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W Kausch - One of the best experts on this subject based on the ideXlab platform.

  • molecfit a general tool for telluric absorption correction ii quantitative evaluation on eso vlt x shooterspectra
    Astronomy and Astrophysics, 2015
    Co-Authors: W Kausch, S Kimeswenger, M Barden, C Szyszka, A M Jones, S Noll, A Smette, H Sana
    Abstract:

    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, I off and I res , 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.

  • molecfit a general tool for telluric absorption correction ii quantitative evaluation on eso vlt x shooter spectra
    arXiv: Instrumentation and Methods for Astrophysics, 2015
    Co-Authors: W Kausch, Noll S A Smette, S Kimeswenger, M Barden, C Szyszka, A M Jones, H Sana, H Horst, F Kerber
    Abstract:

    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, I_off and I_res, 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 moelcfit to the classical method based on a telluric standard star. (Abridged)

D. Mitchell Wilkes - One of the best experts on this subject based on the ideXlab platform.

  • Multiwavelet prefilters. II. Optimal orthogonal prefilters
    IEEE Transactions on Image Processing, 2001
    Co-Authors: Kitti Attakitmongcol, Douglas P. Hardin, D. Mitchell Wilkes
    Abstract:

    For pt.I see IEEE Trans. Circuits Syst. II, vol.45, p.1106-12 (1998). Prefiltering a given discrete signal has been shown to be an essential and necessary step in applications using unbalanced multiwavelets. In this paper, we develop two methods to obtain optimal second-order approximation preserving prefilters for a given orthogonal multiwavelet basis. These procedures use the prefilter construction introduced in Hardin and Roach (1998). The first prefilter optimization scheme exploits the Taylor series expansion of the prefilter combined with the multiwavelet. The second one is achieved by minimizing the energy compaction ratio (ECR) of the wavelet coefficients for an experimentally determined average Input Spectrum. We use both methods to find prefilters for the cases of the DGHM and Chui-Lian (CL) multiwavelets. We then compare experimental results using these filters in an image compression scheme. Additionally, using the DGHM multiwavelet with the optimal prefilters from the first scheme, we find that quadratic Input signals are annihilated by the high-pass portion of the filter bank at the first level of decomposition.

  • Multiwavelet Prefilters II: Optimal orthogonal prefilters
    1999
    Co-Authors: Kitti Attakitmongcol, Douglas P. Hardin, D. Mitchell Wilkes
    Abstract:

    Prefiltering a given discrete signal has been shown to be an essential and necessary step in applications using unbalanced multiwavelets. In this paper, we develop 2 methods to obtain optimal 2nd-order approximation preserving prefilters for a given orthogonal multiwavelet basis. These procedures use the prefilter construction introduced in [4]. The first prefilter optimization scheme exploits the Taylor series expansion of the prefilter combined with the multiwavelet. The second one is achieved by minimizing the energy compaction ratio of the wavelet coefficients for an experimentally determined average Input Spectrum. We use both methods to find prefilters for the cases of the DGHM and Chui-Lian multiwavelets. We then compare experimental results using these filters in an image compression scheme. Additionally, using the DGHM multiwavelet with the optimal prefilters from the first scheme, we find that quadratic Input signals are annihilated by the high-pass portion of filter bank at the first level of decomposition

H Sana - One of the best experts on this subject based on the ideXlab platform.

  • molecfit a general tool for telluric absorption correction ii quantitative evaluation on eso vlt x shooterspectra
    Astronomy and Astrophysics, 2015
    Co-Authors: W Kausch, S Kimeswenger, M Barden, C Szyszka, A M Jones, S Noll, A Smette, H Sana
    Abstract:

    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, I off and I res , 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.

  • molecfit a general tool for telluric absorption correction ii quantitative evaluation on eso vlt x shooter spectra
    arXiv: Instrumentation and Methods for Astrophysics, 2015
    Co-Authors: W Kausch, Noll S A Smette, S Kimeswenger, M Barden, C Szyszka, A M Jones, H Sana, H Horst, F Kerber
    Abstract:

    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, I_off and I_res, 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 moelcfit to the classical method based on a telluric standard star. (Abridged)

S Kimeswenger - One of the best experts on this subject based on the ideXlab platform.

  • molecfit a general tool for telluric absorption correction ii quantitative evaluation on eso vlt x shooterspectra
    Astronomy and Astrophysics, 2015
    Co-Authors: W Kausch, S Kimeswenger, M Barden, C Szyszka, A M Jones, S Noll, A Smette, H Sana
    Abstract:

    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, I off and I res , 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.

