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Author name code: loefdahl
ADS astronomy entries on 2022-09-14
author:"Loefdahl, Mats"
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Title: The European Solar Telescope
Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.;
Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein,
C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz
Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados,
M.; the EST team
2022arXiv220710905Q Altcode:
The European Solar Telescope (EST) is a project aimed at studying
the magnetic connectivity of the solar atmosphere, from the deep
photosphere to the upper chromosphere. Its design combines the knowledge
and expertise gathered by the European solar physics community during
the construction and operation of state-of-the-art solar telescopes
operating in visible and near-infrared wavelengths: the Swedish 1m Solar
Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR,
the French Télescope Héliographique pour l'Étude du Magnétisme
et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope
(DOT). With its 4.2 m primary mirror and an open configuration, EST will
become the most powerful European ground-based facility to study the Sun
in the coming decades in the visible and near-infrared bands. EST uses
the most innovative technological advances: the first adaptive secondary
mirror ever used in a solar telescope, a complex multi-conjugate
adaptive optics with deformable mirrors that form part of the optical
design in a natural way, a polarimetrically compensated telescope design
that eliminates the complex temporal variation and wavelength dependence
of the telescope Mueller matrix, and an instrument suite containing
several (etalon-based) tunable imaging spectropolarimeters and several
integral field unit spectropolarimeters. This publication summarises
some fundamental science questions that can be addressed with the
telescope, together with a complete description of its major subsystems.
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Title: Multi-frame blind deconvolution and phase diversity with
statistical inclusion of uncorrected high-order modes
Authors: Löfdahl, Mats G.; Hillberg, Tomas
2022arXiv220513650L Altcode:
Images collected with ground-based telescopes suffer blurring and
distortions from turbulence in Earth's atmosphere. Adaptive optics (AO)
can only partially compensate for these effects. Neither multi-frame
blind deconvolution (MFBD) nor speckle techniques restore AO compensated
images to the correct power spectrum and contrast. MFBD can only
compensate for a finite number of low-order aberrations, leaving a tail
of uncorrected high-order modes. Speckle restoration of AO-corrected
data depends on calibrations of the AO corrections and assumptions
regarding the height distribution of atmospheric turbulence. We seek
to develop an improvement to MFBD that combines speckle's usage of
turbulence statistics to account for high-order modes with the ability
of MFBD to sense low-order modes that can be partially corrected by AO
and/or include fixed or slowly changing instrumental aberrations. We
modify the image formation model, supplementing the fitted low-order
wavefront aberrations with tails of random high-order aberrations that
follow Kolmogorov statistics, scaled to estimated or measured values of
Fried's parameter, r0, that characterize the strength of the seeing at
the moment of data collection. We refer to this as statistical diversity
(SD). We test MFBD with SD with noise-free synthetic data, simulating
many different r0 and numbers of AO-corrected modes. SD improves the
contrasts and power spectra of restored images, both in accuracy and in
consistency with varying r0, without penalty in processing time. With
focus diversity (FD), the results are almost perfect. SD also reduces
errors in the fitted wavefront parameters. MFBD with SD and FD seems
robust with respect to several percents of error in r0. Adding SD to
MFBD shows great promise for improving contrasts and power spectra in
restored images. Further studies with real data are motivated.
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Title: SSTRED: Data- and metadata-processing pipeline for CHROMIS
and CRISP
Authors: Löfdahl, Mats G.; Hillberg, Tomas; de la Cruz Rodríguez,
Jaime; Vissers, Gregal; Andriienko, Oleksii; Scharmer, Göran B.;
Haugan, Stein V. H.; Fredvik, Terje
2021A&A...653A..68L Altcode: 2018arXiv180403030L
Context. Data from ground-based, high-resolution solar telescopes
can only be used for science with calibrations and processing, which
requires detailed knowledge about the instrumentation. Space-based
solar telescopes provide science-ready data, which are easier to
work with for researchers whose expertise is in the interpretation of
data. Recently, data-processing pipelines for ground-based instruments
have been constructed. <BR /> Aims: We aim to provide observers
with a user-friendly data pipeline for data from the Swedish 1-meter
Solar Telescope (SST) that delivers science-ready data together with
the metadata needed for proper interpretation and archiving. <BR />
Methods: We briefly describe the CHROMospheric Imaging Spectrometer
(CHROMIS) instrument, including its (pre)filters, as well as recent
upgrades to the CRisp Imaging SpectroPolarimeter (CRISP) prefilters and
polarization optics. We summarize the processing steps from raw data
to science-ready data cubes in FITS files. We report calibrations
and compensations for data imperfections in detail. Misalignment
of Ca II data due to wavelength-dependent dispersion is identified,
characterized, and compensated for. We describe intensity calibrations
that remove or reduce the effects of filter transmission profiles
as well as solar elevation changes. We present REDUX, a new version
of the MOMFBD image restoration code, with multiple enhancements and
new features. It uses projective transforms for the registration of
multiple detectors. We describe how image restoration is used with
CRISP and CHROMIS data. The science-ready output is delivered in FITS
files, with metadata compliant with the SOLARNET recommendations. Data
cube coordinates are specified within the World Coordinate System
(WCS). Cavity errors are specified as distortions of the WCS wavelength
coordinate with an extension of existing WCS notation. We establish
notation for specifying the reference system for Stokes vectors with
reference to WCS coordinate directions. The CRIsp SPectral EXplorer
(CRISPEX) data-cube browser has been extended to accept SSTRED output
and to take advantage of the SOLARNET metadata. <BR /> Results: SSTRED
is a mature data-processing pipeline for imaging instruments, developed
and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP
spectropolarimeter. SSTRED delivers well-characterized, science-ready,
archival-quality FITS files with well-defined metadata. The SSTRED
code, as well as REDUX and CRISPEX, is freely available through git
repositories.
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Title: Is the sky the limit?. Performance of the revamped Swedish
1-m Solar Telescope and its blue- and red-beam reimaging systems
Authors: Scharmer, G. B.; Löfdahl, M. G.; Sliepen, G.; de la Cruz
Rodríguez, J.
2019A&A...626A..55S Altcode: 2019arXiv190505588S
We discuss the use of measurements of the solar granulation contrast
as a measure of optical quality. We demonstrate that for data recorded
with a telescope that uses adaptive optics and/or post-processing to
compensate for many low- and high-order aberrations, the RMS granulation
contrast is directly proportional to the Strehl ratio calculated
from the residual (small-scale) wavefront error (static and/or from
seeing). We demonstrate that the wings of the high-order compensated
point spread function for the Swedish 1-m Solar Telescope (SST) are
likely to extend to a radius of not more than about 2″, which is
consistent with earlier conclusions drawn from stray-light compensation
of sunspot images. We report on simultaneous measurements of seeing
and solar granulation contrast averaged over 2 s time intervals at
several wavelengths from 525 nm to 853.6 nm on the red-beam (CRISP
beam) and wavelengths from 395 nm to 484 nm on the blue-beam (CHROMIS
beam). These data were recorded with the SST, which has been revamped
with an 85-electrode adaptive mirror and a new tip-tilt mirror, both of
which were polished to exceptionally high optical quality. Compared
to similar data obtained with the previous 37-electrode adaptive
mirror in 2009 and 2011, there is a significant improvement in image
contrast. The highest 2 s average image contrasts measured in April
2015 through 0.3-0.9 nm interference filters at 525 nm, 557 nm, 630
nm, and 853.5 nm with compensation only for the diffraction limited
point spread function of SST are 11.8%, 11.8%, 10.2%, and 7.2%,
respectively. Similarly, the highest 2 s contrasts measured at 395 nm,
400 nm, and 484 nm in May 2016 through 0.37-1.3 nm filters are 16%,
16%, and 12.5%, respectively. The granulation contrast observed with
SST compares favorably to measured values with SOT on Hinode and with
Sunrise as well as major ground-based solar telescopes. Simultaneously
with the above wideband red-beam data, we also recorded narrowband
continuum images with the CRISP imaging spectropolarimeter. We find
that contrasts measured with CRISP are entirely consistent with the
corresponding wideband contrasts, demonstrating that any additional
image degradation by the CRISP etalons and telecentric optical system
is marginal or even insignificant. Finally, we discuss the origin of
the 48 nm RMS wavefront error needed to bring consistency between the
measured granulation contrast and that obtained from 3D simulations
of convection.
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Title: Mapping the Magnetic Field of Flare Coronal Loops
Authors: Kuridze, D.; Mathioudakis, M.; Morgan, H.; Oliver, R.; Kleint,
L.; Zaqarashvili, T. V.; Reid, A.; Koza, J.; Löfdahl, M. G.; Hillberg,
T.; Kukhianidze, V.; Hanslmeier, A.
2019ApJ...874..126K Altcode: 2019arXiv190207514K
Here, we report on the unique observation of flaring coronal loops at
the solar limb using high-resolution imaging spectropolarimetry from
the Swedish 1 m Solar Telescope. The vantage position, orientation,
and nature of the chromospheric material that filled the flare loops
allowed us to determine their magnetic field with unprecedented accuracy
using the weak-field approximation method. Our analysis reveals coronal
magnetic field strengths as high as 350 G at heights up to 25 Mm above
the solar limb. These measurements are substantially higher than a
number of previous estimates and may have considerable implications
for our current understanding of the extended solar atmosphere.
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Title: CRISPRED: CRISP imaging spectropolarimeter data reduction
pipeline
Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.;
Hillberg, T.; Rouppe van der Voort, L.
2017ascl.soft08003D Altcode:
CRISPRED reduces data from the CRISP imaging spectropolarimeter at
the Swedish 1 m Solar Telescope (SST). It performs fitting routines,
corrects optical aberrations from atmospheric turbulence as well as
from the optics, and compensates for inter-camera misalignments,
field-dependent and time-varying instrumental polarization, and
spatial variation in the detector gain and in the zero level offset
(bias). It has an object-oriented IDL structure with computationally
demanding routines performed in C subprograms called as dynamically
loadable modules (DLMs).
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Title: A Comparison of Solar Image Restoration Techniques for
SST/CRISP Data (Summary)
Authors: Löfdahl, M.
