Context. Absorption by molecules in the Earth’s atmosphere strongly affects ground-based astronomical observations. The resulting absorption line strength and shape depend on the highly variable ...physical state of the atmosphere, i.e. pressure, temperature, and mixing ratio of the different molecules involved. Usually, supplementary observations of so-called telluric standard stars (TSS) are needed to correct for this effect, which is expensive in terms of telescope time. We have developed the software package molecfit to provide synthetic transmission spectra based on parameters obtained by fitting narrow ranges of the observed spectra of scientific objects. These spectra are calculated by means of the radiative transfer code LBLRTM and an atmospheric model. In this way, the telluric absorption correction for suitable objects can be performed without any additional calibration observations of TSS. Aims. We evaluate the quality of the telluric absorption correction using molecfit with a set of archival ESO-VLT/X-Shooter visible and near-infrared spectra. Methods. Thanks to the wavelength coverage from the U to the K band, X-Shooter is well suited to investigate the quality of the telluric absorption correction with respect to the observing conditions, the instrumental set-up, input parameters of the code, the signal-to-noise of the input spectrum, and the atmospheric profiles. These investigations are based on two figures of merit, Ioff and Ires, that describe the systematic offsets and the remaining small-scale residuals of the corrections. We also compare the quality of the telluric absorption correction achieved with molecfit to the classical method based on a telluric standard star. Results. The evaluation of the telluric correction with molecfit shows a convincing removal of atmospheric absorption features. The comparison with the classical method reveals that molecfit performs better because it is not prone to the bad continuum reconstruction, noise, and intrinsic spectral features introduced by the telluric standard star. Conclusions. Fitted synthetic transmission spectra are an excellent alternative to the correction based on telluric standard stars. Moreover, molecfit offers wide flexibility for adaption to various instruments and observing sites.
Atmospheric water vapor is the main limiting factor of atmospheric transparency in the mm and submm wavelength spectral windows. Thus, dry sites are needed for the installation and successful ...operation of radio astronomy observatories exploiting those spectral windows. Temperature and wind are variables of special consideration when planning the installation and operation of large-aperture radio telescopes, as these parameters affect the mechanical response of radio telescopes exposed to the environmental conditions. Temperature, and in particular temperature gradients, induce thermal deformation of mechanical structures, while high wind speeds and gusts induce pointing jitter affecting the tracking accuracy of astronomical sources during the observations. This work summarizes the statistics of precipitable water vapor (PWV), temperature, and wind monitored at sites by the coastal mountain range, as well as on the west slope of the Andes mountain range in the region of Antofagasta, Chile. This information could prove useful for the planning of extended baselines for the Atacama Large Millimeter and Submillimeter Array (ALMA), and/or new radio telescope projects, such as the Atacama Large Aperture Submm/mm Telescope (AtLAST) initiative.
We report new observations of the infrared (IR) spectrum of low-current Th-Ar hollow cathode lamps with the 2 m Fourier transform spectrometer (FTS) at the National Institute of Standards and ...Technology. These observations establish more than 2400 lines that are suitable for use as wavelength standards in the range 900-4500 nm. The line list is used as input for a physical instrument model that provides the wavelength calibration for the Cryogenic High-Resolution IR Echelle Spectrometer (CRIRES), the European Southern Observatory's new high-resolution image IR spectrograph at the Very Large Telescope. We have also observed the variation of the spectrum of Th-Ar lamps as a function of operating current. The results allow us to optimize the spectral output in terms of relative intensity and line density for operation on the telescope. Our results should be generally useful for wavelength calibration in near-IR astronomy, providing a high density of sharp, well-characterized emission lines with the ease and efficiency of operation of a commercial discharge lamp.
Context.
Precise photometric and astrometric measurements on astronomical images require an accurate knowledge of the point spread function (PSF). When the PSF cannot be modelled directly from the ...image, PSF-reconstruction techniques become the only viable solution. So far, however, their performance on real observations has rarely been quantified.
Aims.
In this Letter, we test the performance of a novel hybrid technique, called PRIME, on Adaptive Optics-assisted SPHERE/ZIMPOL observations of the Galactic globular cluster NGC 6121.
Methods.
PRIME couples PSF-reconstruction techniques, based on control-loop data and direct image fitting performed on the only bright point-like source available in the field of view of the ZIMPOL exposures, with the aim of building the PSF model.
Results.
By exploiting this model, the magnitudes and positions of the stars in the field can be measured with an unprecedented precision, which surpasses that obtained by more standard methods by at least a factor of four for on-axis stars and by up to a factor of two on fainter, off-axis stars.
Conclusions.
Our results demonstrate the power of PRIME in recovering precise magnitudes and positions when the information directly coming from astronomical images is limited to only a few point-like sources and, thus, paving the way for a proper analysis of future Extremely Large Telescope observations of sparse stellar fields or individual extragalactic objects.