  • molecfit a general tool for telluric absorption correction ii quantitative evaluation on eso vlt x shooter spectra
    arXiv: Instrumentation and Methods for Astrophysics, 2015
    Co-Authors: W Kausch, Noll S A Smette, S Kimeswenger, M Barden, C Szyszka, A M Jones, H Sana, H Horst, F Kerber
    Abstract:

    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, I_off and I_res, 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 moelcfit to the classical method based on a telluric standard star. (Abridged)

S Hasselmann - One of the best experts on this subject based on the ideXlab platform.

  • an improved algorithm for the retrieval of ocean wave spectra from synthetic aperture radar image spectra
    Journal of Geophysical Research, 1996
    Co-Authors: S Hasselmann, K Hasselmann, C Bruning, Patrick Heimbach
    Abstract:

    An earlier algorithm for retrieving two-dimensional wave spectra from synthetic aperture radar (SAR) image spectra is improved by using a modified cost function and introducing an additional iteration loop in which the first-guess Input Spectrum is systematically updated. For this purpose a spectral partitioning scheme is applied in which the Spectrum is decomposed into a finite number of distinct wave systems. At each iteration step, the individual wave systems of the partitioned nth-guess wave Spectrum are adjusted to agree in mean energy, frequency, and direction with the corresponding mean values of the associated wave systems of the SAR-inverted wave Spectrum. The algorithm retrieves smooth wave spectra, avoiding the discontinuities which tended to arise in the previous algorithm in the transition region near the azimuthal wavenumber cutoff of the SAR image Spectrum. The azimuthal cutoff of the SAR Spectrum is also reproduced more accurately. The greatest improvement of the new retrieval algorithm is obtained when the discrepancies between the initial first-guess wave Spectrum and the observed SAR Spectrum are large. In this case the additional updating loop for the Input Spectrum enables the retrieved Spectrum to adjust such that the simulated SAR Spectrum matches more closely the observed SAR Spectrum. The overall correlation of a large set of simulated SAR spectra with the measured SAR spectra is found to be significantly higher than with the previous algorithm, indicating that the algorithm not only overcomes isolated shortcomings of the earlier algorithm but also yields retrieved wave spectra which are generally more consistent with the Input SAR data. An additional practical advantage of the new algorithm is that it returns spectral partioning parameters which can be used in SAR wave data assimilation schemes.

  • on the nonlinear mapping of an ocean wave Spectrum into a synthetic aperture radar image Spectrum and its inversion
    Journal of Geophysical Research, 1991
    Co-Authors: K Hasselmann, S Hasselmann
    Abstract:

    A new, closed nonlinear integral transformation relation is derived describing the mapping of a two-dimensional ocean wave Spectrum into a synthetic aperture radar (SAR) image Spectrum. The general integral relation is expanded in a power series with respect to orders of nonlinearity and velocity bunching. The individual terms of the series can be readily computed using fast Fourier transforms. The convergence of the series is rapid. The series expansion is also useful in identifying the different contributions to the net imaging process, consisting of the real aperture radar (RAR) cross-section modulation, the nonlinear motion (velocity bunching) effects, and their various interaction products. The lowest term of the expansion with respect to nonlinearity order yields a simple quasi-linear approximate mapping relation consisting of the standard linear SAR modulation expression multiplied by an additional nonlinear Gaussian azimuthal cutoff factor. The cutoff scale is given by the rms azimuthal (velocity bunching) displacement. The same cutoff factor applies to all terms of the power series expansion. The nonlinear mapping relation is inverted using a standard first-guess wave Spectrum as regularization term. This is needed to overcome the basic 180° mapping ambiguity and the loss of information beyond the azimuthal cutoff. The inversion is solved numerically using an iteration technique based on the successive application of the explicit solution for the quasi-linear mapping approximation, with interposed corrections invoking the full nonlinear mapping expression. A straightforward application of this technique, however, generally yields unrealistic discontinuities of the best fit wave Spectrum in the transition region separating the low azimuthal wave number domain, in which useful SAR information is available and the wave Spectrum is modified, from the high azimuthal wave number region beyond the azimuthal cutoff, where the first-guess wave Spectrum is retained. This difficulty is overcome by applying a two-step inversion procedure. In the first step the energy level of the wave Spectrum is adjusted, and the wave number plane rotated and rescaled, without altering the shape of the Spectrum. Using the resulting globally fitted Spectrum as the new first-guess Input Spectrum, the original inversion method is then applied without further constraints in a second step to obtain a final fine-scale optimized Spectrum. The forward mapping relation and inversion algorithms are illustrated for three Seasat cases representing different wave conditions corresponding to weakly, moderately, and strongly nonlinear imaging conditions.