2016ASPC..504..111L Altcode:
Solar images from high-resolution, ground-based telescopes are
corrected for the blurring effects of atmospheric turbulence by use
of adaptive optics and post-facto image restoration. Two classes of
image restoration methods are regularly used today, those based on
Multi-Frame Blind Deconvolution (MFBD; Löfdahl 2002) and those based on
Speckle Interferometry (SI; von der Luhe &Dunn 1987). In a recently
started project, we will compare and evaluate such methods for use with
spectropolarimetric data from the CRisp Imaging SpectroPolarimeter
(CRISP; Scharmer et al. 2008) of the Swedish 1-meter Solar Telescope
(SST; Scharmer et al. 2003). For SST/CRISP data we routinely use the
Multi-Object MFBD (MOMFBD; van Noort et al. 2005) technique to jointly
restore images collected from a wideband camera and from the narrowband
cameras behind the CRISP FPI and polarimetry optics. This crucial step
in the data reduction pipeline of CRISP (CRISPRED; de la Cruz Rodríguez
et al. 2015) is carefully integrated with the application of various
procedures that are designed to reduce effects of imperfections in
the instruments. In order to make the comparison as fair as possible,
we have extended CRISPRED so that the Kiepenheuer-Institut Speckle
Interferometry Package (KISIP; Wöger & von der Lühe 2008),
together with Speckle Deconvolution (SD; Keller & von der Luehe
1992; Mikurda et al. 2006), can serve as a drop in replacement for
MOMFBD. The adaption of SI and SD to CRISPRED will allow us to make fair
comparisons not only of the restored images, but also of derivative
data like Stokes maps and further on to evaluate the consequences
of remaining errors and artifacts for the interpretation of physical
quantities inferred through atmospheric model inversions.
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Title: Off-disk straylight measurements for the Swedish 1-m Solar
Telescope
Authors: Löfdahl, Mats G.
2016A&A...585A.140L Altcode: 2015arXiv151008329L
Context. Accurate photometry with ground-based solar telescopes requires
characterization of straylight. Scattering in Earth's atmosphere and in
the telescope optics are potentially significant sources of straylight,
for which the point spread function (PSF) has wings that reach very
far. This kind of straylight produces an aureola, extending several
solar radii off the solar disk. <BR /> Aims: We want to measure
such straylight using the ordinary science instrumentation. <BR />
Methods: We scanned the intensity on and far off the solar disk by use
of the science cameras in several different wavelength bands on a day
with low-dust conditions. We characterized the far wing straylight by
fitting a model to the recorded intensities involving a multicomponent
straylight PSF and the limb darkening of the disk. <BR /> Results:
The measured scattered light adds an approximately constant fraction
of the local granulation intensity to science images at any position
on the disk. The fraction varied over the day but never exceeded a
few percent. The PSFs have weak tails that extend to several solar
radii, but most of the scattered light originates within ~1'. <BR />
Conclusions: Far-wing scattered light contributes only a small amount
of straylight in SST data. Other sources of straylight are primarily
responsible for the reduced contrast in SST images.
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Title: CRISPRED: A data pipeline for the CRISP imaging
spectropolarimeter
Authors: de la Cruz Rodríguez, J.; Löfdahl, M. G.; Sütterlin, P.;
Hillberg, T.; Rouppe van der Voort, L.
2015A&A...573A..40D Altcode: 2014arXiv1406.0202D
The production of science-ready data from major solar telescopes
requires expertise beyond that of the typical observer. This is
a consequence of the increasing complexity of instruments and
observing sequences, which require calibrations and corrections
for instrumental and seeing effects that are not only difficult to
measure, but are also coupled in ways that require careful analysis
in the design of the correction procedures. Modern space-based
telescopes have data-processing pipelines capable of routinely
producing well-characterized data products. High resolution imaging
spectropolarimeters at ground-based telescopes need similar data
pipelines.We present new methods for flat-fielding spectropolarimetric
data acquired with telecentric Fabry-Perot instruments and a new
approach for accurate camera co-alignment for image restoration. We
document a procedure that forms the basis of current state-of-the-art
processing of data from the CRISP imaging spectropolarimeter at the
Swedish 1 m Solar Telescope (SST). By collecting, implementing, and
testing a suite of computer programs, we have defined a data reduction
pipeline for this instrument. This pipeline, CRISPRED, streamlines the
process of making science-ready data.It is implemented and operated
in IDL, with time-consuming steps delegated to C.CRISPRED will also be
the basis for the data pipeline of the forthcoming CHROMIS instrument.
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Title: Restoration of the contrast in solar images
Authors: Loefdahl, M.
2012IAUSS...6E.305L Altcode:
Solar images from the major ground-based high-resolution telescopes
are routinely processed to compensate for blurring and distortion
caused by turbulence in Earth's atmosphere and only partly removed
by adaptive optics. When the seeing conditions are good enough,
methods based on multi-frame blind deconvolution, phase diversity,
and speckle interferometry deliver images with nearly diffraction
limited resolution. This corresponds to good estimates of the core of
the short-exposure point spread functions (PSFs). Combining many short
exposures results in high-resolution images with high signal to noise
ratio. This approach has been used successfully e.g., for studies
of the velocity and magnetic fields of small-scale photospheric and
chromospheric solar features, and for their temporal evolution. However,
the contrast and spatial power spectra of ground-based solar images
are usually severely under-estimated. We know this from recent advances
in space-based observations and MHD modeling of the photosphere, that
until recently did not agree on the contrast of solar image data. It
is now well established that synthetic data from current codes produce
data with contrasts and power spectra that well represent reality. This
means it is now possible to test methods for estimating the effects
of various sources of reduced contrast. We have recently set out on a
search for such sources present at the Swedish 1-m Solar Telescope. In
this presentation I will discuss some of our findings and some of our
ideas for further testing.
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Title: Sources of straylight in the post-focus imaging instrumentation
of the Swedish 1-m Solar Telescope
Authors: Löfdahl, M. G.; Scharmer, G. B.
2012A&A...537A..80L Altcode: 2011arXiv1110.2663L
Context. Recently measured straylight point spread functions (PSFs) in
Hinode/SOT make granulation contrast in observed data and synthetic
magnetohydrodynamic (MHD) data consistent. Data from earthbound
telescopes also need accurate correction for straylight and fixed
optical aberrations. <BR /> Aims: We aim to develop a method for
measuring straylight in the post-focus imaging optics of the Swedish
1-m Solar Telescope (SST). <BR /> Methods: We removed any influence from
atmospheric turbulence and scattering by using an artificial target. We
measured integrated straylight from three different sources in the same
data: ghost images caused by reflections in the near-detector optics,
PSFs corresponding to wavefront aberrations in the optics by using
phase diversity, and extended scattering PSF wings of unknown origin
by fitting to a number of different kernels. We performed the analysis
separately in the red beam and the blue beam. <BR /> Results: Wavefront
aberrations, which possibly originate in the bimorph mirror of the
adaptive optics, are responsible for a wavelength-dependent straylight
of 20-30% of the intensity in the form of PSFs with 90% of the energy
contained within a radius of 0.6 arcsec. There are ghost images that
contribute at the most a few percent of straylight. The fraction of
other sources of scattered light from the post-focus instrumentation
of the SST is only ~10<SUP>-3</SUP> of the recorded intensity. This
contribution has wide wings with a FWHM ~16” in the blue and ~34”
in the red. <BR /> Conclusions: The present method seems to work well
for separately estimating wavefront aberrations and the scattering
kernel shape and fraction. Ghost images can be expected to remain
at the same level for solar observations. The high-order wavefront
aberrations possibly caused by the AO bimorph mirror dominate the
measured straylight but are likely to change when imaging the Sun. We
can therefore make no firm statements about the origin of straylight
in SST data, but strongly suspect wavefront aberrations to be the
dominant source.
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Title: A tilted interference filter in a converging beam
Authors: Löfdahl, M. G.; Henriques, V. M. J.; Kiselman, D.
2011A&A...533A..82L Altcode: 2011arXiv1108.1234L
Context. Narrow-band interference filters can be tuned toward shorter
wavelengths by tilting them from the perpendicular to the optical
axis. This can be used as a cheap alternative to real tunable filters,
such as Fabry-Pérot interferometers and Lyot filters. At the Swedish
1-meter Solar Telescope, such a setup is used to scan through the
blue wing of the Ca ii H line. Because the filter is mounted in a
converging beam, the incident angle varies over the pupil, which
causes a variation of the transmission over the pupil, different for
each wavelength within the passband. This causes broadening of the
filter transmission profile and degradation of the image quality. <BR
/> Aims: We want to characterize the properties of our filter, at
normal incidence as well as at different tilt angles. Knowing the
broadened profile is important for the interpretation of the solar
images. Compensating the images for the degrading effects will improve
the resolution and remove one source of image contrast degradation. In
particular, we need to solve the latter problem for images that are
also compensated for blurring caused by atmospheric turbulence. <BR />
Methods: We simulate the process of image formation through a tilted
interference filter in order to understand the effects. We test the
hypothesis that they are separable from the effects of wavefront
aberrations for the purpose of image deconvolution. We measure the
filter transmission profile and the degrading PSF from calibration
data. <BR /> Results: We find that the filter transmission profile
differs significantly from the specifications. We demonstrate how to
compensate for the image-degrading effects. Because the filter tilt
effects indeed appear to be separable from wavefront aberrations in a
useful way, this can be done in a final deconvolution, after standard
image restoration with Multi-Frame Blind Deconvolution/Phase Diversity
based methods. We illustrate the technique with real data.
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Title: Evaluation of image-shift measurement algorithms for solar
Shack-Hartmann wavefront sensors
Authors: Löfdahl, M. G.
2010A&A...524A..90L Altcode: 2010arXiv1009.3401L
Context. Solar Shack-Hartmann wavefront sensors measure differential
wavefront tilts as the relative shift between images from different
subapertures. There are several methods in use for measuring these
shifts. <BR /> Aims: We evaluate the inherent accuracy of the methods
and the effects of various sources of error, such as noise, bias
mismatch, and blurring. We investigate whether Z-tilts or G-tilts
are measured. <BR /> Methods: We test the algorithms on two kinds of
artificial data sets, one corresponding to images with known shifts
and one corresponding to seeing with different r<SUB>0</SUB>. <BR />
Results: Our results show that the best methods for shift measurements
are based on the square difference function and the absolute
difference function squared, with subpixel accuracy accomplished by
use of two-dimensional quadratic interpolation. These methods measure
Z-tilts rather than G-tilts.
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Title: Striation and convection in penumbral filaments
Authors: Spruit, H. C.; Scharmer, G. B.; Löfdahl, M. G.
