Optical turbulence vertical distribution with standard and high resolution at Mt Graham (original) (raw)
Related papers
Ground-based and Airborne Telescopes II, 2008
Since November 2004 we measured the optical turbulence (C 2 N profiles) with a Generalized Scidar (GS) placed at the focus of the Vatican Advanced Technology Telescope at Mt.Graham, Arizona. The present statistic consists in measurements related to 43 nights covering different periods of the solar year. In this paper we calculate the statistics of the astroclimatic parameters (C 2 N , seeing ε , isoplanatic angle θ 0 , wavefront coherence time τ 0 ) and we compare these values with those measured above other top level astronomic sites. All profiles are reduced into a form suitable to be used as inputs for adaptive optics point spread function simulations for the conceptual design of the Laser Guide Star Facility supported by a GLAO system of the Large Binocular Telescope. With GS measurements done observing wide binaries (30-35 arcsec), the turbulence in the first kilometer above the ground is characterized with the vertical resolution (200-250 m) required for the optimization of a 4 arcmin field of view AO system. It is the first time that are published measurements of the optical turbulence vertical distribution above a mid-latitude site with such a high vertical resolution and such a high statistical reliability. On 8 of those nights, employing cross-correlation scintillation maps of wide binaries and the method described in Ref.
On the normalization of scintillation autocovariance for generalized SCIDAR
Optics Express, 2009
The Generalized SCIDAR (Scintillation Detection and Ranging) technique consists in the computation of the mean autocorrelation of double-star scintillation images taken on a virtual plane located a few kilometers below the telescope pupil. This autocorrelation is normalized by the autocorrelation of the mean image. Johnston et al. in 2002 [1] pointed out that this normalization leads to an inexact estimate of the optical-turbulence strength C 2 N . Those authors restricted their analysis to turbulence at ground level. Here we generalize that study by calculating analytically the error induced by that normalization, for a turbulent layer at any altitude. An exact expression is given for any telescope-pupil shape and an approximate simple formula is provided for a full circular pupil. We show that the effect of the inexact normalization is to overestimate the C 2 N values. The error is larger for higher turbulent layers, smaller telescopes, longer distances of the analysis plane from the pupil, wider double-star separations, and larger differences of stellar magnitudes. Depending on the observational parameters and the turbulence altitude, the relative error can take values from zero up to a factor of 4, in which case the real C 2 N value is only 0.2 times the erroneous one. Our results can be applied to correct the C 2 N profiles that have been measured using the Generalized SCIDAR technique.
Atmospheric Intensity Scintillation of Stars, I. Statistical Distributions and Temporal Properties
Publications of the Astronomical Society of the Pacific, 1997
Stellar intensity scintillation in the optical was extensively studied at the astronomical observatory on La Palma (Canary Islands). Atmospheric turbulence causes "flying shadows" on the ground, and intensity fluctuations occur both because this pattern is carried by winds and is intrinsically changing. Temporal statistics and time changes were treated in Paper I, and the dependence on optical wavelength in Paper II. This paper discusses the structure of these flying shadows and analyzes the scintillation signals recorded in telescopes of different size and with different (secondary-mirror) obscurations. Using scintillation theory, a sequence of power spectra measured for smaller apertures is extrapolated up to very large (8 m) telescopes. Apodized apertures (with a gradual transmission falloff near the edges) are experimentally tested and modeled for suppressing the most rapid scintillation components. Double apertures determine the speed and direction of the flying shadows. Challenging photometry tasks (e.g., stellar microvariability) require methods for decreasing the scintillation "noise." The true source intensity I(l) may be segregated from the scintillation component in DI(t,l,x,y) postdetection computation, using physical modeling of the temporal, chromatic, and spatial properties of scintillation, rather than treating it as mere "noise." Such a scheme ideally requires multicolor high-speed (Շ10 ms) photometry on the flying shadows over the spatially resolved (Շ10 cm) telescope entrance pupil. Adaptive correction in real time of the two-dimensional intensity excursions across the telescope pupil also appears feasible, but would probably not offer photometric precision. However, such "second-order" adaptive optics, correcting not only the wavefront phase but also scintillation effects, is required for other critical tasks such as the direct imaging of extrasolar planets with large ground-based telescopes. * scint
Recalibrated generalized SCIDAR measurements at Cerro Paranal (the site of the Very Large Telescope)
Monthly Notices of the Royal Astronomical Society, 2012
Generalized SCIDAR (GS) measurements, which were taken at the Paranal Observatory in 2007 November/December in the context of a site qualification for the future European Extremely Large Telescope, are recalibrated to overcome the bias induced on the C 2 N profiles by an incorrect normalization of the autocorrelation of the scintillation maps, which has recently been identified in the GS technique. A complete analysis of the GS-corrected measurements and the corrected errors is performed statistically as well as on individual nights, and for each time period during the nights. The relative errors of the C 2 N profiles can reach up to 60 per cent in some narrow temporal windows and some vertical slabs, with the total seeing up to 12 per cent and the total integrated turbulence J up to 21 per cent. However, the statistical analysis shows that the absolute error of the median values of the total seeing is 0.06 arcsec (relative error 5.6 per cent); for the boundary seeing it is 0.05 arcsec (relative error 5.6 per cent) and for the seeing in the free atmosphere it is 0.04 arcsec (relative error 9 per cent). We find that, in spite of the fact that the relative error increases with the height, the boundary and the free atmosphere seeing contribute in an equivalent way to the error on the total seeing in absolute terms. Moreover, we find that there are no correlations between the relative errors and the value of the corresponding seeing. The absolute error of the median value of the isoplanatic angle is 0.13 arcsec (relative error 6.9 per cent).
