arXiv subjects

Sebastian Schafer

Publications and source records attributed to Sebastian Schafer.

4 recordsLinked to original sources

arXiv2026

Two Fabry-Perots and two calibration units for CARMENES

The wavelength calibration and nightly drift measurements for CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs) are provided by a combination of hollow cathode lamps and two Fabry-P\'{e}rot units. CARMENES consists of two spectrograph, one for the visible part of the spectrum (520-960nm) and one for the near infrared (960-1710nm). Each spectrograph has its own calibration unit and its own Fabry-P\'{e}rot. The calibration units are equipped with Th-Ne, U-Ar and U-Ne hollow cathode lamps as well as a flat field lamp. The Fabry-P\'{e}rots are optimized for the wavelength ranges of the spectrographs and use halogen-tungsten lamps as light sources. The Fabry-P\'{e}rots have a free spectral range of 15GHz for the visible and 12.2GHz for the near infrared which translates to $\sim$17,900 useful emission lines for the visible spectrograph and $\sim$9,700 for the infrared. These lines are used to compute the wavelength solution, and to monitor the instrumental drift during the night. The Fabry-P\'{e}rot units are temperature and pressure stabilized and designed to reach an internal stability of better than 10\,cm/s per night. Here, we present the designs of both Fabry-P\'{e}rot units and the calibration units.

astro-ph.IM↗

arXiv2018

Magnetic Tunnel Junction Performance Under Mechanical Strain

In this work we investigate the effect of the mechanical stress on the performance of magnetic tunnel junctions (MTJ) with perpendicular magnetic anisotropy. We developed a 4-point bending setup, that allows us to apply a constant stress over a large substrate area with access to electrical measurements and external magnetic field. This setup enables us to measure key device performance parameters, such as tunnel magnetoresistance (TMR), switching current ($I_c^{50\%}$) and thermal stability ($Δ$), as a function of applied stress. We find that variations in these parameters are negligible: less than $\SI{2}{\percent}$ over the entire measured range between the zero stress condition and the maximum stress at the point of wafer breakage.

cond-mat.mes-hall↗

arXiv2015

An indirect transmission measurement-based spectrum estimation method for computed tomography

The characteristics of an x-ray spectrum can greatly influence imaging and related tasks. In practice, due to the pile-up effect of the detector, it's difficult to directly measure the spectrum of a CT scanner using an energy resolved detector. An alternative solution is to estimate the spectrum using transmission measurements with a step phantom or other CT phantom. In this work, we present a new spectrum estimation method based on indirect transmission measurement and model spectra mixture approach. The estimated x-ray spectrum was expressed as weighted summation of a set of model spectra, which can significantly reduce the degrees of freedom (DOF) of the spectrum estimation problem. Next, an estimated projection can be calculated with the assumed spectrum. By iteratively updating the unknown weights, we minimized the difference between the estimated projection data and the raw projection data. The final spectrum was calculated with these calibrated weights and the model spectra. Both simulation and experimental data were used to evaluate the proposed method. In the simulation study, the estimated spectra were compared to the raw spectra which were used to generate the raw projection data. For the experimental study, the ground truth measurement of the raw x-ray spectrum was not available. Therefore, the estimated spectrum was compared against spectra generated using the SpekCalc software with tube configurations provided by the scanner manufacturer. The results show the proposed method has potential to accurately estimate x-ray spectra using the raw projection data. The difference between the mean energy of the raw spectra and the mean energy of the estimated spectra were smaller than 0.5 keV for both simulation and experimental data. Further tests show the method was robust with respect to the model spectra generator.

physics.med-ph↗

arXiv2015

A patient-specific scatter artifacts correction method

This paper provides a fast and patient-specific scatter artifact correction method for cone-beam computed tomography (CBCT) used in image-guided interventional procedures. Due to increased irradiated volume of interest in CBCT imaging, scatter radiation has increased dramatically compared to 2D imaging, leading to a degradation of image quality. In this study, we propose a scatter artifact correction strategy using an analytical convolution-based model whose free parameters are estimated using a rough estimation of scatter profiles from the acquired cone-beam projections. It was evaluated using Monte Carlo simulations with both monochromatic and polychromatic X-ray sources. The results demonstrated that the proposed method significantly reduced the scatter-induced shading artifacts and recovered CT numbers.

physics.med-ph↗