186,304 research outputs found
A systematic approach to determining the properties of an iodine absorption cell for high-precision radial velocity measurements
No full textAbsorption cells filled with diatomic iodine are frequently employed as wavelength reference
for high-precision stellar radial velocity determination due to their long-term stability and
low cost. Despite their wide-spread usage in the community, there is little documentation on
how to determine the ideal operating temperature of an individual cell. We have developed a
new approach to measuring the effective molecular temperature inside a gas absorption cell
and searching for effects detrimental to a high-precision wavelength reference, utilizing the
Boltzmann distribution of relative line depths within absorption bands of single vibrational
transitions. With a high-resolution Fourier transform spectrometer, we took a series of 632
spectra at temperatures between 23 and 66◦C. These spectra provide a sufficient basis to test the
algorithm and demonstrate the stability and repeatability of the temperature determination via
molecular lines on a single iodine absorption cell. The achievable radial velocity precision σ RV
is found to be independent of the cell temperature and a detailed analysis shows a wavelength
dependence, which originates in the resolving power of the spectrometer in use and the signal-
to-noise ratio. Two effects were found to cause apparent absolute shifts in radial velocity, a
temperature-induced shift of the order of ∼1 m s−1 K−1 and a more significant effect resulting
in abrupt jumps of ≥50 m s−1 is determined to be caused by the temperature crossing the dew
point of the molecular iodin
Characterization and calibration of a Fourier-transform spectrometer using a laser frequency comb
No full textWe have used a laser frequency comb with a repetition frequency of rep≈1 GHz to measure the drift and dispersion of a Fourier-transform spectrometer (FTS). We used the electronic measurements of rep and CEO to create a reference line list. We measured 28 interferograms and computed the phase and power spectra. The analysis of the interferograms and phase spectra allowed for compensation of several spectroscopic artifacts. In the computed power spectra, we detected ∼64.000 suitable lines in the near-infrared bandwidth Δ=308.79–374.74 THz. The residual dispersion of the FTS can be described by two factors, a linear dispersion and a constant offset. Both are highly correlated and need to be computed simultaneously. The factors were computed from the comparison of a reference with measured line lists. The linear dispersion factor is found to be varying on the order of 10−8 Hz/Hz, while the constant offset is of the order of 107 Hz. Using two factors for calibration, the difference between the reference and the measured line list can be removed completely with an uncertainty of ∼65 kHz corresponding to a precision of 0.5·10−9 Hz/Hz
Radial velocity observations of the 2015 Mar. 20 eclipse A benchmark Rossiter-McLaughlin curve with zero free parameters
No full textSpectroscopic observations of a solar eclipse can provide unique information for solar
and exoplanet research; the huge amplitude of the Rossiter-McLaughlin (RM) effect during
solar eclipse and the high precision of solar radial velocities (RVs) allow detailed
comparison between observations and RV models, and they provide information about the
solar surface and about spectral line formation that are otherwise difficult to obtain. On
March 20, 2015, we obtained 159 spectra of the Sun as a star with the solar telescope and
the Fourier Transform Spectrograph at the Institut für Astrophysik Göttingen, 76 spectra
were taken during partial solar eclipse. We obtained RVs using I2 as wavelength
reference and determined the RM curve with a peak-to-peak amplitude of almost
1.4 km s-1 at
typical RV precision better than 1 m s-1. We modeled the disk-integrated solar RVs using
well-determined parameterizations of solar surface velocities, limb darkening, and
information about convective blueshift from 3D magnetohydrodynamic simulations. We confirm
that convective blueshift is crucial to understand solar RVs during eclipse. Our best
model reproduced the observations to within a relative precision of 10% with residuals
lower than 30 m s-1. We cross-checked parameterizations of velocity fields
using a Dopplergram from the Solar Dynamics Observatory and conclude that disk-integration
of the Dopplergram does not provide correct information about convective blueshift
necessary for m s-1
RV work. As main limitation for modeling RVs during eclipses, we identified limited
knowledge about convective blueshift and line shape as functions of solar limb angle. We
suspect that our model line profiles are too shallow at limb angles larger than
μ =
0.6, resulting in incorrect weighting of the
velocities across the solar disk. Alternative explanations cannot be excluded, such as
suppression of convection in magnetic areas and undiscovered systematics during eclipse
observations. To make progress, accurate observations of solar line profiles across the
solar disk are suggested. We publish our RVs taken during solar eclipse as a benchmark
curve for codes calculating the RM effect and for models of solar surface velocities and
line profiles
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Non-Destructive Testing (NDT) methods used for the study of stone frescoes
No full textThe value of cultural heritage objects is very high, requiring an observation technique that is safe, highly
efficient, and sensitive enough to detect defects that occur in these objects. A combination of non-destructive
testing (NDT) methods that meet these requirements increases the probability of detection (POD) and reduces
the chance of false detection (false positives or false negatives).
In this work, the applicability of optical, infrared and ultrasonic techniques, originally developed for industry, in
the architectural restoration field is examined. As an example, we investigate a brick basis covered with two
plaster layers containing artificial defects (crack and detachment) hidden in the sample. Squares of different
colours have been painted with the last layer comparable to a real fresco, constituting a challenge for
thermographic measurements (e.g., in pulse thermography, data is affected by non-uniform heating and local
variation of thermal emission). The advantages and disadvantages of Holographic Interferometry (Double-
Exposure – DE –), Digital Speckle Photography (DSP), Shearography, Transient Thermal Thermography and
Ultrasonic Testing are discussed with respect to their ability to detect the previously described defects
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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