2010A&A...521A..72S Altcode: 2010arXiv1008.0932S
Observations with the 1-m Swedish Solar Telescope of the flows seen in
penumbral filaments are presented. Time sequences of bright filaments
show overturning motions strikingly similar to those seen along
the walls of small isolated structures in the active regions. The
filaments show outward propagating striations with inclination
angles suggesting that they are aligned with the local magnetic
field. We interpret it as the equivalent of the striations seen in
the walls of small isolated magnetic structures. Their origin is then
a corrugation of the boundary between an overturning convective flow
inside the filament and the magnetic field wrapping around it. The
outward propagation is a combination of a pattern motion due to
the downflow observed along the sides of bright filaments, and the
Evershed flow. The observed short wavelength of the striation argues
against the existence of a dynamically significant horizontal field
inside the bright filaments. Its intensity contrast is explained by
the same physical effect that causes the dark cores of filaments,
light bridges and “canals”. In this way striation represents an
important clue to the physics of penumbral structure and its relation
with other magnetic structures on the solar surface. We put this in
perspective with results from the recent 3-D radiative hydrodynamic
simulations. <P />4 movies are only available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>
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Title: High-order aberration compensation with multi-frame blind
deconvolution and phase diversity image restoration techniques
Authors: Scharmer, G. B.; Löfdahl, M. G.; van Werkhoven, T. I. M.;
de la Cruz Rodríguez, J.
2010A&A...521A..68S Altcode: 2010arXiv1007.1236S
Context. For accurately measuring intensities and determining magnetic
field strengths of small-scale solar (magnetic) structure, knowledge of
and compensation for the point spread function is crucial. For images
recorded with the Swedish 1-meter Solar Telescope (SST), restoration
with multi-frame blind deconvolution (MFBD) and joint phase diverse
speckle (JPDS) methods lead to remarkable improvements in image quality
but granulation contrasts that are too low, indicating additional stray
light. <BR /> Aims: We propose a method to compensate for stray light
from high-order atmospheric aberrations not included in MFBD and JPDS
processing. <BR /> Methods: To compensate for uncorrected aberrations,
a reformulation of the image restoration process is proposed that allows
the average effect of hundreds of high-order modes to be compensated for
by relying on Kolmogorov statistics for these modes. The applicability
of the method requires simultaneous measurements of Fried's parameter
r<SUB>0</SUB>. The method is tested with simulations as well as real
data and extended to include compensation for conventional stray
light. <BR /> Results: We find that only part of the reduction of
granulation contrast in SST images is due to uncompensated high-order
aberrations. The remainder is still unaccounted for and attributed
to stray light from the atmosphere, the telescope with its re-imaging
system and to various high-altitude seeing effects. <BR /> Conclusions:
We conclude that statistical compensation of high-order modes is a
viable method to reduce the loss of contrast occurring when a limited
number of aberrations is explicitly compensated for with MFBD and JPDS
processing. We show that good such compensation is possible with only
10 recorded frames. The main limitation of the method is that already
MFBD and JPDS processing introduces high-order compensation that,
if not taken into account, can lead to over-compensation.
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Title: Wavefront sensing and wavefront reconstruction for the 4m
European Solar Telescope EST
Authors: Berkefeld, Thomas; Soltau, Dirk; del Moro, Dario; Löfdahl,
Mats
2010SPIE.7736E..2JB Altcode: 2010SPIE.7736E..85B
We give an overview of the Adaptive Optics (AO) and Multi-conjugate
Adaptive Optics (MCAO) system of the planned 4m European Solar Telescope
(EST). The parameter space and the problems of solar MCAO working in the
visible are explained. The wavefront reconstruction schemes presently
being considered are explained. First estimates of the expected MCAO
performance for varying parameter sets are given.
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Title: Adaptive optics and MCAO for the 4-m European Solar Telescope
EST
Authors: Soltau, D.; Berkefeld, T.; Sánchez Capuchino, J.; Collados
Vera, M.; Del Moro, D.; Löfdahl, M.; Scharmer, G.
2010SPIE.7736E..0US Altcode: 2010SPIE.7736E..27S
A consortium of more than 20 European solar physics institution from 15
different countries is conducting a design study for a 4 m class solar
telescope which shall be situated at the Canary Islands. In this paper
we introduce the AO and MCAO design concept for EST. A ground layer
deformable mirror is combined with an arrangement of four deformable
layer mirrors. A combination of Shack-Hartmann wave front sensors with
wide and narrow fields of view is used to control the system and to
achieve a corrected field of view of one arcmin.
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Title: CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
Authors: Scharmer, G. B.; Narayan, G.; Hillberg, T.; de la Cruz
Rodriguez, J.; Löfdahl, M. G.; Kiselman, D.; Sütterlin, P.; van
Noort, M.; Lagg, A.
2008ApJ...689L..69S Altcode: 2008arXiv0806.1638S
We discuss penumbral fine structure in a small part of a pore,
observed with the CRISP imaging spectropolarimeter at the Swedish
1-m Solar Telescope (SST), close to its diffraction limit of
0.16”. Milne-Eddington inversions applied to these Stokes data
reveal large variations of field strength and inclination angle over
dark-cored penumbral intrusions and a dark-cored light bridge. The
mid-outer part of this penumbra structure shows ~0.3” wide spines,
separated by ~1.6” (1200 km) and associated with 30° inclination
variations. Between these spines, there are no small-scale magnetic
structures that easily can be identified with individual flux tubes. A
structure with nearly 10° more vertical and weaker magnetic field is
seen midway between two spines. This structure is cospatial with the
brightest penumbral filament, possibly indicating the location of a
convective upflow from below.
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Title: Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution
Authors: Scharmer, G.; Henriques, V.; Hillberg, T.; Kiselman, D.;
Löfdahl, M.; Narayan, G.; Sütterlin, P.; van Noort, M.; de la Cruz
Rodríguez, J.
2008ESPM...12..2.5S Altcode:
We present first observations of sunspots with the imaging
spectropolarimeter CRISP, recently installed at the Swedish 1-m
Solar Telescope (SST) on La Palma. This spectropolarimeter is based
on a high-fidelity dual Fabry-Perot filter system. <P />Two liquid
crystals and a polarizing beam splitter are used to reduce seeing
induced I,Q,U,V crosstalk by simultaneously recording images with
two 1kx1k back-illuminated Sarnoff CCD's. A third CCD simultaneously
records broadband images through the pre-filter of the FPI filter
system, allowing image reconstruction and co-alignment of images of
different polarization states and at different wavelengths in Zeeman
sensitive spectral lines. <P />The first data, recorded in April 2008,
demonstrate the capability of this system to record high cadence,
high S/N polarimetric data with a spatial resolution at or close to
the diffraction limit of the SST at 630 nm, 0.16 arcsec. We discuss
the analysis of first spectropolarimetric data for sunspots, based on
Milne-Eddington inversion techniques.
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Title: SST/CRISP Magnetometry with Fe I 630.2 nm
Authors: Narayan, G.; Scharmer, G. B.; Hillberg, T.; Lofdahl, M.;
van Noort, M.; Sutterlin, P.; Lagg, A.
2008ESPM...122.120N Altcode:
We present recent full Stokes observations in the Fe I 630.2 nm
line with CRISP, an imaging spectropolarimeter at the Swedish 1-m
Solar Telescope (SST). The observations reach a spatial resolution
of 0".16, close to the diffraction limit of the SST, representing
a major improvement over any past ground based or space based
spectropolarimetric data. We describe the data acquisition and reduction
methods and present results of Milne-Eddington(ME) inversions applied
on observations of plage.
---------------------------------------------------------
Title: What are 'Faculae'?
Authors: Berger, T. E.; Title, A. M.; Tarbell, T.; Rouppe van der
Voort, L.; Löfdahl, M. G.; Scharmer, G. B.
2007ASPC..369..103B Altcode:
We present very high resolution filtergram and magnetogram observations
of solar faculae taken at the Swedish 1-meter Solar Telescope (SST)
on La Palma. Three datasets with average line-of-sight angles of 16,
34, and 53 degrees are analyzed. The average radial extent of faculae is
at least 400~km. In addition we find that contrast versus magnetic flux
density is nearly constant for faculae at a given disk position. These
facts and the high resolution images and movies reveal that faculae are
not the interiors of small flux tubes - they are granules seen through
the transparency caused by groups of magnetic elements or micropores
“in front of” the granules. Previous results which show a strong
dependency of facular contrast on magnetic flux density were caused
by bin-averaging of lower resolution data leading to a mixture of
the signal from bright facular walls and the associated intergranular
lanes and micropores. The findings are relevant to studies of total
solar irradiance (TSI) that use facular contrast as a function of disk
position and magnetic field in order to model the increase in TSI with
increasing sunspot activity.
---------------------------------------------------------
Title: Recent High Resolution Observations and Interpretations of
Sunspot Fine Structure
Authors: Scharmer, G. B.; Langhans, K.; Kiselman, D.; Löfdahl, M. G.
2007ASPC..369...71S Altcode:
We review analyses made of highly resolved filtergrams, magnetograms
and Dopplergrams of sunspots, recorded with the Swedish 1-meter Solar
Telescope (SSTSST) on La Palma. Dark cores in penumbral filaments are
shown to be directly linked to peripheral umbral dots and to dark
lanes in light bridges, suggesting similar or related underlying
physics. The visibility of dark cores is found to depend strongly
on the azimuth angle already for spots located at small heliocentric
distances. It is shown that dark cores are clearly visible close to the
center of the Ca II H line, formed approximately 150--200~km above the
photosphere. We conclude that the τ = 1 layer of dark-cored filaments
outlines a strongly warped surface, consistent with the finding that
the magnetic field strength is strongly reduced in dark cores. We show
that several properties of dark-cored filaments derived from SSTSST
data are consistent with results of inversions of low-resolution Stokes
spectra, but also find important discrepancies with the interpretation
that penumbral filaments can be identified with flux tubes. Our data
are consistent with the model proposed by Spruit & Scharmer (2006),
explaining dark cores as signatures of field-free convection occurring
just below the visible surface of the penumbra. We discuss recent
simulations of light bridges and umbral dots, providing additional
support to that model.
---------------------------------------------------------
Title: Contrast Analysis of Solar Faculae and Magnetic Bright Points
Authors: Berger, T. E.; Rouppe van der Voort, L.; Löfdahl, M.
2007ApJ...661.1272B Altcode:
The morphology and contrast of small-scale solar magnetic elements at
four disk positions is analyzed. The data were obtained at the Swedish
1 m Solar Telescope (SST) over 3 yr (2003-2005). Two of the data sets
have disk positions near disk center (average μ=cosθ>0.8) and show
numerous “magnetic bright points” (MBPs), and two are sufficiently
limbward to show prominent “faculae” (average μ<=0.6). The
filtergrams are obtained in the 430.5 nm G band and 436.4 nm
“continuum” bandpasses; the magnetograms are Fe I 630.25 nm Stokes
V images taken with the Solar Optical Universal Polarimeter (SOUP)
tunable filter. In all images we achieve nearly diffraction-limited
resolution (~100 km in the G band). Analysis shows that MBPs and faculae
are distinct radiative signatures of the magnetic field: MBPs have a
constant or slightly decreasing contrast with increasing magnetogram
signal, while facular contrast increases linearly with magnetogram
signal. Faculae are much larger than MBPs, with an average radial
width of 400 km. The observations support recent modeling showing that
faculae are granules seen through the opacity reduction provided by
magnetic elements (or groups thereof), while MBPs are caused by lateral
radiation leakage scattering from deeper layers of the magnetic element.