Aims.Based on a new set of sulphur abundances in very metal-poor stars and an improved analysis of previous data, we aim at resolving current discrepancies on the trend of S/Fe vs. Fe/H and thereby ...gain better insight into the nucleosynthesis of sulphur. The trends of Zn/Fe and S/Zn will also be studied. Methods.High resolution VLT/UVES spectra of 40 main-sequence stars with -3.3 < Fe/H < -1.0 are used to derive S abundances from the weak λ 8694.6 $\ion{S}{i}$ line and the stronger $\lambda \lambda$ 9212.9,9237.5 pair of $\ion{S}{i}$ lines. For one star, the S abundance is also derived from the $\ion{S}{i}$ triplet at 1.046 μm recently observed with the VLT infrared echelle spectrograph CRIRES. Fe and Zn abundances are derived from lines in the blue part of the UVES spectra, and effective temperatures are obtained from the profile of the Hβ line. Results.Comparison of sulphur abundances from the weak and strong $\ion{S}{i}$ lines provides important constraints on non-LTE effects. The high sulphur abundances reported by others for some metal-poor stars are not confirmed; instead, when taking non-LTE corrections into account, the Galactic halo stars distribute around a plateau at S/Fe ~ +0.2 dex with a scatter of 0.07 dex only. Zn/Fe is close to zero for metallicities in the range -2.0 < Fe/H < -1.0 but increases to a level of Zn/Fe ~ +0.1 to +0.2 dex in the range -2.7< Fe/H < -2.0. At still lower metallicities Zn/Fe rises steeply to a value of Zn/Fe ~ +0.5 dex at Fe/H = -3.2. Conclusions.The trend of S/Fe vs. Fe/H corresponds to the trends of Mg/Fe, Si/Fe, and Ca/Fe and indicates that sulphur in Galactic halo stars has been made by α-capture processes in massive SNe. The observed scatter in S/Fe is much smaller than predicted from current stochastic models of the chemical evolution of the early Galaxy, suggesting that either the models or the calculated yields of massive SNe should be revised. We also examine the behaviour of S/Zn and find that departures from the solar ratio are significantly reduced at all metallicities if non-LTE corrections to the abundances of these two elements are adopted. This effect, if confirmed, would reduce the usefulness of the S/Zn ratio as a diagnostic of past star-formation activity, but would bring closer together the values measured in damped Lyman-alpha systems and in Galactic stars.
Composites based on a steel with 40 vol% magnesia or titania were produced with the ceramics-derived extrusion and pressureless sintering. The materials were tested in a laboratory-scale fused-salt ...electrolysis cell with a synthetic cryolite in order to identify their potential use as electrode material in the Hall-Héroult process. The highly corrosive atmosphere and salt melt initiated certain corrosion effects in both material variants. The corrosion depth was determined with 1160 μm for magnesia and 463 μm for titania, respectively, after 8 h corrosion test. The initial corrosion includes the complete penetration of the specimens with cryolite and the dissolution of the ceramic component. A pre-oxidation of the specimens containing magnesia depressed the corrosion depth by 75% due to the formation of an aluminium oxide layer in the composite material during corrosion tests. The reduction in corrosion depth by pre-oxidation was less pronounced for the TiO2 composite materials (−15%).
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•Powder metallurgical processing of high alloyed CrNi-steel with alumina or tiania leading to different microstructures after sintering.•Detail investigation on the corrosion mechanism in steel/Al2O3 and steel/TiO2 composite variants after immersion test in molten cryolite.•Corrosion mechanism controlled by leaching of ceramic component in both variants with remaining metal structures.•Corrosion depth after pre-oxidation of both composite materials reduced.
The state-of-the-art magnesia-based dry vibrateable tundish linings are predominantly bonded by resin. However, besides high energy consumption, the pyrolysis of the resin-containing lining during ...the tundish heat-up process generates hazardous gases and carbon residues, resulting in an unwanted carbon pick by the molten steel after the finished ladle treatment.
Hence, the aim of this study was to develop novel carbon free binder system, therefore combine acid-based with thermally activated binder systems. Their influence on the physical and mechanical properties were investigated. Additionally, utilizing a unique steel casting simulator, the influence of newly developed lining compositions on the steel quality compared to a reference binder system was tested and analyzed. The influence on the inclusion population in the molten steel and the steel quality was evaluated.
Newly developed compositions showed no significant influence on the steel cleanliness, only a minor change in the population could be detected.
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In the fields of high-resolution metrology and manufacturing, effective anti-vibration measures are required to obtain precise and repeatable results. This is particularly true when the amplitudes of ...ambient vibration and the dimensions of the investigated or manufactured structure are comparable, e.g. in sub-micron semiconductor chip production, holographic interferometry, confocal optical imaging, and scanning probe microscopy. In the active anti-vibration system examined, signals are acquired by extremely sensitive vibration detectors, and the vibration is reduced using a feedback controller to drive electrodynamic actuators. This paper deals with the modeling and control of this anti-vibration system. First, a six-degree-of-freedom rigid body model of the system is developed. The unknown parameters of the unloaded system, including actuator transduction constants, spring stiffness, damping, moments of inertia, and the vertical position of the center of mass, are determined by comparing measured transfer functions to those calculated using the updated model. Finally, different strategies for actively controlling the vibration isolation system are considered.
We report on an episode of extremely low precipitable water vapour (PWV) of approximately 0.1 mm with a duration of more than 12 h at European Southern Observatory's Paranal observatory 2635 m above ...sea level (asl). Such conditions are more commonly expected at sites at much higher altitude such as ALMA on the Chajnantor plateau (5000 m asl) or otherwise particularly dry sites such as locations in Antarctica. We provide a full account of the measurements of PWV and other relevant atmospheric parameters. An explanation of the observed conditions is given in terms of the prevailing meteorological pattern. Based on statistical evidence from measurements by VLT spectrographs (UVES and CRIRES) covering more than a decade, we find that PWV <0.2 mm can be expected on less than 1 per cent of the nights, while <0.5 mm is encountered on 6-7 nights per year ( 2 per cent). The scientific potential of using this small but significant fraction of observing time is illustrated in the context of service mode observing.
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