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A. Ziad R. Gredel J. Aceituno J. Borgnino F. Hoyo A. Irbah F. Martin U. Thiele S. Pedraz 《Monthly notices of the Royal Astronomical Society》2005,362(2):455-459
The main atmospheric optical parameters have been measured at the Calar Alto Observatory simultaneously using the Generalized Seeing Monitor (GSM) and a Differential Image Motion Monitor (DIMM) during several nights in 2002 May. The temporal evolution of the seeing, the outer scale, the isoplanatic angle and the coherence time have been analysed. There is excellent agreement between the seeing measurements provided by the two instruments, particularly when the turbulence is slow. Indeed, the GSM measurements are corrected from the exposure time when the DIMM data were recorded for at least 5 ms. From almost three years of DIMM (at 5 m height above ground) data, a seeing of 0.92 arcsec with a standard deviation of 0.31 arcsec has been obtained for this site. The outer scale , the isoplanatic angle θ0 and the coherence time τ0 measured with the GSM are well fitted with log-normal distributions with median values of 22.9 m, 2.27 arcsec and 3.7 ms, respectively. 相似文献
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Shin Oya Hiroshi Terada Yutaka Hayano Makoto Watanabe Masayuki Hattori Yosuke Minowa 《Experimental Astronomy》2016,42(1):85-98
Stellar images have been obtained under natural seeing at visible and near-infrared wavelengths simultaneously through the Subaru Telescope at Mauna Kea. The image quality is evaluated by the full-width at the half-maximum (FWHM) of the stellar images. The observed ratio of FWHM in the V-band to the K-band is 1.54 ± 0.17 on average. The ratio shows tendency to decrease toward bad seeing as expected from the outer scale influence, though the number of the samples is still limited. The ratio is important for simulations to evaluate the performance of a ground-layer adaptive optics system at near-infrared wavelengths based on optical seeing statistics. The observed optical seeing is also compared with outside seeing to estimate the dome seeing of the Subaru Telescope. 相似文献
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In this empirical study, we compare high-resolution observations obtained with the 65-cm vacuum reflector at Big Bear Solar
Observatory (BBSO) in 2005 and with the Dunn Solar Telescope (DST) at the National Solar Observatory/Sacramento Peak (NSO/SP)
in 2006. We measure the correction of the high-order adaptive optics (AO) systems across the field of view (FOV) using the
spectral ratio technique, which is commonly employed in speckle masking imaging, and differential image motion measurements.
The AO correction is typically much larger (10′′ to 25′′) than the isoplanatic angle and can be described by a radially symmetric function with a central core and extended wings.
The full-width at half-maximum (FWHM) of the core represents a measure of the AO correction. The average FWHM values for BBSO
and NSO/SP are 23.5′′ and 18.2′′, respectively. The extended wings of the function show that the AO systems still contribute to an improved speckle reconstruction
at the periphery of the 80′′×80′′ FOV. The major differences in the level of AO correction between BBSO and NSO/SP can be explained by different contributions
of ground-layer- and free-atmosphere-dominated seeing, as well as different FOVs of the wavefront sensors. In addition, we
find an anisotropic spectral ratio in sunspot penumbrae caused by the quasi-one-dimensional nature of penumbral filaments,
which introduces a significant error in the estimation of the Fourier amplitudes during the image restoration process. 相似文献
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Chris Benn 《New Astronomy Reviews》2006,49(10-12):625
NAOMI is the AO system of the 4.2-m William Herschel Telescope on La Palma. It delivers an AO-corrected image to a lenslet array at the focal plane of the optical integral-field spectrograph OASIS. The resulting 1100 spectra are imaged onto a high-QE, low-fringing MITLL3 CCD. A range of spectroscopic and spatial (0.09–0.42 arcsec/lenslet) configurations is available. At wavelength 0.7 μm, the NAOMI-corrected FWHM is typically half that of the natural seeing. Scheduled OASIS observing began in semester 2004B, with 9 programmes awarded a total of 26 nights during the first year of operation. A Rayleigh laser guide star is under development, with first light expected summer 2006. In conjunction with NAOMI/OASIS, this will provide a unique facility: AO-corrected optical integral-field spectroscopy anywhere on the northern sky. 相似文献
