A. Richard Thompson

Interference and Radioastronomy

Radioastronomy began in 1933 with Karl Janskys accidental discovery of radio emission from the center of the Galaxy as he was studying the effect of thunderstorms on transatlantic telecommunications. Six decades later, telecommunications and other radio services are threatening the future of radioastronomy. Whether searching for the signature of a protogalaxy, studying the maseremission signposts of star formation or pursuing answers to any of the myriad questions of modern astrophysics, todays radioastronomer is often frustrated by man‐made radio interference. From satellites, radar, radio and television transmitters, and wireless personal communication systems of all sorts to microwave ovens, computers and even garage door openers, the same technology that lets us study the universe at radio wavelengths is producing a flood of man‐made signals. Figure 1 illustrates how noisy the radio spectrum has become at wavelengths around 1 meter. (The remedy offered in the figure is, alas, just an astronomers fa...

Radio Frequencies: Policy and Management

The electromagnetic spectrum is a valued shared resource. Its scientific use allows us to learn about our universe, measure and monitor our planet, and communicate scientific data. The use of the spectrum is managed by national, regional, and global regulatory frameworks. There are increasing demands for new or extended allocations because of vast technological advances in the past few years. Understanding spectrum management is important in the successful planning and execution of missions and instruments, as well as in determining the potential source of radio frequency interference in existing data and instruments, and in working to ameliorate its impact. This paper provides a summary of this framework for radio scientists and engineers.

Antennas and Arrays

This chapter opens with a brief review of some basic considerations of antennas. The main part of the chapter is concerned with the configurations of antennas in interferometers and synthesis arrays. It is convenient to classify array designs as follows: 1. Arrays with nontracking antennas 2. Interferometers and arrays with antennas that track the sidereal motion of a source: Linear arrays Arrays with open-ended arms (crosses, T-shaped arrays, and Y-shaped arrays) Arrays with closed configurations (circles, ellipses, and Reuleaux triangles) VLBI arrays Planar arrays.

High-Resolution Imaging Spectroscopy at Terahertz Frequencies

HISAT, a multi-element heterodyne interferometer attached to Space Station Freedom, will provide spectroscopic images with unprecedented detail of those submillime-ter lines of C, O and C+ which are critical diagnostics of UV excitation in the Galaxy. With the arcsecond angular resolution achievable from the space station, HISAT will reveal: — The distribution of sources of ultraviolet radiation in the Galaxy; — The effective temperature of the UV radiation as a function of galactocentric radius; — The chemical and isotopic enhancement of atomic carbon and oxygen with galactic radius; — The propagation of UV radiation in molecular clouds and its stimulative, or inhibitive, effect on star formation; —The density structure, dumpiness or fragmentation, of molecular clouds throughout the Galaxy. HISAT has been selected by NASA for a concept-phase study.

Very-Long-Baseline Interferometry

The technique of very long baseline interferometry (VLBI) has undergone two decades of steady growth and refinement since its inception in 1967. In the beginning, only crude measurements of visibility on single baselines were possible. Now 18-station arrays have been used to produce images with dynamic ranges exceeding 2000:1; relative motions of cosmic masers have been tracked at the microarcsecond level of accuracy; and angular size measurements have been made with baseline lengths up to 2 two earth diameters with an orbiting satellite as a receiving element.

Propagation Effects: Ionized Media

Three distinct ionized media, or plasmas, affect the propagation of radio signals passing through them: the Earth’s ionosphere; the interplanetary medium, also known as the solar wind; and the interstellar medium of our Galaxy. The effects of scattering in other galaxies or in the media between galaxies are not usually important. There are several essential differences between neutral and ionized media with regard to propagation. For neutral media, the index of refraction is greater than unity and is unaffected by magnetic fields. In ionized media, the index of refraction is less than unity and is strongly affected by magnetic fields. Most plasma phenomena scale as ν−2, and their effects can be avoided or mitigated, if desired, by observations at high frequency. Absorption plays an important role in neutral media but very little in ionized media since most radio astronomical observations occur at frequencies far above the plasma frequency. Descriptions of scattering phenomena in both types of media are based on Kolmogorov theory. However, the situation in the neutral troposphere is greatly simplified because the turbulent layer lies close to the observer, and only phase fluctuations develop. The ionized media lie far from the observer, and both phase and amplitude fluctuations are often present in the wavefront when it reaches the observer.

Calibration and Imaging

This chapter is concerned with the calibration and Fourier transformation of visibility data, mainly as applied to Earth-rotation synthesis. Methods for the evaluation of the visibility measurements on a rectangular grid of points, necessary for the use of the discrete Fourier transform as implemented with the fast Fourier transform (FFT) algorithm, are discussed. Phase and amplitude closure conditions, which are valuable calibration tools, are also described. Analysis of the causes of certain types of image defects is given. Special consideration is given for certain observing modes, such as spectral line, and conversion of frequency to velocity is described. In addition, methods of extracting astronomical information directly from visibility data by model fitting are described. These techniques are important even with arrays having excellent (u, v) coverage. Some methods of calculating Fourier transforms before the advent of the FFT are discussed in Appendix 10.3.

Further Imaging Techniques

This chapter is concerned with techniques of processing that are largely nonlinear and include deconvolution, that is removing, to the extent possible, the limitations of the visibility measurements. There are two principal deficiencies in the visibility data that limit the accuracy of synthesis images. These are (1) the limited distribution of spatial frequencies in u and v and (2) errors in the visibility measurements. The limited spatial frequency coverage can be improved by deconvolution processes such as CLEAN that allow the unmeasured visibility to take nonzero values within some general constraints on the image. Calibration can be improved by adaptive techniques in which the antenna gains, as well as the required image, are derived from the visibility data. Wide-field imaging, multifrequency imaging, and compressed sensing are also discussed.

Propagation Effects: Neutral Medium

The neutral gas in the atmosphere has a significant effect on signals passing through it.We are concernedwith three types of effects.

Geometrical Relationships, Polarimetry, and the Interferometer Measurement Equation

In this chapter, we start to examine some of the practical aspects of interferometry. These include baselines, antenna mounts and beam shapes, and the response to polarized radiation, all of which involve geometric considerations and coordinate systems. The discussion is concentrated on Earth-based arrays with tracking antennas, which illustrate the principles involved, although the same principles apply to other systems such as those that include one or more antennas in Earth orbit.