490 research outputs found
The Digital Zenith Camera - A New High-Precision and Economic Astrogeodetic Observation System for Real-Time Measurement of Deflections of the Vertical
During the last few years, new developments in the field of geodetic astronomy have been sparsely published. This might be due to the fact that the determination of deflections of the vertical still required relatively large efforts, both in time and in manpower, thus keeping the costs per point at a high level. Recently, the development of new high performance image sensors (CCD) at a reasonable price level enabled and initiated fundamental improvements in astrogeodetic observation instrumentations in terms of efficiency, automation, accuracy, and real-time capability. This promising development leads to a revitalization of astrogeodetic methods and offers very encouraging prospects for local high-precision astrogeodetic gravity field and geoid determinations.In this paper, two slightly different versions of the digital zenith camera, initially developed at the Institutfur Erdmessung, University of Hannover, are presented as high-precision state-of-the-art instruments. Using modern CCD technology for imaging stars and a GPS receiver, these systems allow the direct determination of the direction of the plumb line and thus its deflection from the ellipsoidal normal within a fully automated procedure in real-time. In addition to a description of the systemâs design and performance, the processing steps are presented: image data acquisition, data transfer and processing giving deflections of the vertical immediately after measurement
Status of Geodetic Astronomy at the Beginning of the 21st Century
At the beginning of the 21st century, a significant technological change took place in geodetic astronomy. The use of digital imaging sensors strongly improved the degree of automation, efficiency and accuracy of methods for the observation of the direction of the plumb line and its vertical deflection. This paper outlines the transition of astrogeodetic techniques and applications from the analogue to the digital era and addresses instrumental developments and recently completed projects. Particular attention is given to Digital Zenith Camera Systemsrepresenting astrogeodetic state-of-the-art instrumentation. Moreover, accuracy issues, present application examples for highly-precise astrogeodetic gravity field determinations and some future applications are described
Modern Determination of Vertical Deflections Using Digital Zenith Cameras
At the beginning of the 21st century, a significant technological change took place in geodetic astronomy. In Zurich and Hannover, digital zenith camera systems were developed based on digital imaging sensors (charge-coupled device) that strongly improved the degree of automation, efficiency, and accuracy of the observation of the direction of the plumb line and its vertical deflection. This paper describes the instrumental design of the new digital zenith camera systems and gives an overview of the data processing with focus on the models used for astrometric data reduction and tilt correction. Results of frequently repeated observations of vertical deflections and comparison measurements show an accuracy of vertical deflection measurements of better than 0.1 arc sec. Application examples for vertical deflection data from zenith camera observations, such as the high-precision local gravity field determination in engineering projects and gravity field validation are summarized
Recommended from our members
Comparative Study of Contact Repulsion in Control and Mutant Macrophages Using a Novel Interaction Detection
In this paper, a novel method for interaction detection is presented to compare the contact dynamics of macrophages in the Drosophila embryo. The study is carried out by a framework called macrosight, which analyses the movement and interaction of migrating macrophages. The framework incorporates a segmentation and tracking algorithm into analysing the motion characteristics of cells after contact. In this particular study, the interactions between cells is characterised in the case of control embryos and Shot mutants, a candidate protein that is hypothesised to regulate contact dynamics between migrating cells. Statistical significance between control and mutant cells was found when comparing the direction of motion after contact in specific conditions. Such discoveries provide insights for future developments in combining biological experiments with computational analysis
Radii and Distances of Cepheids, I., Method and Measurement Errors
We develop a formulation of the Baade-Wesselink method which uses the Fourier
coefficients of the observables. We derive an explicit, analytic expression to
determine the mean radius from each Fourier order. The simplicity of this
method allows us to derive the uncertainty in the mean radius due to
measurement errors.
Using simulations and a recent dataset we demonstrate that the precision of
the radius measurement with optical magnitudes is in most cases limited by the
accuracy of the measurement of the phase difference between the light and the
color index curve. In this case it is advantageous to determine the inverse
radius, because it has normal errors.Comment: 18 pages, postscript, accepted for publication in Ap
Atmospheric dynamics and the mass loss process in red supergiant stars
Red supergiant stars represent a key phase in the evolution of massive stars.
