Fundamental and Applied Aspects of Astronomical “Seeing”

Coulman, C. E.

doi:10.1146/annurev.aa.23.090185.000315

Cited by 99 publications

(48 citation statements)

References 80 publications

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“…Kolmogorov turbulence, which describes how kinetic energy injected at an outer length scale rout cascades to increasingly smaller scales until finally dissipated at an inner length scale rin, predicts β = 5/3 in the domain rin r rout. This scaling has been demonstrated to be a reasonable approximation for the ISM over scales r ∼ 10 2 km to > 1 AU (Armstrong, Rickett & Spangler 1995), and also for the troposphere with r < ∆h, where ∆h is the thickness of the turbulent layer (Coulman 1985;Carilli & Holdaway 1997).…”

Section: Theoretical Backgroundmentioning

confidence: 77%

meqsilhouette: a mm-VLBI observation and signal corruption simulator

Blecher

Deane

Bernardi

et al. 2016

Mon. Not. R. Astron. Soc.

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The Event Horizon Telescope (EHT) aims to spatially resolve the silhouette (or shadow) of the supermassive black holes in the Galactic Centre (Sgr A ) and M87. The primary scientific objectives are to test general relativity in the strong-field regime and to probe accretion and jet-launch physics at event-horizon scales. This is made possible by the technique of Very Long Baseline Interferometry (VLBI) at (sub)millimetre wavelengths, which can achieve angular resolutions of order ∼ 10 µ-arcsec. However, this approach suffers from unique observational challenges, including scattering in the troposphere and interstellar medium; rapidly time-variable source structure in both polarized and total intensity; as well as non-negligible antenna pointing errors. In this, the first paper in a series, we present the MeqSilhouette software package which is specifically designed to accurately simulate EHT observations. It includes realistic descriptions of a number of signal corruptions that can limit the ability to measure the key science parameters. This can be used to quantify calibration requirements, test parameter estimation and imaging strategies, and investigate systematic uncertainties that may be present. In doing so, a stronger link can be made between observational capabilities and theoretical predictions, with the goal of maximising scientific output from the upcoming order of magnitude increase in EHT sensitivity.

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Section: Theoretical Backgroundmentioning

confidence: 77%

meqsilhouette: a mm-VLBI observation and signal corruption simulator

Blecher

Deane

Bernardi

et al. 2016

Mon. Not. R. Astron. Soc.

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“…This theory has been developed and applied extensively to problems of optical seeing [e.g., Roddier (1981), Woolf (1982), Coulman (1985)] and to infrared interferometry (Sutton et al 1982). We confine the discussion here to a few central ideas concerning the structure function of phase and indicate how it is related to other functions that are used to characterize atmospheric turbulence.…”

Section: Kolmogorov Turbulencementioning

confidence: 99%

Propagation Effects: Neutral Medium

Thompson

Moran

Swenson

2017

Astronomy and Astrophysics Library

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The neutral gas in the atmosphere has a significant effect on signals passing through it. We are concerned with three types of effects. First, the large-scale structures in the media give rise to refractive effects. These effects, which can be analyzed in terms of geometrical optics and Fermat's principle, are the deflection of the radio waves and the change of the propagation velocity. Second, radiation can be absorbed. Finally, radiation can be scattered by the turbulent structure of the media. The phenomenon of scattering results in scintillation, or seeing.In the troposphere, water vapor plays a particularly important role in radio propagation. The refractivity of water vapor is about 20 times greater in the radio range than in the near-infrared or optical regimes. The phase fluctuations in radio interferometers at centimeter, millimeter, and submillimeter wavelengths are caused predominantly by fluctuations in the distribution of water vapor. Water vapor is poorly mixed in the troposphere, and the total column density of water vapor cannot be accurately sensed from surface meteorological measurements. Uncertainties in the water vapor content are a serious limitation to the accuracy of VLBI measurements. Small-scale (< 1 km) fluctuations in water vapor distribution limit the angular resolution of connected-element interferometers in the absence of wavefront correction techniques. Furthermore, spectral lines of water vapor cause substantial absorption at frequencies above 100 GHz and usually render the troposphere highly opaque at frequencies between 1 and 10 THz (300 and 30 m). Thus, any discussion of the neutral atmosphere must be primarily concerned with the effects of water vapor. Propagation in the neutral atmosphere from the point of view of radio communications is discussed by Crane (1981) and Bohlander et al. (1985).Our interest in the propagation media arises because the media degrade interferometric measurements of radio sources. Alternately, observations of radio sources can be used to probe the characteristics of the propagation media. Radio interferometric measurements have been used widely for this purpose.

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“…See, for example, [5] for discussion of this type of turbulent mixing, also [6] and references therein. We are interested in S g (|f |), which is somewhat trickier to calculate than S T (|f |), since it involves a correlation between points separated in space as well as time.…”

Section: Appendix A: the Temperature Noise Spectrummentioning

confidence: 99%

Tumbleweeds and airborne gravitational noise sources for LIGO

Creighton¹

2008

Class. Quantum Grav.

104

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The test masses in gravitational-wave detectors will be sensitive not only to astrophysical gravitational waves, but also to the fluctuating Newtonian gravitational forces of moving masses in the ground and air around the detector. These effects are often referred to as gravity gradient noise. This paper considers the effects of gravity gradients from density perturbations in the atmosphere, and from massive airborne objects near the detector. These have been discussed previously by Saulson, who considered the effects of background acoustic pressure waves and of massive objects moving smoothly past the interferometer; the gravity gradients he predicted would be too small to be of serious concern even for advanced interferometric gravitational-wave detectors. In this paper I revisit these phenomena, considering transient atmospheric shocks, and estimating the effects of sound waves or objects colliding with the ground or buildings around the test masses. I also consider another source of atmospheric density fluctuations: temperature perturbations that are advected past the detector by the wind. I find that background acoustic noise and temperature fluctuations still produce gravity gradient noise that is below the noise floor even of advanced interferometric detectors, although temperature perturbations carried along non-laminar streamlines could produce noise that is within an order of magnitude of the projected noise floor at 10 Hz. A definitive study of this effect may require better models of the wind flow past a given instrument. I also find that transient shockwaves in the atmosphere could potentially produce large spurious signals, with signal-to-noise ratios in the hundreds in an advanced interferometric detector. These signals could be vetoed by means of acoustic sensors outside of the buildings. Massive wind-borne objects such as tumbleweeds could also produce gravity gradient signals with signal-to-noise ratios in the hundreds if they collide with the interferometer buildings, so it may be necessary to build fences preventing such objects from approaching within about 30m of the test masses.

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Fundamental and Applied Aspects of Astronomical “Seeing”

Cited by 99 publications

References 80 publications

meqsilhouette: a mm-VLBI observation and signal corruption simulator

meqsilhouette: a mm-VLBI observation and signal corruption simulator

Propagation Effects: Neutral Medium

Tumbleweeds and airborne gravitational noise sources for LIGO

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