---------------------------------------------------------
Title: Observations of dark-cored filaments in sunspot penumbrae
Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.
2007A&A...464..763L Altcode:
Context: The recent discovery of dark-cored penumbral filaments suggests
that we are resolving the building blocks of sunspot penumbrae. Their
properties are largely unknown but provide important clues to
understanding penumbral fine structure. <BR />Aims: Our observations
provide new constraints for the different scenarios put forward to
explain the structure of sunspot penumbrae. <BR />Methods: We present
an analysis of dark-cored penumbral filaments, based on intensity
filtergrams (G-band, continuum and Ca II H line wing), magnetograms
and Dopplergrams, obtained at heliocentric distances between 15° and
55°. <BR />Results: In general, the visibility of dark cores degrades
with increasing heliocentric distance. Based on Ca II H wing images we
conclude that this is due to a geometrical 3D-effect and not due to a
simple formation height effect. Only in the center-side penumbra are
dark-cored filaments visible at all observed heliocentric distances. We
observe that dark-cored filaments frequently split in the umbra, forming
a Y-shape that disappears after a few minutes, leaving a shortened
filamentary structure and a bright dot in the umbra. The dark-cored
filaments have life times ≥ 90 min. The dark cores are related to
a much weaker and a more horizontal magnetic field than their lateral
brightenings. Where the dark-cored filaments appear in the umbra, the
magnetic field is inclined by 40° with respect to the solar surface
normal for both the dark core and the bright edges. With increasing
distance from the umbra, the magnetic field inclination in the dark
cores increases rapidly within a few thousand km. Both the magnetic
field strength and inclination in the lateral brightenings show very
small variations with spot-center radial distance. The velocity field
possesses a strong horizontal component within the dark cores. The
absolute line-of-sight (LOS) velocity is larger within the dark cores
than in their lateral brightenings. The Evershed flow apparently is
present primarily in the dark cores.
---------------------------------------------------------
Title: A comparative study of the contrast of solar magnetic elements
in CN and CH
Authors: Zakharov, V.; Gandorfer, A.; Solanki, S. K.; Löfdahl, M.
2007A&A...461..695Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar image restoration
Authors: Löfdahl, M. G.; van Noort, M. J.; Denker, C.
2007msfa.conf..119L Altcode:
Image restoration is used to repair solar images degraded by the
turbulence in Earth's atmosphere. Restoration algorithms are based on
models of the optical system that produce the images from the solar
source of radiation, through Earth's atmosphere and telescope/instrument
optics, to the detectors recording the data. In this review, these
model components are discussed in the context of two very different
classes of image restoration methods, i.e., Speckle Imaging and Phase
Diversity/Multi-Frame Blind Deconvolution, which have been successfully
used during the last two decades. The strengths and weaknesses of
these two approaches are discussed, as well as some variants and
recent progress.
---------------------------------------------------------
Title: Solar Image Restoration by use of Multi-Object Multi-Frame
Blind Deconvolution
Authors: van Noort, M.; Rouppe van der Voort, L.; Löfdahl, M.
2006ASPC..354...55V Altcode:
We present examples of the application of the image restoration
method of Multi-Object Multi-Frame Blind Deconvolution to observations
obtained with the Swedish one-meter Solar Telescope on La Palma. This
restoration method is an extension of Joint Phase Diverse Speckle
image restoration. Multiple realizations of multiple objects
can now be restored jointly, facilitating near-perfect alignment
between different objects. This greatly reduces false signals in the
determination of derived quantities, such as magnetograms, Dopplergrams
and G-band-continuum difference images.
---------------------------------------------------------
Title: Rapid Temporal Variability of Faculae: High-Resolution
Observations and Modeling
Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort,
L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G.
2006ApJ...646.1405D Altcode:
We present high-resolution G-band observations (obtained with the
Swedish 1 m Solar Telescope) of the rapid temporal variability of
faculae, which occurs on granular timescales. By combining these
observations with magnetoconvection simulations of a plage region, we
show that much of this variability is not intrinsic to the magnetic
field concentrations that are associated with faculae, but rather
a phenomenon associated with the normal evolution and splitting of
granules. We also show examples of facular variability caused by
changes in the magnetic field, with movies of dynamic behavior of
the striations that dominate much of the facular appearance at 0.1"
resolution. Examples of these dynamics include merging, splitting,
rapid motion, apparent fluting, and possibly swaying.
---------------------------------------------------------
Title: A comparative study of the contrast of solar magnetic elements
in CN and CH
Authors: Zakharov, V.; Gandorfer, A.; Solanki, S. K.; Löfdahl, M.
2005A&A...437L..43Z Altcode:
Photospheric bright points were investigated in three different
wavelength bands using interference filters centered at 436.5 nm
(continuum), 430.5 nm (Fraunhofer's G-band dominated by absorption due
to CH), and 388.7 nm (absorption band of CN). Such bright points serve
as proxies of small-scale solar magnetic elements. Near diffraction
limited imaging was achieved by real-time frame selection and
subsequent joint phase diverse speckle reconstruction. Comparison of
the filtergrams of NOAA0670 taken in CH and CN shows that the contrast
of bright points is on average 1.4 times <P />higher in CN than in
G-band, which is in good quantitative agreement with the predictions
of Berdyugina et al. (2003, A&A, 412, 513) and Rutten et al. (2001,
ASP Conf. Ser., 236, 445).
---------------------------------------------------------
Title: Inclination of magnetic fields and flows in sunspot penumbrae
Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.;
Berger, T. E.
2005A&A...436.1087L Altcode:
An observational study of the inclination of magnetic fields and
flows in sunspot penumbrae at a spatial resolution of 0.2 arcsec is
presented. The analysis is based on longitudinal magnetograms and
Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma
using the Lockheed Solar Optical Universal Polarimeter birefringent
filter. Data from two sunspots observed at several heliocentric
angles between 12 ° and 39 ° were analyzed. We find that the
magnetic field at the level of the formation of the Fe i-line wing
(630.25 nm) is in the form of coherent structures that extend radially
over nearly the entire penumbra giving the impression of vertical
sheet-like structures. The inclination of the field varies up to
45 ° over azimuthal distances close to the resolution limit of the
magnetograms. Dark penumbral cores, and their extensions into the outer
penumbra, are prominent features associated with the more horizontal
component of the magnetic field. The inclination of this dark penumbral
component - designated B - increases outwards from approximately 40 °
in the inner penumbra such that the field lines are nearly horizontal
or even return to the solar surface already in the middle penumbra. The
bright component of filaments - designated A - is associated with the
more vertical component of the magnetic field and has an inclination
with respect to the normal of about 35 ° in the inner penumbra,
increasing to about 60 ° towards the outer boundary. The magnetogram
signal is lower in the dark component B regions than in the bright
component A regions of the penumbral filaments. The measured rapid
azimuthal variation of the magnetogram signal is interpreted as being
caused by combined fluctuations of inclination and magnetic field
strength. The Dopplergrams show that the velocity field associated with
penumbral component B is roughly aligned with the magnetic field while
component A flows are more horizontal than the magnetic field. The
observations give general support to fluted and uncombed models of
the penumbra. The long-lived nature of the dark-cored filaments makes
it difficult to interpret these as evidence for convective exchange
of flux tubes. Our observations are in broad agreement with the two
component model of Bellot Rubi et al. (2003), but do not rule out the
embedded flux tube model of Solanki & Montavon (1993).
---------------------------------------------------------
Title: High resolution magnetogram measurements of solar faculae
Authors: Berger, T. E.; Rouppe van der Voort, L.; Lofdahl, M. G.
2005AGUSMSP31A..02B Altcode:
We present new images of magnetic elements near the solar limb
("faculae") along with magnetogram measurements and contrast
profiles. Imaging magnetogram observations were made of AR 10377 at μ
= cosθ = 0.6 on 06-June-2003 using the Swedish 1-m Solar Telescope;
the data are the highest spatial resolution magnetogram measurements of
faculae to date. Contrary to previous lower resolution measurements,
we find no correlation between facular contrast and magnetic flux
density at a given disk position. Increasing magnetic flux density
in a region implies an increasing prevalence of micropores. Previous
observations which lacked sufficient spatial resolution to discern
dark micropore "floors" from bright facular walls find a strong
non-linear dependence of facular contrast on magnetic flux density,
with decreasing contrast beyond a certain flux density. We show instead
that the observed contrast of bright facular walls is independent of
magnetic flux density when properly segmented from dark micropores. The
observations are useful for examining the detailed structure of faculae
including the dark lanes found on the disk-center side of many faculae
(explained by several recent 3D MHD numerical simulations). The average
radial profile for 678 faculae segmented from the dataset is very
nearly gaussian with a FWHM radial extent of 265 km and an extended
tail on the limbward side, as predicted by current MHD simulations.
---------------------------------------------------------
Title: Solar Image Restoration By Use Of Multi-frame Blind
De-convolution With Multiple Objects And Phase Diversity
Authors: Van Noort, Michiel; Der Voort, Luc Rouppe Van; Löfdahl,
Mats G.
2005SoPh..228..191V Altcode: 2005SoPh..228..191N
An extension of Joint Phase Diverse Speckle image restoration is
presented. Multiple realizations of multiple objects having known
wavefront relations with each other can now be restored jointly. As
the alignment of the imaging setup does not change, near-perfect
alignment can be achieved between different objects, thus greatly
reducing false signals in the determination of derived quantities,
such as magnetograms, Dopplergrams, etc. The method was implemented
in C++ as an image restoration server, to which worker clients can
connect and disconnect randomly, so that a large number of CPUs can be
used to speed up the restorations. We present a number of examples of
applications of the restoration method to observations obtained with
the Swedish 1-m Solar Telescope on La Palma.
---------------------------------------------------------
Title: Solar magnetic elements at 0.1 arcsec resolution. General
appearance and magnetic structure
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Löfdahl,
M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
Title, A.; Scharmer, G.