On the comparison between MASS and generalized-SCIDAR techniques
Monthly Notices of the Royal Astronomical Society, 2014
The Multi Aperture Scintillation Sensor (MASS) and the Generalized-Scintillation Detection and Ranging (Generalized SCIDAR) are two instruments conceived to measure optical turbulence (OT) vertical distribution on the whole troposphere and low stratosphere (∼ 20 km) widely used in the astronomical context. In this paper we perform a detailed analysis/comparison of measurements provided by the two instruments and taken during the extended site testing campaign carried out on 2007 at Cerro Paranal and promoted by the European Southern Observatory (ESO). The main and final goal of the study is to provide a detailed estimation of the measurements reliability i.e dispersion of turbulence measurements done by the two instruments at different heights above the ground. This information is directly related to our ability in estimating the absolute value of the turbulence stratification. To better analyze the uncertainties between the MASS and the GS we took advantage of the availability of measurements taken during the same campaign by a third independent instrument (DIMM -Differential Imaging Motion Monitor) measuring the integrated turbulence extended on the whole 20 km. Such a cross-check comparison permitted us to define the reliability of the instruments and their measurements, their limits and the contexts in which their use can present some risk.
The oriented scintillation spectrometer experiment on GRO
Advances in Space Research, 1991
Key results obtained by the OSSE instrument during the rst four years of the Compton Gamma Ray Observatory mission are presented. OSSE has undertaken extended observations of the gamma ray emission from the galactic center region and found the positron annihilation radiation to be consistent with a two-component model: a spheroidal component located at the galactic center and a w eaker galactic disk component. Simultaneous observations with the SIGMA imaging instrument h a v e provided the rst low-energy gamma ray spectrum of the di use continuum emission from the galactic center region. Results on galactic sources include the spectral observations of two new rotation-powered pulsars, PSR 1509-58 and Vela, the discovery of 110 keV cyclotron emission from the Be X-ray binary A0535+26, and the discovery that galactic black hole transients have t w o spectral classes: thermal and power-law. Extragalactic sources also have two spectral classes. Seyfert galaxies are typi ed by thermal spectral with exponential cuto s from 50-300 keV, and blazars which exhibit power-law spectra that extend into the EGRET energy range. Blazar spectra also often have a spectral break in the MeV region. OSSE has also obtained several observations of supernovae, including the rst detection of 57 Co from SN 1987A, hard X-ray emission from a shock-heated pre-SN wind in SN 1993J, and upper limits for 44 Ti and 56 Co emission from Cas A and SN 1991T resp. Finally, recent observations to con rm the COMPTEL observation of 4.4 and 6.1 MeV line emission from the Orion region have provided only upper limits, thereby placing constraints on the intensity and or distribution of the emission.
Two-scale solution for atmospheric scintillation
Journal of the Optical Society of America A, 1985
We demonstrate that a recently derived approximate solution to the fourth-moment equation that was thought to be valid only for strong scattering is in fact valid for all values of the scattering parameter. We use a modified Kolmogorov spectrum for the index-of-refraction fluctuations and present results in two dimensions for comparison with numerical solutions. We also present results in three dimensions that should represent a quantitatively accurate, theoretical description of the atmospheric-scintillation problem. d -A-M = JJ__ H(s, w)J(, -w, )d&l
Remote Sensing
The storm onset on 7 September 2017, triggered several variations in the ionospheric electron density, causing severe phase fluctuations at polar latitudes in both hemispheres. In addition, although quite rare at high latitudes, clear amplitude scintillations were recorded by two Global Navigation Satellite System receivers during the main phase of the storm. This work attempted to investigate the physical mechanisms triggering the observed amplitude scintillations, with the aim of identifying the conditions favoring such events. We investigated the ionospheric background and other conditions that prevailed when the irregularities formed and moved, following a multi-observations approach. Specifically, we combined information from scintillation parameters and recorded by multi-constellation (GPS, GLONASS and Galileo) receivers located at Concordia station (75.10°S, 123.35°E) and SANAE IV base (71.67°S, 2.84°W), with measurements acquired by the Special Sensor Ultraviolet Spectrograp...