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There is a collection of about 100 years of Ca-K line spectroheliograms at the Kodaikanal Observatory (KKL) obtained on daily basis with a single instrument that can be used to study long term variations of various chromospheric features. All the Ca-K images have been digitized using specially developed digitizers with uniform and highly stable light source, high quality lens and 4k×4k format CCD camera. The digitization has been carried out in a room with controlled temperature and humidity. The digitized data are in 16-bit format with pixel resolution of 0.86 arcsec. The digitized images have been calibrated by a process that includes flat-fielding, density to intensity conversion, centering the image, and rotation of the image to make the solar north pole in the fixed direction. Then we applied correction for the limb darkening effect and also made the background in the image uniform. The image background was normalized to unity that enabled us to use the intensity contrast to identify different features, such as plages, enhanced (EN), active (AN), and quite network on images and classified them by using different image contrast and area threshold values. After several experiments with different threshold values for different features and careful analysis of a large number of images, we could fix the threshold values of intensity contrast larger than 1.35 and area larger than 1 arcmin2 for plages, larger than 1.35 but area less than 1 arcmin2 for EN, and between 1.25?–?1.35 for AN. We compared the quarterly averaged and half yearly averaged plage areas obtained from KKL with the Mount Wilson (MWO) data and sunspot number. We find that the plage area extracted from the KKL is highly correlated with the MWO plage area, though there is a slight difference between the two data set in cycle 19. The plage area of KKL is also highly correlated with the sunspot number. The areas of EN and AN are also found to have smaller quasi-periodic variations apart from the solar cycle variations. 相似文献
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P. N. Brandt 《Solar physics》1969,7(2):187-203
The frequency analysis of image motion (IM) at the solar limb was carried out in the frequency range from 0.5 to 50 Hz using a photoelectric equipment. For a telescopic aperture of 35 cm and a bandwidth of 0.65 Hz a typical frequency spectrum under average observing conditions shows a decrease of amplitude from 2 arcsec at 0.5 Hz to 0.4 arcsec at 5 Hz, 0.03 arsec at 50 Hz (and < 0.01 arcsec at 500 Hz). Visually estimated values of image steadiness seem to be in better agreement with the r.m.s. value of image motion (scattering parameter ) than with the amplitude at a certain frequency (Figures 5a, b). The influence of IM on the quality of photographic pictures or on spectra of solar fine structures is calculated as a function of exposure time. Table II gives the IM scattering parameters (0.01 arcsec to 4 arcsec) calculated for exposure times from 0.001 to 0.5 sec — valid for a time average. The modulation transfer functions (MTF, one-dimensional) derived from the IM scattering parameters are presented in Figure 7 together with the MTF for a diffraction-limited telescope of 35 cm aperture at 6000 Å. Exposure times of less than approximately 0.01 sec (certain within a factor of 2) render the influence of IM negligible compared to the MTF of the objective used for this investigation.Mitteilungen aus dem Fraunhofer Institut Nr. 88. 相似文献
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Michael Dopita John Hart Peter McGregor Patrick Oates Gabe Bloxham Damien Jones 《Astrophysics and Space Science》2007,310(3-4):255-268
This paper describes the Wide Field Spectrograph (WiFeS) under construction at the Research School of Astronomy and Astrophysics
(RSAA) of the Australian National University (ANU) for the ANU 2.3 m telescope at the Siding Spring Observatory. WiFeS is
a powerful integral field, double-beam, concentric, image-slicing spectrograph designed to deliver excellent throughput, wavelength
stability, spectrophotometric performance and superb image quality along with wide spectral coverage throughout the 320–950 nm
wavelength region. It provides a 25×38 arcsec field with 0.5 arcsec sampling along each of twenty five 38×1 arcsec slitlets.