2004A&A...428..613B Altcode:
New observations of solar magnetic elements in a remnant active region
plage near disk center are presented. The observations were obtained at
the recently commissioned Swedish 1-m Solar Telescope on La Palma. We
examine a single 430.5 nm G-band filtergram that resolves ∼70 km
(0.1 arcsec) structures and find new forms of magnetic structures
in this particular region. A cotemporal Ca II H-line image is used
to examine the low-chromosphere of network elements. A cotemporal Fe
I 630.25 nm magnetogram that resolves structures as small as 120 km
(0.18 arcsec) FWHM with a flux sensitivity of approximately 130 Mx
cm<SUP>-2</SUP> quantifies the magnetic structure of the region. A
Ni I 676.8 nm Dopplergram establishes relative velocity patterns
associated with the network features with an accuracy of about 300 m
s<SUP>-1</SUP>. We find that magnetic flux in this region as seen in
both the magnetogram and the G-band image is typically structured into
larger, amorphous, “ribbons” which are not resolved into individual
flux tubes. The measured magnetic flux density in the ribbon structures
ranges from 300 to 1500 Mx cm<SUP>-2</SUP>, the higher values occurring
at localized concentrations embedded within the ribbons. The Dopplergram
indicates relative downflows associated with all magnetic elements
with some indication that higher downflows occur adjacent to the peak
magnetic flux location. The mean absolute magnetic flux density of the
remnant plage network is about 130 Mx cm<SUP>-2</SUP>; in the lowest
flux regions of the field-of-view, the mean absolute flux density is
approximately 60 Mx cm<SUP>-2</SUP>. Within these quiet regions we do
not find evidence of pervasive kilo-gauss strength magnetic elements
as seen in recent high resolution internetwork studies. In general,
the observations confirm recent 3-dimensional numerical simulations
which show that the magnetic field in high-density regions such as
plage is concentrated in complex structures that are not generally
composed of discrete magnetic flux tubes. <P />Appendices are only
available in electronic form at http://www.edpsciences.org
---------------------------------------------------------
Title: Observations of solar magnetic elements with 0.1" resolution
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Lofdahl,
M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
Title, A. M.; Scharmer, G.
2004AAS...204.2005B Altcode: 2004BAAS...36..686B
New observations of solar magnetic elements in a remnant active region
plage near disk center are presented. The observations were taken with
the Swedish 1-meter Solar Telescope on La Palma. We examine a single
430.5 nm G-band filtergram that resolves ∼70 km (0.”1) structures
and find new forms of magnetic structures in this particular region. A
simultaneous Ca II H-line image is used to examine the low-chromosphere
of network elements. A simultaneous Fe I 630.25 nm magnetogram
that resolves structures as small as 120 km (0.”18) FWHM with a
flux sensitivity of approximately 130 Mx cm<SUP>-2</SUP> quantifies
the magnetic structure of the region. A Ni I 676.8 nm Dopplergram
establishes relative velocity patterns associated with the network
features with an accuracy of about 300 m s<SUP>-1</SUP>. Magnetic flux
in this region as seen in both the magnetogram and the G-band image
is typically structured into larger, amorphous, “ribbons” with a
wide range of flux density values, rather than isolated kilogauss
flux tubes. We also present filtergrams and magnetograms of magnetic
elements at the solar limb showing that solar faculae are resolved
into bright granular walls that appear to project 350 to 500 km above
the photosphere.
---------------------------------------------------------
Title: Innovative Information Technology for Space Weather Research
Authors: Wang, H.; Qu, M.; Shih, F.; Denker, C.; Gerbessiotis, A.;
Lofdahl, M.; Rees, D.; Keller, C.
2004AAS...204.5209W Altcode: 2004BAAS...36..755W
Solar activity is closely related to the near earth environment --
summarized descriptively as space weather. Changes in space weather
have adverse effect on many aspects of life and systems on earth and in
space. Real-time, high-quality data and data processing would be a key
element to forecast space weather promptly and accurately. Recently,
we obtained a funding from US National Science Foundation to apply
innovative information technology for space weather prediction. <P />(1)
We use the technologies of image processing and pattern recognition,
such as image morphology segmentation, Support Vector Machines (SVMs),
and neural networks to detect and characterize three important solar
activities in real-time: filament eruptions, flares, and emerging flux
regions (EFRs). Combining the real time detection with the recent
statistical study on the relationship among filament eruptions,
flares, EFRs, coronal mass ejections (CMEs), and geomagnetic storms,
we are establishing real time report of solar events and automatic
forecasting of earth directed CMEs and subsequent geomagnetic storms. <P
/>(2) We combine state-of-art parallel computing techniques with phase
diverse speckle imaging techniques, to yield near real-time diffraction
limited images with a cadence of approximately 10 sec. We utilize the
multiplicity of parallel paradigms to optimize the calculation of phase
diverse speckle imaging to improve calculation speed. With such data,
we can monitor flare producing active regions continuously and carry
out targeted studies of the evolution and flows in flare producing
active regions. <P />(3) We are developing Web based software tools to
post our processed data, events and forecasting in real time, and to
be integrated with current solar activity and space weather prediction
Web pages at BBSO. This will also be a part of Virtual Solar Observatory
(VSO) being developed by the solar physics community. <P />This research
is supported by NSF ITR program.
---------------------------------------------------------
Title: Penumbral structure at 0.1 arcsec resolution. I. General
appearance and power spectra
Authors: Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Kiselman,
D.; Scharmer, G. B.
2004A&A...414..717R Altcode:
We analyse sunspot filtergrams of unprecedented quality obtained by
\citet{scharmer02dark} with the Swedish 1-m Solar Telescope on La
Palma. The observations comprise images in three different wavelength
bands: 488, 436, and 430 nm (G-band). We find that there are still
unresolved penumbral filaments which must have widths smaller than
80 km. The fine structuring along the filaments is limited. Penumbral
grains have internal structure and look like they are split or crossed
by narrow dark structures. We calculate intensity power spectra of the
penumbra from images that are corrected for seeing using the Phase
Diversity technique. The effects of high order aberrations that are
not corrected for are estimated to be too low to be consistent with
a flat power spectrum. The penumbral power spectra do not show any
signs of bumps or peaks that could correspond to a preferred scale
at 0\farcs35 for the width of penumbral filaments. We argue that the
power spectrum is not a very reliable source of information concerning
preferred scales.
---------------------------------------------------------
Title: Intensity Oscillations in the upper transition region above
active region plage
Authors: de Pontieu, B.; Erdelyi, R.; de Wijn, A.; Loefdahl, M.
2003AGUFMSH42B0540D Altcode:
Although there are now many observations showing the presence
of oscillations in the corona, almost no observational studies have
focused on the bright upper transition region (TR) emission (so-called
moss) above active region plage. Here we report on a wavelet analysis
of observations (made with TRACE, the Transition Region and Coronal
Explorer) of strong ( ∼ 5-15%) intensity oscillations in the upper
TR footpoints of hot coronal loops. They show a range of periods from
200 to 600 seconds, typically persisting for 4 to 7 cycles. These
oscillations are not associated with sunspots, as they usually occur at
the periphery of plage regions. A majority of the upper TR oscillations
are directly associated with upper chromospheric oscillations observed
in Hα , i.e., periodic flows in spicular structures. The presence of
such strong oscillations at low heights (of order 3,000 km) provides
an ideal opportunity to study the propagation of oscillations from
photosphere and chromosphere into the TR and corona, and improve
our understanding of the magnetic connectivity in the chromosphere
and TR. In addition, we use new high resolution observations of the
photosphere and chromosphere, taken with the Swedish Solar Telescope, to
shed light on the source of chromospheric mass flows such as spicules.
---------------------------------------------------------
Title: Observations of magnetoconvection in Sunspots with 100 km
resolution
Authors: Berger, T. E.; Löfdahl, M. G.; Scharmer, G.; Title, A. M.
2003SPD....34.1108B Altcode: 2003BAAS...35..828B
We present new observations from the Swedish 1-meter Solar Telescope
(SST) on La Palma with ∼0.1 arcsecond ( ∼100 km) resolution:
the highest resolution yet achieved in solar observations. We focus
on sunspot and active region magnetoconvective phenomena using G-band
4305 Å, 4877 Å continuum, 7507 Å TiO bandhead, and Ca II 3968 Å
H-line filtergram movies. The G-band data are post-processed using
Joint Phase Diverse Speckle wavefront restoration to create a full
diffraction limited time series. Sunspot light-bridges are shown to
have dark lanes less than 300 km in width that are coherent along
the entire length of the bridge. Similarly, we find elongated dark
“canals” in plage regions, particularly near pores, that appear to be
highly modified intergranular downflow lanes. The canals are less than
200 km in width and are much more coherent than intergranular lanes
in non-magnetic regions, often retaining their basic structure for
more than one granular turn-over time. Both the light-bridge central
lane and the canals appear to be the result of highly constrained
flow structure in strong magnetic field regions -- an aspect of solar
magnetoconvection that has not previously been observed. This reseach
was supported by funding from the Royal Swedish Academy of Sciences,
a SOHO Guest Investigator subcontract to California State University
Northridge, and the NASA TRACE contract NAS5-38099 at Lockheed Martin.
---------------------------------------------------------
Title: Adaptive optics system for the new Swedish solar telescope
Authors: Scharmer, Goran B.; Dettori, Peter M.; Lofdahl, Mats G.;
Shand, Mark
2003SPIE.4853..370S Altcode:
The 1-meter Swedish solar telescope is a new solar telescope that was
put in operation on the island of La Palma in the Canary Islands at the
end of May 2002. The goal of this telescope is to reach its diffraction
limited resolution of 0.1 arcsec in blue light. This has already
been achieved by use of a low-order adaptive optics (AO)system. This
paper describes the AO system initially developed for the former
50-cm Swedish Vacuum Solar Telescope (SVST) and further improved for
the new telescope. Both systems use a combination of bimorph modal
mirrors and Shack-Hartmann wavefront sensors. Unique to these systems
are that they rely on a single workstation or a PC to do all the
computations required to extract and pre-process the images, measure
their positions using cross correlation techniques and for controlling
the deformable mirror. This is in the present system possible by using
the PERR instruction available on Compaq's Alpha architecture and in
the new system using the PSADDBW instruction, available on Pentium 4
and Athlon processors. We describe both these systems with an emphasis
on the performance, the ease of support and upgrades of performance. We
also describe the optimization of the electrode geometry for the new
37-electrode bimorph mirror, supplied by AOPTIX Technologies, Inc.,
for controlling Karhunen--Loeve modes. Expected performance, based on
closed-loop simulations, is discussed.
---------------------------------------------------------
Title: Phase diverse speckle inversion applied to data from the
Swedish 1-meter solar telescope
Authors: Lofdahl, Mats G.; Scharmer, Goran B.