Recent radiative hydrodynamic simulations suggest that their atmospheres may be
the location of large-scale convective motions. As supergiant convection is
expected to generate supersonic motions and shocks, we seek constraints on
these atmospheric motions and their possible relation with mass-loss rates. We
present high-resolution, visible spectroscopy of a sample of red supergiants
(spectral type M I) and analyse them with a tomographic technique. We observe
steep velocity gradients, characterising both upward and downward supersonic
motions, which are time variable on time scales of a few hundred days. These
convective motions will generate turbulent pressure, which will strongly
decrease the effective gravity. We suggest that this decrease, combined with
radiative pressure on molecular lines, initiate the mass loss in red supergiant
stars.Comment: Accepted by Astronomy and Astrophysics. 12 pages, 14 figure
Velocity Dispersion of Dissolving OB Associations Affected by External Pressure of Formation Environment
This paper presents a possible way to understand dissolution of OB
associations (or groups). Assuming rapid escape of parental cloud gas from
associations, we show that the shadow of the formation environment for
associations can be partially imprinted on the velocity dispersion at their
dissolution. This conclusion is not surprising as long as associations are
formed in a multiphase interstellar medium, because the external pressure
should suppress expansion caused by the internal motion of the parental clouds.
Our model predicts a few km s as the internal velocity dispersion.
Observationally, the internal velocity dispersion is km s which
is smaller than our prediction. This suggests that the dissipation of internal
energy happens before the formation of OB associations.Comment: 6 pages. AJ accepte
Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud
For seven years, the EROS-2 project obtained a mass of photometric data on
variable stars. We present a peculiar Cepheid-like star, in the direction of
the Small Magellanic Cloud, which demonstrates unusual photometric behaviour
over a short time interval. We report on data of the photometry acquired by the
MARLY telescope and spectroscopy from the EFOSC instrument for this star,
called EROS2 J005135-714459(sm0060n13842), which resembles the unusual Cepheid
HR 7308. The light curve of our target is analysed using the Analysis of
Variance method to determine a pulsational period of 5.5675 days. A fit of
time-dependent Fourier coefficients is performed and a search for proper motion
is conducted. The light curve exhibits a previously unobserved and spectacular
change in both mean magnitude and amplitude, which has no clear theoretical
explanation. Our analysis of the spectrum implies a radial velocity of 104 km
s and a metallicity of -0.40.2 dex. In the direction of right
ascension, we measure a proper motion of 17.46.0 mas yr using EROS
astrometry, which is compatible with data from the NOMAD catalogue. The nature
of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may
have detected a non-zero proper motion for this star, which would imply that it
is a foreground object. Its radial velocity, pulsational characteristics, and
photometric data, however, suggest that it is instead a Cepheid-like object
located in the SMC. In such a case, it would present a challenge to
conventional Cepheid models.Comment: Correction of typos in the abstrac
The MACHO Project LMC Variable Star Inventory. VI. The Second-overtone Mode of Cepheid Pulsation From First/Second Overtone (FO/SO) Beat Cepheids
MACHO Project photometry of 45 LMC FO/SO beat Cepheids which pulsate in the
first and second overtone (FO and SOo, respectively) has been analysed to
determine the lightcurve characteristics for the SO mode of Cepheid pulsation.
We predict that singly-periodic SO Cepheids will have nearly sinusoidal
lightcurves; that we will only be able to discern SO Cepheids from fundamental
(F) and (FO) Cepheids for P <= 1.4 days; and that the SO distribution will
overlap the short-period edge of the LMC FO Cepheid period-luminosity relation
(when both are plotted as a function of photometric period).
We also report the discovery of one SO Cepheid candidate,
MACHO*05:03:39.670:04:32, with a photometric period of 0.775961 +/- 0.000019
days and an instrumental amplitude of 0.047 +/- 0.009 mag in V.Comment: 23 pages, 7 Encapsulated PostScript figures. Accepted for publication
in the Astrophysical Journa
- âŠ