300 research outputs found
Atmosphere-like turbulence generation with surface-etched phase-screens
We built and characterized an optical system that emulates the optical
characteristics of an 8m-class telescope like the Very Large Telescope. The
system contains rotating glass phase-screens to generate realistic
atmosphere-like optical turbulence, as needed for testing multi-conjugate
adaptive optics systems. In this paper we present an investigation of the
statistical properties of two phase-screens etched on glass-plate surfaces,
obtained from Silios Technologies. Those etched screens are highly transmissive
(above 85%) from 0.45 to 2.5 microns. From direct imaging, their Fried
parameter r0 values (0.43+-0.04 mm and 0.81+-0.03 mm, respectively, at 0.633
microns) agree with the expectation to within 10%. This is also confirmed by a
comparison of measured and expected Zernike coefficient variances. Overall, we
find that those screens are quite reproducible, allowing sub-millimetre r0
values, which were difficult to achieve in the past. We conclude that the
telescope emulator and phase-screens form a powerful atmospheric turbulence
generator allowing systematic testing of different kinds of AO instrumentation.Comment: 10 pages, 8 figures, 3 mpeg movies. Submitted to Optics Expres
First high-resolution images of the Sun in the 2796 \AA{} Mg II k line
We present the first high-resolution solar images in the Mg II k 2796 \AA{}
line. The images, taken through a 4.8 \AA{} broad interference filter, were
obtained during the second science flight of SUNRISE in June 2013 by the SuFI
instrument. The Mg II k images display structures that look qualitatively very
similar to images taken in the core of Ca II H. The Mg II images exhibit
reversed granulation (or shock waves) in the internetwork regions of the quiet
Sun, at intensity contrasts that are similar to those found in Ca II H. Very
prominent in Mg II are bright points, both in the quiet Sun and in plage
regions, particularly near disk center. These are much brighter than at other
wavelengths sampled at similar resolution. Furthermore, Mg II k images also
show fibril structures associated with plage regions. Again, the fibrils are
similar to those seen in Ca II H images, but tend to be more pronounced,
particularly in weak plage.Comment: Accepted for publication in The Astrophysical Journal Letter
SUNRISE/IMaX observations of convectively driven vortex flows in the Sun
We characterize the observational properties of the convectively driven
vortex flows recently discovered on the quiet Sun, using magnetograms,
Dopplergrams and images obtained with the 1-m balloon-borne Sunrise telescope.
By visual inspection of time series, we find some 3.1e-3 vortices/(Mm^2 min),
which is a factor of 1.7 larger than previous estimates. The mean duration of
the individual events turns out to be 7.9 min, with a standard deviation of 3.2
min. In addition, we find several events appearing at the same locations along
the duration of the time series (31.6 min). Such recurrent vortices show up in
the proper motion flow field map averaged over the time series. The typical
vertical vorticities are <= 6e-3 1/sec, which corresponds to a period of
rotation of some 35 min. The vortices show a preferred counterclockwise sense
of rotation, which we conjecture may have to do with the preferred vorticity
impinged by the solar differential rotation.Comment: To appear in ApJL. 5 Figs, 4 pages. The two animations associated
with the work can be downloaded from
http://www.iac.es/proyecto/solarhr/imaxvortex.html References updated in V
Ground-Based Coronagraphy with High Order Adaptive Optics
We summarize the theory of coronagraphic optics, and identify a dimensionless
fine-tuning parameter, F, which we use to describe the Lyot stop size in the
natural units of the coronagraphic optical train and the observing wavelength.
We then present simulations of coronagraphs matched to adaptive optics (AO)
systems on the Calypso 1.2m, Palomar Hale 5m and Gemini 8m telescopes under
various atmospheric conditions, and identify useful parameter ranges for AO
coronagraphy on these telescopes. Our simulations employ a tapered, high-pass
filter in spatial frequency space to mimic the action of adaptive wavefront
correction. We test the validity of this representation of AO correction by
comparing our simulations with recent K-band data from the 241-channel Palomar
Hale AO system and its dedicated PHARO science camera in coronagraphic mode.Comment: To appear in ApJ, May 2001 (28 pages, 10 figs
Surface waves in solar granulation observed with {\sc Sunrise}
Solar oscillations are expected to be excited by turbulent flows in the
intergranular lanes near the solar surface. Time series recorded by the IMaX
instrument aboard the {\sc Sunrise} observatory reveal solar oscillations at
high resolution, which allow studying the properties of oscillations with short
wavelengths. We analyze two times series with synchronous recordings of Doppler
velocity and continuum intensity images with durations of 32\thinspace min and
23\thinspace min, resp., recorded close to the disk center of the Sun to study
the propagation and excitation of solar acoustic oscillations. In the Doppler
velocity data, both the standing acoustic waves and the short-lived,
high-degree running waves are visible. The standing waves are visible as
temporary enhancements of the amplitudes of the large-scale velocity field due
to the stochastic superposition of the acoustic waves. We focus on the
high-degree small-scale waves by suitable filtering in the Fourier domain.