2003SPIE.4853..567L Altcode:
We report on the use of a new joint phase diverse speckle code,
an implementation of a method where a single object and individual
phases are estimated from several pairs of phase diverse data. The
code was used on 430.5 nm G-band data collected with the newly
installed Swedish 1-meter solar telescope in La Palma, equipped with
a low-order adaptive optics system. We describe the algorithm briefly,
show wavefront statistics and object estimates from the processing and
discuss the results. We demonstrate a resolution of 0.12 arc seconds
for a time sequence and a large field of view, which is a break-through
for ground based solar telescopes.
---------------------------------------------------------
Title: First Results from the Swedish 1-m Solar Telescope
Authors: Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Rouppe van
der Voort, L. H. M.
2003ASPC..307....3S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Multi-frame blind deconvolution with linear equality
constraints
Authors: Löfdahl, Mats G.
2002SPIE.4792..146L Altcode: 2002physics...9004L
The Phase Diverse Speckle (PDS) problem is formulated mathematically
as Multi Frame Blind Deconvolution (MFBD) together with a set of Linear
Equality Constraints (LECs) on the wavefront expansion parameters. This
MFBD-LEC formulation is quite general and, in addition to PDS, it
allows the same code to handle a variety of different data collection
schemes specified as data, the LECs, rather than in the code. It also
relieves us from having to derive new expressions for the gradient
of the wavefront parameter vector for each type of data set. The
idea is first presented with a simple formulation that accommodates
Phase Diversity, Phase Diverse Speckle, and Shack-Hartmann wavefront
sensing. Then various generalizations are discussed, that allows many
other types of data sets to be handled.
---------------------------------------------------------
Title: Dark cores in sunspot penumbral filaments
Authors: Scharmer, Göran B.; Gudiksen, Boris V.; Kiselman, Dan;
Löfdahl, Mats G.; Rouppe van der Voort, Luc H. M.
2002Natur.420..151S Altcode:
Sunspot umbrae-the dark central regions of the spots-are surrounded
by brighter filamentary penumbrae, the existence of which remains
largely inexplicable. The penumbral filaments contain magnetic fields
with varying inclinations and are associated with flowing gas, but
discriminating between theoretical models has been difficult because
the structure of the filaments has not hitherto been resolved. Here
we report observations of penumbral filaments that reveal dark cores
inside them. We cannot determine the nature of these dark cores,
but their very existence provides a crucial test for any model of
penumbrae. Our images also reveal other very small structures, in line
with the view that many of the fundamental physical processes in the
solar photosphere occur on scales smaller than 100km.
---------------------------------------------------------
Title: Observation and Modelling of Micropore Formation in Active
Network Regions
Authors: Berger, T. E.; Löfdahl, M. G.; Bercik, D. J.
2002AAS...200.9101B Altcode: 2002BAAS...34Q.953B
We present phase-diversity corrected G-band 4305 Å and 4364 Å
continuum image time series showing the formation of a micropore in
a small active region near disk center. The data were acquired at
the Swedish Vacuum Solar Telescope on La Palma in June of 1997 and
post-processed using the Phase Diverse Speckle (PDS) algorithm to
produce diffraction limited images throughout the majority of both
time series. The micropore dataset comprises a 29x29 Mm field of view
and spans 5.1 hours with a 38 second cadence. The micropore forms in
a strong sink area that can be seen to “collect" many G-band bright
points over the first 2 hours of the observation. During this time
there is an occasional darkening at the sink point that may be the
first unstable phase of the micropore formation. Once a stable dark
pore forms in the flowfield, it grows to a maximum diameter of 1.2 Mm
in approximately 1.9 hours. The pore persists for another 35 minutes
before apparently being broken up by the intergranular flowfield. The
total “lifetime" of the stable pore phase is 2.5 hours. A separate
nearby micropore of 1.5 Mm maximum diameter exists for the entire 5.2
hour data span. We show G-band and continuum movies of the micropore
formation, correlation tracking flowfield analyses, G-band bright
point tracking results, and area versus time plots for the micropore
formation lifetime. The observational data are compared with fully
compressible 3D MHD numerical simulations which show the development
of a similar micropore structure within the computational domain. This
research was supported by NASA SR&T grant NASW-98008, The Royal
Swedish Academy of Sciences, NSF and NASA funding at Michigan State
University, and Lockheed Martin IRAD funding.
---------------------------------------------------------
Title: Two dual-wavelength sequences of high-resolution solar
photospheric images captured over several hours and restored by use
of phase diversity
Authors: Löfdahl, M. G.; Berger, T. E.; Seldin, J. H.
2001A&A...377.1128L Altcode:
The collection, seeing compensation, and temporal filtering of two
high-resolution time-sequences of solar photospheric images are
described. 44arcsecx80 arcsec image sequences of co-spatial and
co-temporal 430.5 nm G band and 436.4 nm continuum filtergrams,
were obtained with the 47.5 cm Swedish Vacuum Solar Telescope. One
data set covers 5 hours of photospheric evolution; the other covers
8 hours. The field-of-view contains both an enhanced network region,
a large pore, and in the longer sequence, a pore forming event. The
mean time between frames is <40 s. With a few exceptions, the G
band frames are partitioned phase-diverse speckle (PPDS) restorations
of three realizations of the atmospheric turbulence acquired rapidly in
sequence. Due to strict simultaneity and closely spaced wavelengths, the
G band wavefronts, compensated for fixed aberration differences, could
also be used for deconvolving the corresponding continuum data. For
some of the data, collected during periods of particularly bad seeing,
restorations made with a related method, joint phase-diverse speckle,
were substituted for the PPDS restorations.
---------------------------------------------------------
Title: Workstation-based solar/stellar adaptive optics system
Authors: Scharmer, Goran B.; Shand, Mark; Lofdahl, Mats G.; Dettori,
Peter M.; Wei, Wang
2000SPIE.4007..239S Altcode:
The microprocessors used in off-the-shelf workstations double in
performance every eighteen months. The Swedish Vacuum Solar Tower (SVST)
uses off-the-shelf workstations for all aspects of its on-line telescope
control and data acquisition. Since 1995 workstation performance
has been adequate for a correlation tracker of solar granulation
controlling a tip- tilt corrector. In 2000 workstation performance
permits the construction of a 20 - 50 subimage Shack-Hartmann based low-
latency adaptive optics system. It is argued that workstations provide a
cost-effective, upgradable, low-risk and flexible means of construction
of stellar and solar adaptive optics systems. We give an overview
of the adaptive optics system installed at the SVST in May 1999. The
system uses a bimorph modal mirror with 19 electrodes from Laplacian
Optics. For use with extended targets, such as solar fine structure,
cross- correlations with 16 X 16-pixel sub-images are used. For use with
point sources, a centroiding algorithm is implemented. The work station
used is capable of completing all processing required by the adaptive
optics system in 0.5 ms (cross-correlations) or 0.3 ms (centroiding),
with potential for significant performance improvements.
---------------------------------------------------------
Title: Resolving piston ambiguities when phasing a segmented mirror
Authors: Lofdahl, Mats G.; Eriksson, Henrik
2000SPIE.4013..774L Altcode:
Wavefront sensing in monochromatic light is insensitive to segment
piston errors that are a whole number of waves. If the wavefront
sensing is performed in several wavelengths, this ambiguity can be
resolved. We give an algorithm for finding the correct phase, given
multiple measurements in different wavelengths. Using this algorithm,
the capture range of a wavefront sensor can be extended from on the
order of +/- (lambda) /2 in piston to several waves. This relaxes
the demands on an initial, coarse alignment method. The extended
capture range depends on the selection of wavelengths available for
phase measurements and the expected accuracy of the wavefront sensing
method used.
---------------------------------------------------------
Title: Predictor approach to closed-loop phase-diversity wavefront
sensing
Authors: Lofdahl, Mats G.; Scharmer, Goran B.
2000SPIE.4013..737L Altcode:
We present a novel and fast method for utilizing wavefront information
in closed-loop phase-diverse image data. We form a 2D object-independent
error function using the images at different focus positions together
with OTFs of the diffraction limited system. Each coefficient in an
expansion of the wavefront is estimated quickly and independently by
calculating the inner produce of a corresponding predictor function and
the error function. This operation is easy to parallelize. The main
computational burden is in pre- processing, when the predictors are
formed. This makes this method fast and therefore attractive for closed
loop operation. Calculating the predictors involves error function
derivatives with respect to the wavefront parameters, statistics of
the parameters, noise levels and other known characteristics of the
optical system. The predictors are optimized so that the RMS error in
the wavefront parameters is minimized rather than consistency between
estimated quantities with image data. We present simulation results that
are relevant to the phasing of segmented mirrors in a space telescope,
such as the NGST.
---------------------------------------------------------
Title: Calibration of a Deformable Mirror and Strehl Ratio
Measurements by Use of Phase Diversity
Authors: Löfdahl, Mats G.; Scharmer, Göran B.; Wei, Wang
2000ApOpt..39...94L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Orthogonalization of Basis Functions for Diagonalized
Wavefront Sensing
Authors: Löfdahl, M.
1999ASPC..183..320L Altcode: 1999hrsp.conf..320L
No abstract at ADS
---------------------------------------------------------
Title: Experimental results from a multiple telescope imaging array.
Authors: Kendrick, R. L.; Bell, R.; Benson, L.; Cuneo, P.; Duncan,
A. L.; Holmes, B.; Lofdahl, M.; Mitchell, K.; Reardon, B.; Sigler,
R.; Stone, R.; Stubbs, D.; Zarifis, V.
1999aero....5..415K Altcode:
A nine telescope imaging phased array is being demonstrated at the
Lockheed-Martin Advanced Technology Center in Palo Alto. The array
consists of nine afocal telescopes arranged in a y-formation that are
combined to a common focus in a Fizeau interferometer configuration. The
authors have demonstrated diffraction limited performance over a 150
micro-radian field of view with broadband (500 to 800 nm) illumination.
---------------------------------------------------------
Title: The Multi Aperture Imaging Array
Authors: Zarifis, V.; Bell, R. M., Jr.; Benson, L. R.; Cuneo, P. J.;
Duncan, A. L.; Herman, B. J.; Holmes, B.; Sigler, R. D.; Stone, R. E.;
Stubbs, D. M.; Kendrick, R. L.; Paxman, R. G.; Seldin, J. H.; Löfdahl,
M. G.
1999ASPC..194..278Z Altcode: 1999wfoi.conf..278Z
A multi-aperture telescope imaging phased array is being demonstrated at
the Lockheed Martin Advanced Technology Center in Palo Alto. The sparse
array consists of nine afocal telescopes that are combined to a common
focus in a Fizeau interferometer configuration. We have demonstrated
diffraction limited performance over a 150 micro-radian field of
view with broad band (550 to 800 nm) illumination. The relatively
high bandwidth closed loop phasing is achieved using phase diversity
wavefront sensing techniques. Applications using a filled array for
astronomical observations will also be discussed.