The output format is optimized to match the 4096×4096 pixel CCD detectors in each of two cameras individually optimized for
the blue and the red ends of the spectrum, respectively. A process of “interleaved nod-and-shuffle” will be applied to permit
quantum noise-limited sky subtraction. Using VPH gratings, spectral resolutions of 3000 and 7000 are provided. The full spectral
range is covered in a single exposure at R=3000, and in two exposures in the R=7000 mode. The use of transmissive coated optics, VPH gratings and optimized mirror coatings ensures a throughput (including
telescope atmosphere and detector) >30% over a wide spectral range. The concentric image-slicer design ensures an excellent
and uniform image quality across the full field. To maximize scientific return, the whole instrument is configured for remote
observing, pipeline data reduction, and the accumulation of calibration image libraries. 相似文献
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Simon P. Driver Warrick J. Couch Steven Phillipps & Rodney Smith 《Monthly notices of the Royal Astronomical Society》1998,301(2):357-368
We present detailed simulations of long-exposure CCD images. The simulations are used to explore the validity of the statistical method for reconstructing the luminosity distribution of galaxies within a rich cluster, i.e., by the subtraction of field number-counts from those of a sight-line through the cluster. In particular, we use the simulations to establish the reliability of our observational data to be presented in Paper III. Based on our intended CCD field-of-view (6.5 × 6.5 arcmin2 ) and a 1σ detection limit of 26 mag arcsec−2 , we conclude that the luminosity distribution can be robustly determined over a wide range of absolute magnitude (−23 < M R < −16) provided: (a) the cluster has an Abell richness 1.5 or greater; (b) the redshift of the cluster lies in the range 0.1 < z < 0.3; (c) the seeing is better than FWHM 1.25 arcsec, and (d) the photometric zero-points are accurate to within Δ m = ± 0.12. If these conditions are not met, then the recovered luminosity distribution is unreliable. Finally, although the method clearly has limitations, within these limitations the technique represents an extremely promising probe of galaxy evolution and environmental dependences. 相似文献
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A. Tokovinin T. Travouillon 《Monthly notices of the Royal Astronomical Society》2006,365(4):1235-1242
New measurements of optical turbulence profile at the Cerro Pachón observatory in Chile are analysed jointly with previously published data to model the variations of the intensity and thickness of the ground layer and free atmosphere under a variety of observing conditions. This work is motivated by the need to predict statistically the performance of ground-layer adaptice optics. We find that the ground-layer profile can be represented by a decaying exponent with a scale height of 20–40 m, increasing to 100 m under bad conditions. The zone from 6 to 500 m contributes typically about 61 per cent to the total integral, the latter causing a median seeing of 0.77 arcsec. Turbulence integrals in the ground layer and in free atmosphere vary independently of each other, in 50 per cent of cases they deviate by less than 1.8 times from their respective median values. The existence of periods with low turbulence in the free atmosphere and their importance for adaptive optics is stressed. 相似文献
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Strong evidence that Io's sodium emission is due to resonant scattering is given by our observations which show a monotonic increase of emission intensity with residual solar intensity. In addition we detected no emission during three eclipse observations of Io. We propose a resonant scattering model with two spacial components comprising an optically thick atmosphere extending 103 km above Io's surface surrounded by an optically thin cloud which forms a partial torus around Jupiter. In this model a flux of 107 cm?2 sec?1 sodium atoms are sputtered from Io's surface by heavy energetic ions which are accelerated in a plasma sheath around Io. The atoms sputtered from the surface collide with atoms in Io's atmosphere so the equipartition of kinetic energy is established. The total sodium abundance is about 3 × 1013 cm?2. During Io's day, sodium and other atmospheric constituents are ionized, giving rise to the ionosphere observed by Pioneer 10. Atoms escape by means of Jeans escape from the critical level, which is at the top of the atmosphere and the base of the cloud. We have observed sodium emission 6arcsec (6 Io diameters) above and below Io's orbital plane and 23arcsec toward Jupiter in Io's orbital plane. No emission was detected at maximum elongation 180° from Io. We interpret these results to mean that atoms escaping from Io form a partial torus whose thickness is about 12 arcsec and whose length is at least one-fifth of Io's orbital circumference. 相似文献
