovery of a new companion
strengthens the connection between hot Jupiters and binary stars. We place the
systems on a color-magnitude diagram and derive masses for the companions which
turn out to be roughly 0.5 solar mass stars.Comment: Accepted to Astronomical Journal, 16 pages, 5 Figure
Know the Star, Know the Planet. V. Characterization of the Stellar Companion to the Exoplanet Host HD 177830
HD 177830 is an evolved K0IV star with two known exoplanets. In addition to the planetary companions it has a late-type stellar companion discovered with adaptive optics imagery. We observed the binary star system with the PHARO near-IR camera and the Project 1640 coronagraph. Using
the Project 1640 coronagraph and integral field spectrograph we extracted a spectrum of the stellar
companion. This allowed us to determine that the spectral type of the stellar companion is a M4±1V.
We used both instruments to measure the astrometry of the binary system. Combining these data
with published data, we determined that the binary star has a likely period of approximately 800
years with a semi-major axis of 100-200 AU. This implies that the stellar companion has had little or
no impact on the dynamics of the exoplanets. The astrometry of the system should continue to be
monitored, but due to the slow nature of the system, observations can be made once every 5-10 years
A Creative Writing Case Study of Gender-Based Violence in Coach Education: Stacey’s Story
Creative nonfiction writing is the literary technique employed in this article to explore insights and assist our understanding of an “alleged” sexual assault in a sport coach education environment. Creative nonfiction employs various narrative tools: characters, setting, figurative language, sequences of events, plot, sub-plot, and dialogue, designed to render the sensitive and controversial elements of sexual assault significant. Readers are, therefore, invited to engage with Stacey’s Story and reflect on the actions of both the perpetrator(s) and the victim. While there are risks associated with the sharing of stories, especially those which are considered dangerous, it is envisaged that Stacey’s Story will be viewed as an opportunity to develop more critical responses and advance our understanding of gender-based violence in sport
Improved models of upper-level wind for several astronomical observatories
An understanding of wind speed and direction as a function of height are
critical to the proper modeling of atmospheric turbulence. We have used
radiosonde data from launch sites near significant astronomical observatories
and created mean profiles of wind speed and direction and have also computed
Richardson number profiles. Using data from the last 30 years, we extend the
1977 Greenwood wind profile to include parameters that show seasonal variations
and differences in location. The added information from our models is useful
for the design of adaptive optics systems and other imaging systems. Our
analysis of the Richardson number suggests that persistent turbulent layers may
be inferred when low values are present in our long term averaged data.
Knowledge of the presence of these layers may help with planning for adaptive
optics and laser communications.Comment: 21 pages, 15 Figures, 8 table
An ion gating, bunching, and potential re-referencing unit
A novel design to achieve the gating, bunching, and potential re-referencing of an ion beam, suitable for use in a photofragment spectrometer, is presented. The device simultaneously performs all three functions in a simple, compact, and easily aligned unit. It requires only a single digital signal and one high voltage supply for operation, and provides higher flux density than previous designs. The unit uses lensing to perform beam gating, an approach which has not been reported previously. The design does not require grids, and does not introduce divergence into the ion beam. Experimental results for the combined gating, bunching, and re-referencing unit are presented, and compared with modeled performance.This work is funded by a Major Equipment Grant
from the Australian National University. One of the authors
(E.H.R.) is the recipient of an Australian Postgraduate
Award
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