1,720,981 research outputs found
Speckle reconstruction of photometric data observed with adaptive optics
No full textTo achieve the highest spatial resolution for ground-based observations one has to correct for degradations by the Earth's atmosphere. This can be done by on-line and post-facto techniques. Here we combine observations with Adaptive Optics (AO) and speckle reconstruction. As possible techniques we present two modified versions (methods B and C) of the Göttingen speckle masking code and describe their application to observations of a solar active region obtained with the Swedish 1-m Solar Telescope (La Palma) by means of AO. By comparing the results with those obtained from the conventional method (method A), the influence of AO on the post-facto reconstruction is revealed. One has to account for the field dependence of the AO correction and the fitting errors of the calculated surface of the deformable mirror with respect to the according wave fronts. Both new approaches enhance the contrast in the lockpoint region less than the conventional method. Method C uses simulated AO corrected speckle transfer functions (STFs). This method shows the most uniform image quality over the whole field of view and indicates the importance of the atmospheric model assumptions and the partial character of the WF correction by AO with appropriate WF error statistics. However, the consideration of field dependent speckle transfer functions in the classical speckle reconstruction (Method B) can serve as a first approach to avoid an intensity over-amplification around the AO-lockpoint region, although the rms of the contrast in the reconstructed images tends to rise slightly towards the outer parts of the field of view. Even the apparently small differences between the speckle transfer functions, used in the different reconstruction methods, cause the modulus of the estimated intensity differences to vary by up to 22%
On super-resolution in astronomical imaging
No full textOur work is an attempt to fulfil one of the aims of astronomical imaging, that is, to obtain information at high angular resolution. Stars as point sources can be represented by Dirac δ-functions whose Fourier transforms contain information about the position and the (integrated) intensity at all angular frequencies. Thus, we can deconvolve unresolved images of star fields even at angular distances smaller than the diffraction limit of the telescope with which the observations are performed. We give an example of reconstruction of the image of two stars with an angular separation of a factor 2.5 less than . However, we find that super-resolution is feasible only for point sources. For extended objects the information about intensity fluctuations at angular frequencies is irretrievably lost. We discuss the impossibility of super-resolution for the Sun using a numerically simulated image of solar granulation. However, one can enhance the contrast of solar images, though without increasing angular resolution beyond the diffraction limit.
The flux-gap between bright and dark solar magnetic structures
No full textThe upper size limit of solar small-scale magnetic flux
concentrations (“G-band bright points”, BP) is reconsidered from
speckle-reconstructed images taken at the 1-m SST on La Palma.
The size-histogram shows a sharp drop towards 250 km diameter,
variation of the noise filter threshold diminishes that value due
to segmentation of the elongated structures. A further artificial
segmentation of still elongated (i.e. not round) BP indicates that
the upper limit may well be below 200 km diameter, corresponding
to a flux smaller than Mx which is more than
40 times smaller than that of smallest dark (mini-) pores. BP with
diameters of 130 km would already yield to a flux gap of two
orders of magnitude. The drop of BP numbers between the histogram
maximum and the 90 km resolution limit achieved is found to depend
on the low-pass filtering and is thus probably virtual. Higher spatial
resolution data will still increase the flux gap between bright and
dark solar magnetic flux concentrations which might be a signature
of differently deep rooting in the solar atmosphere
Spectropolarimetry of a sunspot at disk centre
No full textThe magnetic, thermal and velocity structure of a sunspot
at the solar disk centre () is investigated by
inverting the full Stokes profiles of three infrared lines. A single
magnetic component atmosphere is assumed with height gradients of the
physical quantities. Since the line-of-sight (LOS) is perpendicular to
the solar surface, differential optical or projection effects do not
interfere, as often is the case for the usual observations at oblique
LOS. We find a symmetric configuration of the field and flow and the
downward motion that increases with radial distance by up to 3 km s-1
near the outer penumbral border. The magnetic field is found to be highly
axially symmetric without any indication of azimuthal vortices. A tight
relation between field strength and inclination is obtained with a
gradient of /1000 G independent of height. The penumbra shows
“spines” hosting a pronounced negative correlation between field strength
and inclination in the sense that steeper and stronger magnetic fields
are related to brightenings in the line cores but not in the continuum.
We discuss the dependence of the obtained results on different assumptions
of parasitic light, and present indications of its overestimation by the
inversion code.
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
The new Göttingen Fabry-Pérot spectrometer for two-dimensional observations of the Sun
No full textStudies of small-scale dynamics and magnetic fields in the solar atmosphere require spectroscopy and polarimetry with high spatial resolution. For this purpose, spectrometers based on Fabry-Pérot interferometers (FPIs) have advantages over slit spectrographs. They possess a high throughput and allow fast two-dimensional, narrow-band imaging and image reconstruction of the data. In the present contribution we describe an upgrade, essentially renewal, of the Göttingen FPI spectrometer achieved during the first half of 2005. A new etalon from IC Optical Systems Ltd. (formerly Queensgate), England, with 70 mm free aperture for high spectral resolution has been mounted. New CCD detectors from LaVision GmbH (Göttingen) with powerful computer hard- and software were implemented. We consider the product of signal-to-noise ratio, frame rate, and field of view as a measure of the efficiency. At low light levels, e.g. in narrow-band speckle applications, this product has increased by a factor ~60 compared to the old system. In addition, several spectral regions can now be scanned quasi-simultaneously. We present first results obtained with the upgraded spectrometer. The efforts are undertaken to provide an up-to-date post-focus instrument for the new German 1.5 m GREGOR solar telescope presently under construction at the Observatorio del Teide on Tenerife.
The structure of a penumbral connection between solar pores
No full textHigh resolution 2D-spectro-polarimetric observations
have been used to analyse the magnetic field and flow topologies
of a penumbral connection between two opposite polarity solar
pores. A filamentary structured Evershed-like material flow from
one pore to the other along the magnetic field lines has been
detected. The flow channels are co-spatial with bright penumbral
filaments close to the pore which feeds the flow and the clear
brightness-velocity relation vanishes close to the pore which
represents the sink of the flow. The boundary between umbra and
penumbra of the two pores show significant differences: bright
comet-like penumbral grains represent endings of penumbral
filaments at the flow sources whereas no such grains were found at
the sinks of the flow. Furthermore, a systematic variation of the
asymmetries of measured Stokes V profiles across the penumbral
connection have been found. The obtained results are in accordance
with the widely-accepted uncombed penumbra hypothesis and the moving
flux tube model
Variations on the Author
Get PDF“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
Get PDFWe 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
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