1,720,987 research outputs found
Phase-correction algorithm for Fourier transform spectroscopy of a laser frequency comb
No full textFourier-transform spectrographs (FTS) are among the most important tools for high-resolution spectroscopy over a broad spectral bandwidth. Usually, the frequency axis of an FTS is calibrated with relatively few atomic lines and an absolute wavelength reference, which is often a stabilized He–Ne laser. Normally, the phase-spectrum is measured using a continuous light source to enable phase correction. Laser frequency combs (LFC) provide a much higher stability. Their spectrum consists of closely spaced narrow lines, which are very well suited for the characterization and calibration of an FTS. Due to the pulsed nature of the LFC, however, the phase spectrum cannot be measured in the same way as for continuous light sources. We show how a proper phase spectrum from an FTS measurement of an LFC can be obtained and how the strongly varying phase spectrum noise can be filtered. We analyzed a narrow spectral band 10.200–12.500 cm−1 in which we detected ∼60.000 lines with sufficient intensity. Only with an accurate truncation of the interferogram and a proper shifting, the complex structure of the phase spectrum is revealed. For phase filtering, we adapted Mertz’s algorithm and show how the instrumental line shape is significantly improved
Spectral envelope control for a flat frequency comb spectrum
No full textLaser frequency combs have properties which make them promising spectrograph calibration light sources. One drawback for this application is the high dynamic range in the supercontinuum spectra of some frequency combs. We aim to flatten the spectrum of a Ti:sapphire laser frequency comb to improve the calibration performance for a Fourier transform spectrograph. For this, we develop a compact Fourier transform optical pulse shaping setup, which enables control of the spectral envelope via dispersion of the light onto a spatial light modulator. We demonstrate, that this setup allows us to flatten the comb spectrum from a dynamic range of 20 dB to less than 6 dB in the wavelength range 739-939 nm. For 86 % of the wavelength range, the dynamic range is below 1 dB
Data analysis methods for laser frequency comb line position measurements with a Fourier transform spectrograph
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Validation of the ESPRESSO wavelength calibration using iodine absorption cell spectra
No full textABSTRACT High-quality wavelength calibration is crucial for science cases like radial-velocity studies of exoplanets, the search for a possible variation of fundamental constants, and the redshift drift experiment. However, for state-of-the-art spectrographs, it has become difficult to verify the wavelength calibration on sky because no astrophysical source provides spectra with sufficiently stable or accurate wavelength information. We therefore propose to use iodine absorption cells to validate the wavelength calibration. Observing a bright and featureless star through the iodine cell emulates an astrophysical target with exactly known spectral features that can be analysed like any other science target, allowing to verify the wavelength calibration derived from the internal calibration sources and to identify systematics in the data processing. As demonstration, we temporarily installed an absorption cell at ESPRESSO. Employing a full forward modelling approach of the spectrum, including the instrumental line-spread function, we demonstrate wavelength calibration accuracy at the level of a few . We also show that wavelength measurements do depend on the geometry of the light-injection into the spectrograph fibers. This highlights the importance of probing exactly the same light path as science targets, something not possible with internal calibration sources alone. We also demonstrate excellent radial-velocity stability at the {{< 20\, \mathrm{cm\,s^{-1}}}} level in a full end-to-end fashion, from sky to data product. Our study therefore showcases the great potential of absorption cells for the verification and long-term monitoring of the wavelength calibration as well as the unique insights they can provide
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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