---------------------------------------------------------
Title: Dynamics of Magnetic Flux Elements in the Solar Photosphere
Authors: van Ballegooijen, A. A.; Nisenson, P.; Noyes, R. W.; Löfdahl,
M. G.; Stein, R. F.; Nordlund, Å.; Krishnakumar, V.
1998ApJ...509..435V Altcode: 1998astro.ph..2359V
The interaction of magnetic fields and convection is investigated in
the context of the coronal heating problem. We study the motions of
photospheric magnetic elements using a time series of high-resolution
G-band and continuum filtergrams obtained at the Swedish Vacuum
Solar Telescope at La Palma. The G-band images show bright points
arranged in linear structures (“filigree”) located in the lanes
between neighboring granule cells. We measure the motions of these
bright points using an object tracking technique, and we determine
the autocorrelation function describing the temporal variation of
the bright point velocity. The correlation time of the velocity is
about 100 s. To understand the processes that determine the spatial
distribution of the bright points, we perform simulations of horizontal
motions of magnetic flux elements in response to solar granulation
flows. Models of the granulation flow are derived from the observed
granulation intensity images using a simple two-dimensional model
that includes both inertia and horizontal temperature gradients; the
magnetic flux elements are assumed to be passively advected by this
granulation flow. The results suggest that this passive advection model
is in reasonable agreement with the observations, indicating that on
a timescale of 1 hr the flux tubes are not strongly affected by their
anchoring at large depth. Finally, we use potential-field modeling
to extrapolate the magnetic and velocity fields to larger height. We
find that the velocity in the chromosphere can be locally enhanced at
the separatrix surfaces between neighboring flux tubes. The predicted
velocities are several km s<SUP>-1</SUP>, significantly larger than
those of the photospheric flux tubes. The implications of these results
for coronal heating are discussed.
---------------------------------------------------------
Title: Fine Structures of Magnetic Field in Solar Quiet Region
Authors: Zhang, H.; Scharmer, G.; Lofdahl, M.; Yi, Z.
1998SoPh..183..283Z Altcode:
In this paper, we present a time series of Fe i λ5250.2 Å photospheric
filtergrams and corresponding magnetograms in a quiet region. The
relationship between fine structures of granulation and magnetic fields
is analyzed. It is found that although most bright filigree features in
photospheric filtergrams are related to corresponding magnetic features,
they are generally not cospatial. It is also found that some bright
features and their corresponding photospheric magnetic fields show
fast changes within several minutes.
---------------------------------------------------------
Title: Measurements of Solar Magnetic Element Dispersal
Authors: Berger, Thomas E.; Löfdahl, Mats G.; Shine, Richard A.;
Title, Alan M.
1998ApJ...506..439B Altcode:
The dispersal of magnetic elements in the solar photospheric flow
field is studied by tracking individual “magnetic bright points”
(MBPs) identified in a G-band 4305 Å filtergram time series obtained
at the 50 cm Swedish Vacuum Solar Telescope on La Palma, Spain. The time
series spans approximately 70 minutes with a field of view of 29" × 29"
near disk center. All images in the time series are restored to near
the telescope diffraction limit (~0.2" in the G band) using partitioned
phase diverse speckle techniques. Regions of enhanced magnetic network
and quiet Sun are examined. In the network region, automated tracking
of individual MBPs reveals approximately Gaussian diffusion, with
indications for slightly “superdiffusive” dispersal. The inferred
Gaussian diffusion coefficient is 60.4 +/- 10.9 km<SUP>2</SUP>
s<SUP>-1</SUP>. In the quiet-Sun region, local correlation tracking
velocity measurements show the dispersal of artificial tracers to be
non-Gaussian over most of our data set with indications of an asymptotic
approach to a 285 km<SUP>2</SUP> s<SUP>-1</SUP> Gaussian diffusion.
---------------------------------------------------------
Title: Fast phase diversity wavefront sensing for mirror control
Authors: Lofdahl, Mats G.; Duncan, Alan L.; Scharmer, Goran B.
1998SPIE.3353..952L Altcode:
We show with simulation experiments that closed-loop phase- diversity
can be used without numerical guard-bands for wavefront sensing
of low-order wavefronts from extended objects using broad-band
filters. This may allow real-time correction at high bandwidth for
certain applications. We also present a proper maximum likelihood
treatment of Shack- Hartmann data, which includes an imaging model to
extract curvature information from the lenslet images. We demonstrate
by simple simulations that this approach should allow higher-order
wavefront information to be extracted than with traditional
Shack-Hartmann wavefront sensing for a given number of lenslets.
---------------------------------------------------------
Title: Phase diversity experiment to measure piston misalignment on
the segmented primary mirror of the Keck II Telescope
Authors: Lofdahl, Mats G.; Kendrick, Richard L.; Harwit, Alex;
Mitchell, Keith E.; Duncan, Alan L.; Seldin, John H.; Paxman, Richard
G.; Acton, Daniel S.
1998SPIE.3356.1190L Altcode:
We are developing a technique to measure segment misalignment of large
telescopes based on wavefront estimation using phase-diverse images. We
report the current results of an experiment to measure piston errors on
the Keck II primary segmented mirror, through atmospheric turbulence,
using phase-diverse phase retrieval. The segment piston errors are
separated from the random turbulence by averaging phase estimates from
many frames. Phase estimates from real data collected with segments
intentionally moved in piston reproduce the observed speckle patterns
well. However, average phase maps do not reveal the segment piston
errors. Simulations show that the observed data were collected in
a regime of turbulence where the current algorithm often fails, but
would be expected to work very well when the adaptive optics system
is operating. There is reason to believe that we can eventually make
the algorithm work with these or similar data if apparent mismatches
between the data and our current imaging model are removed.
---------------------------------------------------------
Title: Preparation of a Dual Wavelength Sequence of High-Resolution
Solar Photospheric Images Using Phase Diversity
Authors: Löfdahl, Mats G.; E. Berger, Thomas; Shine, Richard S.;
Title, Alan M.
1998ApJ...495..965L Altcode:
The collection, seeing compensation, and temporal filtering of
a high-resolution time-sequence of solar photospheric images is
described. A 70 minute time series of cospatial and cotemporal G
band 4305 Å and wideband 4686 Å filtergrams was obtained with
the 50 cm Swedish Vacuum Solar Telescope on the island of La Palma,
Spain. The 29" × 70" field-of-view near disk center contains both
an enhanced network region and an (apparently) nonmagnetic “quiet”
region of granulation. The mean time between frames is 23.5 s. Each
frame is created with partitioned phase-diverse speckle restoration of
three realizations of the atmospheric turbulence acquired rapidly in
sequence. The result is high-resolution movies of the solar photosphere,
good enough to allow detection, tracking, and analysis of ~0.2" bright
points. This analysis is the subject of a companion paper.
---------------------------------------------------------
Title: Measurements of Solar Magnetic Element Motion from
High-Resolution Filtergrams
Authors: Berger, Thomas E.; Löfdahl, Mats G.; Shine, Richard S.;
Title, Alan M.
1998ApJ...495..973B Altcode:
Solar photospheric flowfield properties on sub-0.5" scales are
measured using local correlation tracking (LCT) and object tracking of
magnetic bright points (MBPs: photospheric bright points associated
with magnetic elements). The dataset is a 70 minute time series
of cospatial and cotemporal G-band 4305 Å and wideband 4686 Å
filtergrams obtained with the 50 cm Swedish Vacuum Solar Telescope on
the island of La Palma, Spain. We examine a 29" × 70" field of view
(FOV) near disk center and compare a 29" × 29" magnetic network
subfield and a 27" × 27" apparently nonmagnetic “quiet-Sun”
subfield. The mean time between frames is 23.75 s. Each frame is
created by partitioned phase-diverse speckle restoration of three
image pairs acquired rapidly in sequence. Angular resolution is ~0.4"
or less in all frames of the restored data set. <P />From LCT on a 0.4"
grid with 0.83" FWHM apodization, we find the flow speeds to be Rayleigh
distributed with a mode of 550 m s<SUP>-1</SUP> and an average of 950
m s<SUP>-1</SUP> in the network FOV; in the quiet FOV the modal speed
is 700 m s<SUP>-1</SUP> with a mean of 1100 m s<SUP>-1</SUP>. Within
the network FOV, a “magnetic region” defined by the loci of tracked
MBPs exhibit even greater alteration: rms contrast of the region is
8% higher in the G band compared to areas outside, LCT speeds are
reduced by a factor of 1.6, and the convective flow structures are
smaller and much more chaotic. Mesogranulation is entirely absent in
the magnetic region. The modal and mean speeds of 534 tracked MBPs
are 300 m s<SUP>-1</SUP> and 1470 m s<SUP>-1</SUP>, respectively. MBPs
split and merge with mean times of 320 and 404 s respectively. The mean
lifetime of MBPs in the data set is 9.33 minutes although approximately
5% of the objects can be tracked for the entire 70 minute duration of
the time series.
---------------------------------------------------------
Title: Phase-Diversity Restoration of two Simultaneous 70-minute
Photospheric Sequences.
Authors: Lofdahl, M. G.; Berger, T. E.; Shine, R. A.; Title, A. M.
1997SPD....28.0218L Altcode: 1997BAAS...29..896L
Seeing effects have been corrected in two cospatial and cotemporal
70-minute sequences of images collected in the G-band 4305 Angstroms and
wideband 4686 Angstroms. The data were obtained with the 50 cm Swedish
Vacuum Solar Telescope on the island of La Palma, Spain. The 29arcsecx
70arcsec field-of-view (FOV) near disk center contains both an enhanced
network region and an (apparently) non-magnetic “quiet” region of
granulation. The mean time between restored frames is 23.5 s. Each of
the 180 images is created with Phase-Diverse Speckle (PDS) imaging,
using two different focus positions sampled at the best three snapshots
of the atmospheric turbulence (seeing) during a 20-second selection
window. Wavefronts are estimated for each focused--defocused image
pair and a restored frame is produced from all six images. The average
resolution in the restored sequence is about 0farcs4 (corresponding to
spatial frequencies up to half the diffraction limit of the telescope),
which is good enough to allow detection of ~ 0farcs2 bright points. The
data is used for statistical measurements of magnetic element speed,
interaction frequency, and lifetime (see accompanying poster by
T. E. Berger et al). We show destretched and space-time filtered
movies of both the G-band and continuum images, as well as raw data
to demonstrate the effect of the restoration process. This work was
supported by NASA contracts NAS5-30386 at Stanford and NAS8-39747 and
Independent Research Funds at Lockheed-Martin. MGL was supported by
the Swedish Science Research Council.