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The Imaging Magnetograph eXperiment (IMaX) for?the?Sunrise Balloon-Borne Solar Observatory 总被引:1,自引:0,他引:1
V. Mart��nez?Pillet J. C. del Toro Iniesta A. ��lvarez-Herrero V. Domingo J. A. Bonet L. Gonz��lez?Fern��ndez A. L��pez?Jim��nez C. Pastor J. L. Gasent?Blesa P. Mellado J. Piqueras B. Aparicio M. Balaguer E. Ballesteros T. Belenguer L. R. Bellot?Rubio T. Berkefeld M. Collados W. Deutsch A. Feller F. Girela B. Grauf R. L. Heredero M. Herranz J. M. Jer��nimo H. Laguna R. Meller M. Men��ndez R. Morales D. Orozco?Su��rez G. Ramos M. Reina J. L. Ramos P. Rodr��guez A. S��nchez N. Uribe-Patarroyo P. Barthol A. Gandorfer M. Knoelker W. Schmidt S. K. Solanki S. Vargas?Dom��nguez 《Solar physics》2011,268(1):57-102
The Imaging Magnetograph eXperiment (IMaX) is a spectropolarimeter built by four institutions in Spain that flew on board the Sunrise balloon-borne solar observatory in June 2009 for almost six days over the Arctic Circle. As a polarimeter, IMaX uses fast polarization modulation (based on the use of two liquid crystal retarders), real-time image accumulation, and dual-beam polarimetry to reach polarization sensitivities of 0.1%. As a spectrograph, the instrument uses a LiNbO3 etalon in double pass and a narrow band pre-filter to achieve a spectral resolution of 85 mÅ. IMaX uses the high-Zeeman-sensitive line of Fe i at 5250.2 Å and observes all four Stokes parameters at various points inside the spectral line. This allows vector magnetograms, Dopplergrams, and intensity frames to be produced that, after reconstruction, reach spatial resolutions in the 0.15??C?0.18 arcsec range over a 50×50 arcsec field of view. Time cadences vary between 10 and 33 s, although the shortest one only includes longitudinal polarimetry. The spectral line is sampled in various ways depending on the applied observing mode, from just two points inside the line to 11 of them. All observing modes include one extra wavelength point in the nearby continuum. Gauss equivalent sensitivities are 4 G for longitudinal fields and 80 G for transverse fields per wavelength sample. The line-of-sight velocities are estimated with statistical errors of the order of 5??C?40 m?s?1. The design, calibration, and integration phases of the instrument, together with the implemented data reduction scheme, are described in some detail. 相似文献
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D. A. Schwartz D. E. Harris H. Landt A. Siemiginowska H. L. Marshall J. M. Gelbord E. S. Perlman M. Georganopoulos M. Birkinshaw D. M. Worrall C. C. Cheung L. Stawarz S. G. Jorstad A. P. Marscher Y. Uchiyama C. M. Urry 《Astrophysics and Space Science》2007,311(1-3):341-345
We present a Chandra image of the quasar, jet, and lobes of PKS 1354+195 (=4C 19.44). The radio jet is 18 arcsec long, and appears to be very
straight. The length gives many independent spatial resolution elements in the Chandra image while the straightness implies that the geometrical factors are constant along the jet although their values are uncertain.
We also have 4 frequency radio images with half to one arcsecond angular resolution, and use HST and Spitzer data to study
the broad band spectral energy distributions. The X-ray and radio spectra are both consistent with a spectrum f
ν
∝
ν
−0.7 for the integrated jet. Using that spectral index, the model of inverse Compton scattering of electrons on the cosmic microwave
background (IC/CMB) gives magnetic field strengths and Doppler factors that are relatively constant along the jet. Extended
X-ray emission is evident in the direction of the otherwise unseen counter-jet. X-ray emission continues past the radio jet
to the South, and is detected within both the southern and northern radio lobes. 相似文献
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T. Shimizu S. Nagata S. Tsuneta T. Tarbell C. Edwards R. Shine C. Hoffmann E. Thomas S. Sour R. Rehse O. Ito Y. Kashiwagi M. Tabata K. Kodeki M. Nagase K. Matsuzaki K. Kobayashi K. Ichimoto Y. Suematsu 《Solar physics》2008,249(2):221-232
The Hinode Solar Optical Telescope (SOT) is the first space-borne visible-light telescope that enables us to observe magnetic-field
dynamics in the solar lower atmosphere with 0.2 – 0.3 arcsec spatial resolution under extremely stable (seeing-free) conditions.
