Bibcode
Puschmann, K. G.; Beck, C.
Bibliographical reference
Astronomy and Astrophysics, Volume 533, id.A21
Advertised on:
9
2011
Journal
Citations
24
Refereed citations
19
Description
Context. Ground-based imaging and imaging spectropolarimetric data are
often subjected to post-facto reconstruction techniques to improve the
spatial resolution. Aims: We test the effects of reconstruction
techniques on two-dimensional data to determine the best approach to
improve our data. Methods: We obtained an 1-h time-series of
spectropolarimetric data in the Fe i line at 630.25 nm with the
Göttingen Fabry-Pérot Interferometer (FPI) that are
accompanied by imaging data in the blue continuum at 431.3 nm and Ca ii
H at 396.85 nm. We apply both speckle and (multi-object) multi-frame
blind deconvolution ((MO)MFBD) techniques. We use the "Göttingen"
speckle and speckle deconvolution codes and the MOMFBD code in the
implementation of Van Noort et al. (2005). We compare the resulting
spatial resolution and investigate the impact of the image
reconstruction on spectral characteristics of the Göttingen FPI
data. Results: The speckle reconstruction and MFBD perform
similar for our imaging data with nearly identical intensity contrasts.
MFBD provides a better and more homogeneous spatial resolution at the
shortest wavelength when applied to a series of image bursts. The MOMFBD
and speckle deconvolution of the intensity spectra lead to similar
results, but our choice of settings for the MOMFBD yields an intensity
contrast smaller by about 2% at a comparable spatial resolution. None of
the reconstruction techniques introduces significant artifacts in the
intensity spectra. The speckle deconvolution (MOMFBD) has a rms noise in
Stokes V/I of 0.32% (0.20%). The deconvolved spectra thus require a high
significance threshold of about 1.0% to separate noise peaks from true
signal. A comparison to spectra with a significantly higher
signal-to-noise (S/N) ratio and to spectra from a magneto-hydrodynamical
simulation reveals that the Göttingen FPI can only detect about 30%
of the polarization signal present in quiet Sun areas. The distribution
of NCP values for the speckle-deconvolved data matches that of
observations with higher S/N better than MOMFBD, but shows seemingly
artificially sharp boundaries and unexpected changes of the sign.
Conclusions: For our imaging data, both MFBD and speckle reconstruction
are equivalent, with a slightly better and more stable performance of
MFBD. For the spectropolarimetric data, the higher intensity contrast of
the speckle deconvolution is balanced by the smaller amplification of
the noise level in the MOMFBD at a comparable spatial resolution. The
noise level prevents the detection of weak and diffuse magnetic fields.
Future efforts should be directed to improve the S/N of the
Göttingen FPI spectra for spectropolarimetric observations to lower
the final significance thresholds.
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