---------------------------------------------------------
Title: Measurements of Magnetic Element Dynamics in the Network
Authors: Berger, T. E.; Lofdahl, M. G.; Shine, R. A.; Title, A. M.
1997SPD....28.0219B Altcode: 1997BAAS...29..896B
Statistical measurements of magnetic element speed, interaction
frequency, and lifetime in an enhanced network region at disk center
are presented. The primary dataset is a 70 min time series of G-band
4305 Angstroms filtergrams taken at the 50 cm Swedish Vacuum Solar
Telescope (SVST) on the island of La Palma, Spain. A second time
series of 4686 Angstroms wide-band continuum filtergrams, cospatial
and simultaneous to within several milliseconds with the G-band
images, is also studied. The field-of-view is near Sun center and
includes a region of enhanced network activity with many G-band bright
points. Both time series are corrected for seeing to very near the
telescope diffraction limit by the technique of Phase Diverse Speckle
(PDS) restoration (see accompanying poster by M. G. Lofdahl et al). We
show destretched and space-time filtered movies of both the G-band and
continuum images in two fields-of-view: a region of quiet granulation
and the region of enhanced network activity. Within the network, local
correlation tracking measurements on a 0\farcs4 grid show the RMS
speed to be 778 m s(-1) ; outside the network the RMS speed is 1168 m
s(-1) . Corkflow simulations show that normal convective flow patterns
(granulation and mesogranulation) are absent in the network. Magnetic
elements move with modal and mean speeds of 100 m s(-1) and 815 m s(-1)
, respectively. The mean interaction frequency (time between either
merging or splitting) is 220 s. The mean lifetime of tracked magnetic
elements in the network is 9.33 min although 5% of the elements are
tracked for the entire 70 min of the times series. This work was
supported by NASA contracts NAS5-30386 at Stanford and NAS8-39747 and
Independent Research Funds at Lockheed-Martin. MGL was supported by
the Swedish Science Research Council.
---------------------------------------------------------
Title: Evaluation of Phase-Diversity Techniques for Solar-Image
Restoration
Authors: Paxman, Richard G.; Seldin, John H.; Loefdahl, Mats G.;
Scharmer, Goeran B.; Keller, Christoph U.
1996ApJ...466.1087P Altcode:
Phase-diversity techniques provide a novel observational method for
overcoming the effects of turbulence and instrument-induced aberrations
in ground-based astronomy. Two implementations of phase-diversity
techniques that differ with regard to noise model, estimator,
optimization algorithm, method of regularization, and treatment of edge
effects are described. Reconstructions of solar granulation derived
by applying these two implementations to common data sets are shown to
yield nearly identical images. For both implementations, reconstructions
from phase-diverse speckle data (involving multiple realizations of
turbulence) are shown to be superior to those derived from conventional
phase-diversity data (involving a single realization). Phase-diverse
speckle reconstructions are shown to achieve near diffraction-limited
resolution and are validated by internal and external consistency tests,
including a comparison with a reconstruction using a well-accepted
speckle-imaging method.
---------------------------------------------------------
Title: Phase diversity wavefront sensing and image restoration
applied to high-resolution solar observations
Authors: Löfdahl, M. G.
1996PhDT........94L Altcode:
An implementation of a phase-diverse speckle imaging (PDS)
technique for reducing the effects of aberrations in solar images is
described. Such aberrations usually occur in the Earth's atmosphere
and in telescopes. PDS is a post-processing technique for measuring
such aberrations and for deblurring the images. <P />The code has
been extensively tested. Realistic simulations indicate that the
systematic errors are small. The wavefront and object estimates
calculated from real solar data, collected with the Swedish Vacuum
Solar Telescope (SVST), are spatially and temporally consistent with
expectations for anisoplanatism and the assumption of no evolution of
the solar features on the time scale of a few seconds. Implementation
invariance is demonstrated by comparison of the output with that of
a separately developed implementation. External reference for the
wavefront determination is provided by comparison with theoretical
predictions of temporal variation of the telescopic aberrations
at the SVST. High-quality image restorations can be made with much
less data than is needed by the more established method of speckle
interferometry. <P />The usefulness of the technique for astronomical
purposes is demonstrated by the successful restoration and analysis
of a 29"x29" 70-minute time sequence of solar granulation and bright
points. The resolution in the restored data is sufficient to allow
the evolution and motion of bright points to be followed in detail.
---------------------------------------------------------
Title: Wavefront sensing and image restoration from focused and
defocused solar images.
Authors: Löfdahl, M. G.; Scharmer, G. B.
1994A&AS..107..243L Altcode:
We have implemented a least-squares technique for recovering phase
information from simultaneously recorded focused and defocused
images. The inversions are made from small subfields in order to
deal with anisoplanatic image formation, such as occurring through
the Earth's atmosphere. Parameters corresponding to the alignment
of the focused-defocused images are determined simultaneously with
aberration parameters. Simulations show that the method can recover
wavefronts of up to 1/2 wave rms and that 15-21 Zernike coefficients
can be obtained from 3"x3"-5"x5" solar granulation images using a 50 cm
telescope and with noise levels of 0.4% of the average intensity. In
general, the accuracy of the restored images is better than expected
from the number of Zernike polynomials used to represent the wave
front. We have applied the method to sequences of 100 8-bit solar
granulation images obtained with the Swedish Vacuum Solar Telescope
(SVST) in La Palma using subfields of 3"x3"-5"x5". These data enable
a number of consistency tests, all of which demonstrate that the
technique works on real data. Using averaged images obtained from each
sequence we find that derived alignment parameters are consistent to
within 0.02" and that wavefronts derived from different subfields and
different sequences recorded close in time are virtually identical. The
wavefronts derived from averaged images are also virtually identical
to the average of wavefronts derived from individual images. These
measurements of telescope aberrations suggest that astigmatism and
coma are the major aberrations. These aberrations vary with time
in a way which is consistent with a major contribution from the
telescope objective and/or the first folding mirror of the alt-az
tower telescope. Wavefronts derived from individual images show 50-90%
correlation between Zernike coefficients 4-14 derived from nearby
but independent subfields. Mosaics of 13 x 11 independently derived
wavefronts from single images show smooth variations across a 12"x10"
field-of-view. These results are consistent with the impression
that degradation of image quality is more or less uniform across the
image. Restored images in a sequence show a high degree of consistency
and much more fine structure than the corresponding observed images, but
occasional fringe-like artifacts can be seen. Using the results of two
inversions to restore the scene removes such artifacts. We conclude that
this technique provides adequate wavefront information on telescopic
and atmospheric wavefront aberrations and substantial improvements
in image quality. The ease of implementation as well as its tolerance
to experimental errors and low cost makes it an excellent complement
to or even substitute for adaptive optics for many applications. The
technique is particularly well-suited for solar telescopes, where
wavefront sensing over a large field-of-view is important.
---------------------------------------------------------
Title: Application of phase-diversity to solar images
Authors: Lofdahl, Mats G.; Scharmer, Goran B.
1994SPIE.2302..254L Altcode:
We have implemented a least-squares technique for recovering phase
information and alignment parameters from simultaneously obtained
focused and defocused solar images. Small subfields are used,
in order to deal with anisoplanatism. The method is applied to
sequences of 100 8-bit solar granulation images. These data enable
a number of consistency tests, all of which demonstrate that the
technique works. Alignment parameters derived from averaged images
in a sequence are highly consistent and wavefronts derived from
different subfields and different sequences recorded close in time are
virtually identical. The wavefronts derived from averaged images are
also virtually identical to the average of wavefronts derived from
individual images. These aberrations vary with time in a way which
is consistent with a major contribution from the moving elements of
the alt-az tower telescope. Independently derived wavefronts from
single images show high correlation between neighboring subfields and
smooth variations across large fields-of-view, consistent with the
impression that the image quality is more or less uniform across the
image. Restored images in a sequence show a high degree of consistency
and much more fine structure than the corresponding observed images.
---------------------------------------------------------
Title: Phase-Diversity Restoration of Solar Images
Authors: Löfdahl, Mats G.; Scharmer, Göran B.
1993rtpf.conf...89L Altcode:
We have implemented a least-squares linearization technique for
recovering phase information from simultaneously recorded focused
and defocused solar images. The inversion is made from small
subfields, in order to deal with anisoplanatic image formation,
such as occuring through the earth's atmosphere. Simulations show
that the method can recover wavefronts of approximately 1/4 wave
rms and that noise levels corresponding to 8-bit images allow better
reconstruction of the images than of the derived wavefronts. We find
that cross-correlation techniques cannot be used to align the focused
and defocused images because of cross-talk with coma and similar
asymmetric aberrations. Therefore we simultaneously determine alignment
and aberration parameters. We have applied the method to 8-bit solar
granulation images obtained with the Swedish Vacuum Solar Telescope
in La Palma using subfields of 3".1 x 3".1. The least-squares fits
indicate wavefronts of approximately 0.12-0.16 waves rms. Derived
alignment parameters from different subfields and different images
are consistent, indicating that the wavefront is well determined by
the data. Wavefronts derived from different sub fields of a single
image in general show similar aberrations whereas derived wavefronts
from different images only show occasional similarities, indicating
a significant wavefront aberration from the lower atmosphere. The
reconstructed granulation images show enhanced fine structure and
an increase of the rms contrast by ~2% as compared to the observed
focused image, but the rms contrasts of consequtive images indicate
small errors in the derived rms wavefronts.
---------------------------------------------------------
Title: The ROYACS User Library.
Authors: Löfdahl, M.
1992lest.rept...65L Altcode:
The main purpose with the ROYACS User Library is to provide ANA users at
the Stockholm Observatory with suitable subprograms for data reduction
of solar images. Other useful routines have also been included in
order to improve the programming environment.
---------------------------------------------------------
Title: Swedish solar telescope: Short summary of instrumentation
and observation techniques
Authors: Scharmer, Goran; Lofdahl, Mats
1991AdSpR..11e.129S Altcode: 1991AdSpR..11..129S
A short summary of the design concepts of the Swedish Solar Telescope
at La Palma, is given along with the most important parts of the
instrumentation and observing techniques. Our experiences from using
high-speed read-out CCDs for solar observations are also discussed. The
advantages of this data aquisition system are that it allows real-time
frame selection for achieving high spatial resolution, that several
cameras can be slaved by one seeing monitor, and that bursts of digital
images can be recorded for full spatial coverage of small parts of
active regions.