Investigating the propagation and excitation of - and -modes with wave
numbers \thinspace 1/Mm we find that also exploding granules
contribute to the excitation of solar -modes in addition to the contribution
of intergranular lanes.Comment: 12 pages, 4 figures, to appear in a special volume on Sunrise in
Astrophysical Journal Letter
Ground-based coronagraphy with high-order adaptive optics
We simulate the actions of a coronagraph matched to diffraction-limited adaptive optics (AO) systems on the Calypso 1.2 m, Palomar Hale 5 m and Gemini 8.1 m telescopes, and identify useful parameter ranges for AO coronagraphy on these systems. We model the action of adaptive wavefront correction with a tapered, high-pass filter in spatial frequency rather than a hard low frequency cutoff, and estimate the minimum number of AO channels required to produce sufficient image quality for coronagraphic suppression within a few diffraction widths of a central bright object (as is relevant to e.g., brown dwarf searches near late-type dwarf stars). We explore the effect of varying the occulting image- plane stop size and shape, and examine the trade-off between throughput and suppression of the image halo and Airy rings. We discuss our simulations in the context of results from the 241-channel Palomar Hale AO coronagraph system, and suggest approaches for future AO coronagraphic instruments on large telescopes
Fully resolved quiet-Sun magnetic flux tube observed with the Sunrise IMaX instrument
Until today, the small size of magnetic elements in quiet Sun areas has
required the application of indirect methods, such as the line-ratio technique
or multi-component inversions, to infer their physical properties. A consistent
match to the observed Stokes profiles could only be obtained by introducing a
magnetic filling factor that specifies the fraction of the observed pixel
filled with magnetic field. Here, we investigate the properties of a small
magnetic patch in the quiet Sun observed with the IMaX magnetograph on board
the balloon-borne telescope Sunrise with unprecedented spatial resolution and
low instrumental stray light. We apply an inversion technique based on the
numerical solution of the radiative transfer equation to retrieve the
temperature stratification and the field strength in the magnetic patch. The
observations can be well reproduced with a one-component, fully magnetized
atmosphere with a field strength exceeding 1 kG and a significantly enhanced
temperature in the mid- to upper photosphere with respect to its surroundings,
consistent with semi-empirical flux tube models for plage regions. We therefore
conclude that, within the framework of a simple atmospheric model, the IMaX
measurements resolve the observed quiet-Sun flux tube.Comment: Accepted for publication in The Astrophysical Journal Letters on Aug
11 201
Bright points in the quiet Sun as observed in the visible and near-UV by the balloon-borne observatory Sunrise
Bright points (BPs) are manifestations of small magnetic elements in the
solar photosphere. Their brightness contrast not only gives insight into the
thermal state of the photosphere (and chromosphere) in magnetic elements, but
also plays an important role in modulating the solar total and spectral
irradiance. Here we report on simultaneous high-resolution imaging and
spectropolarimetric observations of BPs using Sunrise balloon-borne observatory
data of the quiet Sun at disk center. BP contrasts have been measured between
214 nm and 525 nm, including the first measurements at wavelengths below 388
nm. The histograms of the BP peak brightness show a clear trend toward broader
contrast distributions and higher mean contrasts at shorter wavelengths. At 214
nm we observe a peak brightness of up to five times the mean quiet-Sun value,
the highest BP contrast so far observed. All BPs are associated with a magnetic
signal, although in a number of cases it is surprisingly weak. Most of the BPs
show only weak downflows, the mean value being 240 m/s, but some display strong
down- or upflows reaching a few km/s.Comment: Accepted for publication in The Astrophysical Journal Letters on
September 08 201
Transverse component of the magnetic field in the solar photosphere observed by Sunrise
We present the first observations of the transverse component of photospheric
magnetic field acquired by the imaging magnetograph Sunrise/IMaX. Using an
automated detection method, we obtain statistical properties of 4536 features
with significant linear polarization signal. Their rate of occurrence is 1-2
orders of magnitude larger than values reported by previous studies. We show
that these features have no characteristic size or lifetime. They appear
preferentially at granule boundaries with most of them being caught in downflow
lanes at some point in their evolution. Only a small percentage are entirely
and constantly embedded in upflows (16%) or downflows (8%).Comment: Accepted for the Sunrise Special Issue of ApJ
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