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Astrometry and photometry with wide-field imagers
Full text linkSeveral projects in astronomy require detectors with a high number of resolution elements on the sky. For this reason, many observatories equipped their telescopes with a particular kind of detectors, the so-called wide-field imagers, that fulfil this requirement. In this thesis we show that, with a careful data analysis based on an accurate point-spread-function (PSF) modelling and geometric-distortion correction, it is possible to obtain high astrometric and photometric accuracy over wide field of views (FoVs). The prescriptions discussed in the thesis can be extended to the most of the detectors that are or will be placed at ground and space observatories.
In the first part of the thesis, we mainly focus on detectors working in the near-infrared (NIR) regime. We want to concentrate on NIR imaging because of the increasing interest of the astronomical community for this wavelength range, which will be the baseline for JWST. In the second part of the thesis we also present a couple of applications to optical wide-field imagers (LBC@LBT and the CCD mounted at the Asiago Schmidt telescope).
We start by investigating the astrometric and photometric performance of the NIR wide-field imager HAWK-I@VLT. We adapt to HAWK-I the techniques originally developed for the Hubble Space Telescope (HST) imagers to obtain high-precision astrometry and photometry, and then adapted to the ground-based, wide-field imager mounted at the 2.2-m MPI/ESO telescope. We accurately model HAWK-I PSFs and solve for the geometric distortion of the detector. With this careful data reduction, we are able to reach an astrometric accuracy of a few milliarcsec (mas) over the entire FoV of the instrument. Together with the distortion-correction package, we also create astro-photometric catalogues of seven fields (four stellar clusters, two extragalactic fields and one region toward the Galactic centre). Furthermore, to test the reached astrometric accuracy, we compute relative proper motions for stars in two globular clusters (M 22 and M 4) and successfully separate cluster members from background/foreground objects. Proper-motion-selected colour-magnitude diagrams of the globular cluster M 22 allow us to study its multiple stellar populations, finding that the two stellar populations hosted in the sub-giant branches of M 22 have the same radial distribution from the cluster centre out to 9 arcmin, within our uncertainty.
We then move to the wide-field imager VIRCAM@VISTA. Again, we export the tools made for HAWK-I to this imager and solve for the distortion of the detector. The geometric-distortion correction of VIRCAM is quite challenging because the tangential-plane projection effects are not negligible over a FoV larger than 1 sq. deg on the sky. We start by using the 2MASS catalogue as reference frame, and then we auto-calibrate the geometric distortion as done for the HAWK-I detector. This way, we are able to correct the geometric distortion of VIRCAM to an astrometric accuracy of about 8 mas. Finally, we use the `VISTA Variables in the Vía Láctea' (VVV) data to compute M 22 globular-cluster relative proper motions. VVV observations are not designed to such high-astrometric-accuracy purposes, but, with our tools, we are able to reach a proper-motion precision of ~1.4 mas/yr and separate cluster and field stars, as well as to measure the motion difference between Galactic bulge and disc stars toward the direction of M 22.
In the last part of the thesis we describe our new project, focused to exploit the data coming from the planet-hunting K2 mission, the re-designed Kepler mission after the problems that occurred to its spacecraft. The analysis of crowded environments using K2 data can be very complex with classical (aperture-based) photometric approaches. Our method is instead specifically developed to deal with these particular regions (stellar clusters and toward the Galactic centre) and its key ingredients are PSF astrometry and photometry, high-angular-resolution input catalogues and PSF-based neighbour subtraction.
We first address the problem of the K2 undersampled PSFs which fine structures, if not properly modelled, can introduce systematic errors that worsen both astrometry and photometry. To this aim, we follow the iterative method made to model HST undersampled PSFs. We then use a high-angular-resolution, ground-based catalogue to identify all detectable objects in the field and, for each of them, we measure their flux after we subtract all close-by neighbours. This way, we increase the number of measurable sources in the field and obtain a more reliable estimate of their flux. Most importantly, for variable stars, eclipsing binaries and exoplanet candidates, this method leads to a more reliable value of the true amplitude or eclipse/transit depth of their light curves because we reduce light-dilution effects. This is particularly interesting for exoplanets, as the true radius of the transiting objects would be otherwise under-estimated.
We apply this method to the first K2 Campaign that covered two open clusters (M 35 and NGC 2158) and extract about 50000 light curves from only one channel of K2. This number is more than two times the total number of sources usually analysed over the entire FoV (76 channels) in a typical K2 Campaign. For bright stars, we reach a photometric precision of ~30 parts per million, comparable with other works in the literature focused on isolated stars. At the faint end, we measure objects down to 5 magnitudes fainter than in any published work with K2 data so far, and show that the PSF photometry performs better than aperture photometry in this magnitude regime. The improvement here described is more significant in crowded regions. Within the field of these two clusters we also find more than 2000 variable stars.
All the projects developed during this thesis have also interesting long-term perspectives, since they can be seen as complementary or in preparation to future space-based missions like TESS and PLATO. Candidate exoplanets from Kepler/K2 (but also from the forthcoming TESS) observations can potentially be interesting targets for JWST, and then E-EL
High-precision astrometry with VVV - I. An independent reduction pipeline for VIRCAM@VISTA
No full textA PSF-based approach to Kepler/K2 data - I. Variability within the K2 Campaign 0 star clusters M 35 and NGC 2158
No full textKepler and K2 data analysis reported in the literature is mostly based on aperture photometry. Because of Kepler's large, undersampled pixels and the presence of nearby sources, aperture photometry is not always the ideal way to obtain high-precision photometry, and, because of this, the data set has not been fully exploited so far. We present a new method that builds on our experience with undersampled HST images. The method involves a point-spread function (PSF) neighbour-subtraction and was specifically developed to exploit the huge potential offered by the K2 ‘super-stamps’ covering the core of dense star clusters. Our test-bed targets were the NGC 2158 and M 35 regions observed during the K2 Campaign 0. We present our PSF modelling and demonstrate that, by using a high-angular-resolution input star list from the Asiago Schmidt telescope as the basis for PSF neighbour subtraction, we are able to reach magnitudes as faint as KP ≃ 24 with a photometric precision of 10 per cent over 6.5 h, even in the densest regions. At the bright end, our photometric precision reaches ∼30 parts per million. Our method leads to a considerable level of improvement at the faint magnitudes (KP ≳ 15.5) with respect to the classical aperture photometry. This improvement is more significant in crowded regions. We also extracted raw light curves of ∼60 000 stars and detrended them for systematic effects induced by spacecraft motion and other artefacts that harms K2 photometric precision. We present a list of 2133 variables
A PSF-based approach to Kepler/K2data – III. Search for exoplanets and variable stars within the open cluster M 67 (NGC 2682)
Full text linkVariable stars in one open cluster within the Kepler/K2-Campaign-5 field: M 67 (NGC 2682)
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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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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