To achieve precise measurements of the polarization with diffraction-limited images, stable pointing of the telescope (<0.09 arcsec,
3σ) is required for solar images exposed on the focal plane CCD detectors. SOT has an image stabilization system that uses image
displacements calculated from correlation tracking of solar granules to control a piezo-driven tip-tilt mirror. The system
minimizes the motions of images for frequencies lower than 14 Hz while the satellite and telescope structural design damps
microvibration in higher frequency ranges. It has been confirmed from the data taken on orbit that the remaining jitter is
less than 0.03 arcsec (3σ) on the Sun. This excellent performance makes a major contribution to successful precise polarimetric measurements with 0.2 – 0.3 arcsec
resolution.
K. Kobayashi now at NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA. 相似文献
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Aidan J. ROSS Andrew STEELE Marc D. FRIES Lukas KATER Hilary DOWNES Adrian P. JONES Caroline L. SMITH Peter M. JENNISKENS Michael E. ZOLENSKY Muawia H. SHADDAD 《Meteoritics & planetary science》2011,46(3):364-378
This work is the first detailed study of carbon phases in the ureilite Almahata Sitta (sample #7). We present microRaman data for diamond and graphite in Almahata Sitta, seven unbrecciated ureilites, and two brecciated ureilites. Diamond in Almahata Sitta was found to be distinct from that in unbrecciated and brecciated ureilites, although diamond in unbrecciated and brecciated ureilites is indistinguishable. Almahata Sitta diamond shows a peak center range of 1318.5–1330.2 cm?1 and a full width at half maximum (FWHM) range of 6.6–17.4 cm?1, representing a shock pressure of at least 60 kbar. The actual peak shock pressure may be higher than this due to postshock annealing, if shock synthesis is the source of ureilite diamonds. Diamond in unbrecciated and brecciated ureilites have peak center wave numbers closer to terrestrial kimberlite diamond, but show a wider range of FWHM than Almahata Sitta. The larger peak shift observed in Almahata Sitta may indicate the presence of lonsdaleite. Alternatively, the lower values in brecciated ureilites may be evidence of an annealing step either following the initial diamond‐generating shock or as a consequence of heating during reconsolidation of the breccia. Graphite in Almahata Sitta shows a G‐band peak center range of 1569.1–1577.1 cm?1 and a G‐band FWHM range of 24.3–41.6 cm?1 representing a formation temperature of 990 ± 120 °C. Amorphous carbon was also found. We examine the different theories for diamond formation in ureilites, such as chemical vapor deposition and shock origin from graphite, and explore explanations for the differences between Almahata Sitta and other ureilites. 相似文献
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The aim of the present study is to investigate the short-term periodicity in the solar radius measurements and to compare
with the short periods in sunspot numbers, sunspot areas and flare index data. The spectral analysis of data sets covering
a time interval from 26 February 2000 to 26 October 2007 during Solar Cycle 23 were made by using the Date Compensated Discrete
Fourier Transform (DCDFT). The power spectrum of solar radius data corrected for the seeing effect gives an evident peak at
25.7 days with the amplitude of 0.034 arcsec, which is slightly different from the peaks of 26.2 and 26.7 days produced by
sunspot numbers and sunspot areas data, respectively. Besides, the main peak of 25.7 days detected in the power spectrum of
solar radius data is in agreement with the period of 25.5 days, suggested to be the fundamental period of the Sun by Bai and
Sturrock (in Nature
350, 141, 1991). 相似文献