2,253 research outputs found
Sub-Hz line width diode lasers by stabilization to vibrationally and thermally compensated ULE Fabry-Perot cavities
We achieved a 0.5 Hz optical beat note line width with ~ 0.1 Hz/s frequency
drift at 972 nm between two external cavity diode lasers independently
stabilized to two vertically mounted Fabry-Perot (FP) reference cavities.
Vertical FP reference cavities are suspended in mid-plane such that the
influence of vertical vibrations to the mirror separation is significantly
suppressed. This makes the setup virtually immune for vertical vibrations that
are more difficult to isolate than the horizontal vibrations. To compensate for
thermal drifts the FP spacers are made from Ultra-Low-Expansion (ULE) glass
which possesses a zero linear expansion coefficient. A new design using Peltier
elements in vacuum allows operation at an optimal temperature where the
quadratic temperature expansion of the ULE could be eliminated as well. The
measured linear drift of such ULE FP cavity of 63 mHz/s was due to material
aging and the residual frequency fluctuations were less than 40 Hz during 16
hours of measurement. Some part of the temperature-caused drift is attributed
to the thermal expansion of the mirror coatings. High-frequency thermal
fluctuations that cause vibrations of the mirror surfaces limit the stability
of a well designed reference cavity. By comparing two similar laser systems we
obtain an Allan instability of 2*10-15 between 0.1 and 10 s averaging time,
which is close to the theoretical thermal noise limit.Comment: submitted to Applied Physics
TRACK-a new algorithm and open-source tool for the analysis of pursuit-tracking sensorimotor integration processes.
In daily life, sensorimotor integration processes are fundamental for many cognitive operations. The pursuit-tracking paradigm is an ecological and valid paradigm to examine sensorimotor integration processes in a more complex environment than many established tasks that assess simple motor responses. However, the analysis of pursuit-tracking performance is complicated, and parameters quantified to examine performance are sometimes ambiguous regarding their interpretation. We introduce an open-source algorithm (TRACK) to calculate a new tracking error metric, the spatial error, based on the identification of the intended target position for the respective cursor position. The identification is based on assigning cursor and target direction changes to each other as key events, based on the assumptions of similarity and proximity. By applying our algorithm to pursuit-tracking data, beyond replication of known effects such as learning or practice effects, we show a higher precision of the spatial tracking error, i.e., it fits our behavioral data better than the temporal tracking error and thus provides new insights and parameters for the investigation of pursuit-tracking behavior. Our work provides an important step towards fully utilizing the potential of pursuit-tracking tasks for research on sensorimotor integration processes. [Abstract copyright: © 2023. The Author(s).
The neurophysiology of continuous action monitoring.
Monitoring actions is essential for goal-directed behavior. However, as opposed to short-lasting, and regularly reinstating monitoring functions, the neural processes underlying continuous action monitoring are poorly understood. We investigate this using a pursuit-tracking paradigm. We show that beta band activity likely maintains the sensorimotor program, while theta and alpha bands probably support attentional sampling and information gating, respectively. Alpha and beta band activity are most relevant during the initial tracking period, when sensorimotor calibrations are most intense. Theta band shifts from parietal to frontal cortices throughout tracking, likely reflecting a shift in the functional relevance from attentional sampling to action monitoring. This study shows that resource allocation mechanisms in prefrontal areas and stimulus-response mapping processes in the parietal cortex are crucial for adapting sensorimotor processes. It fills a knowledge gap in understanding the neural processes underlying action monitoring and suggests new directions for examining sensorimotor integration in more naturalistic experiments. [Abstract copyright: © 2023 The Author(s).
Reconstruction of F2 layer peak electron density based on operational vertical total electron content maps
Observation and modeling of energetic particles at synchronous orbit on July 29, 1977
In the twelve hours following a worldwide storm, there was a series of at least four magnetospheric substorms, the last and largest of which exhibited an expansion phase onset at approximately 1200 UT. Data from six spacecraft in three general local time groupings (0300, 0700, and 1300 LT) were examined and vector magnetic field data and energetic electron and ion data from approximately 15 keV to 2 MeV were employed
Laser frequency comb techniques for precise astronomical spectroscopy
Precise astronomical spectroscopic analyses routinely assume that individual
pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons.
Intra-pixel sensitivity (IPS) variations may already cause small systematic
errors in, for example, studies of extra-solar planets via stellar radial
velocities and cosmological variability in fundamental constants via quasar
spectroscopy, but future experiments requiring velocity precisions approaching
~1 cm/s will be more strongly affected. Laser frequency combs have been shown
to provide highly precise wavelength calibration for astronomical
spectrographs, but here we show that they can also be used to measure IPS
variations in astronomical CCDs in situ. We successfully tested a laser
frequency comb system on the Ultra-High Resolution Facility spectrograph at the
Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded
in a single CCD exposure, we find that the average IPS deviates by <8 per cent
if it is assumed to vary symmetrically about the pixel centre. We also
demonstrate that series of comb exposures with absolutely known offsets between
them can yield tighter constraints on symmetric IPS variations from ~100
pixels. We discuss measurement of asymmetric IPS variations and absolute
wavelength calibration of astronomical spectrographs and CCDs using frequency
combs.Comment: 11 pages, 7 figures. Accepted for publication in MNRA
Gradual diffusion and punctuated phase space density enhancements of highly relativistic electrons: Van Allen Probes observations
Abstract The dual-spacecraft Van Allen Probes mission has provided a new window into mega electron volt (MeV) particle dynamics in the Earth\u27s radiation belts. Observations (up to E ~10 MeV) show clearly the behavior of the outer electron radiation belt at different timescales: months-long periods of gradual inward radial diffusive transport and weak loss being punctuated by dramatic flux changes driven by strong solar wind transient events. We present analysis of multi-MeV electron flux and phase space density (PSD) changes during March 2013 in the context of the first year of Van Allen Probes operation. This March period demonstrates the classic signatures both of inward radial diffusive energization and abrupt localized acceleration deep within the outer Van Allen zone (L ~4.0 ± 0.5). This reveals graphically that both competing mechanisms of multi-MeV electron energization are at play in the radiation belts, often acting almost concurrently or at least in rapid succession. Key Points Clear observations to higher energy than ever before Precise detection of where and how acceleration takes place Provides new eyes on megaelectron Volt
A new L-dwarf member of the moderately metal-poor triple system HD 221356
We report on the discovery of a fourth component in the HD 221356 star
system, previously known to be formed by an F8V, slightly metal-poor primary
([Fe/H]=-0.26), and a distant M8V+L3V pair. In our ongoing common proper motion
search based on VISTA Hemisphere Survey (VHS) and 2MASS catalogues, we have
detected a faint (J=13.76+/-0.04 mag) co-moving companion of the F8 star
located at angular separation of 12.13+/-0.18 arcsec (position angle of
221.8+/-1.7), corresponding to a projected distance of ~312 AU at 26 pc.
Near-infrared spectroscopy of the new companion, covering the 1.5-2.4 micron
wavelength range with a resolving power of R~600, indicates an L1+/-1 spectral
type. Using evolutionary models the mass of the new companion is estimated at
~0.08 solar masses, which places the object close to the stellar-substellar
borderline. This multiple system provides an interesting example of objects
with masses slightly above and below the hydrogen burning mass limit. The low
mass companions of HD 221356 have slightly bluer colours than field dwarfs with
similar spectral type, which is likely a consequence of the sub-solar
metallicity of the system.Comment: 7 pages, 4 figures, accepted for publication in MNRA
- …