We study the distribution of noise in images synthesized by radio interferometers, focusing particularly on the noise due to source power fluctuations. For a total power interferometer, the r.m.s. fluctuation in the image consists of a term that is constant all over the map, which is the uncorrelated system noise divided by the number of telescopes in the interferometer, and a term that is the image itself. For a correlation interferometer the expression is not so simple but is qualitatively similar. Our results are consistent with intuitive expectations in various observing situations. We find similarities in self-noise at optical and radio wavelengths.

We discuss the distribution of source- and receiver- noise in radio synthesis images and show that the source-noise is maximum at the position of the source but also appears in the off-source region because of the sidelobes. Analytical expressions are derived for the rms noise at any location of both “total-power” and “correlation” images. We show that under certain conditions, deconvolution can remove the source noise from the off-source region in snap-shot images. Some of the results are verified experimentally.

The interferometric mapping of astronomical objects relies on the van-Cittert Zernike theorem, one of the major results of the theory of partially coherent light [see, Bom and Wolf (1980), chapter 10]. This theorem states that the degree of spatial coherence of the field from a distant spatially incoherent source is proportional to the Fourier transform of the intensity distribution across the source. Measurement of the degree of spatial coherence, by, for example, measuring the visibility of interference fringes, allows the object to be mapped by making an inverse Fourier transform. (For a full description of this technique see Thompson, Moran and Swenson, 1986.)

In this paper I present a summary of the results an investigation into what happens when the distant source is not spatially coherent (James, 1990). Using a heuristic model of a spherically symmetric partially coherent source, an analytic expression for the error in the measurement of the effective radius, expressed as a function of coherence area, can be obtained.

The conceptual guidelines in designing mm radio-telescope antennas are enlightened by some recent progress in their mechanical construction, and opto-radio-geometrical adjustment. This paper reviews concepts, illustrated in many existing solutions, including the new “phaseretrieval” holography for single dish adjustment (D.Morris, IRAM, 1982): 2 amplitude only maps at focus and out of focus, with enough dynamic range and signal to noise ratio, give surface errors within hours.

History of correlators for radio interferrometer is briefly reviewed. Advantages and disadvantages of various methods are discussed and the most appropriate ways are given for various interferrometers. Challenges to the next generation short-wavelength interferrometer are also discussed.

Optical fibres have been used extensively within the Australia Telescope to carry IF data, LO reference, timing signals and computer communications. This paper discusses the various options considered in the design of a system which should assist similar projects in making the best use of optical fibres.

The Westerbork Synthesis Radio Telescope will be equipped with new front ends. These front ends will cover 8 frequency bands in the range from 250 MHz to 8.6 GHz. For the frequency bands above 1.2 GHz the sensitivity of the instrument will be drastically improved. Two independent local oscillator systems make it possible to observe in two frequency bands simultaneously.

The dynamic range of an image obtained using an interferometer array depends on the (interferometer) gain closure errors. This paper discusses hardware imperfections which cause closure errors and how these errors can be minimised.

In pulse calibration of an interferometer a series of short pulses is inserted into the receiver input to produce a spectrum of equi-spaced lines. The use of these for calibration, and the proposed system for the VLBA, are described.

A review of the recent developments in the design, construction and performance of cryogenicallycoolable, high-electron-mobility transistor (HEMT, MODFET) amplifiers and their application in compact cryogenic receivers for radio astronomy applications is presented.

Magnetic tape recorders developed for VLBI have taken advantage of the best available technology to achieve high data rates and large storage capacity. The Mark I, II and III VLBI recording systems have data rates of 0.72, 4 and 224 Mbits/sec respectively. The recorder developed for the VLBA has a data rate of 256 Mbits/sec and can store 5.5 Terabits of data on one tape.

The technique of Burst Mode (BM) VLBI is described and its particular application to the observations of pulsars is explored. The wideband pulsed radiation from these objects is exploited to acquire data at two ends of an interferometer baseline in high-speed bursts and only during the pulsar on-times. The result at the VLBI correlator is an improvement in sensitivity by as much as the inverse pulsar duty cycle, and is a consequence of the bandwidth expansion offered by BM and the intrinsic use of gating. As a result, precision astrometrical studies of pulsars will benefit through the greatly increased instantaneous bandwidths achieved by pulsar BM. The problems, implications, and limitations of acquiring and processing pulsar BM data are discussed.

A common problem in producing interferometer maps of objects with structure on the scale-size of the primary beam is the acquisition of UV data corresponding to telescope spacings of less than one telescope diameter. One technique for solving this “short-spacing” problem is to divide the aperture of one of the telescopes into smaller sub-apertures and measuring the visibilities between these sub-apertures. This technique is being tested using the Owens Valley Millimeter Interferometer.

FFT based 2D Dygital Lens is discussed comparing with Fourier synthesis. The sensitivity of constructing Large Array will be 50mJy. Precise manipuration of the radiation field is possible in it.

Phase and amplitude is controlled by manipulating the digitized radiation field. Application to the phase switching, and the calibration of phase and amplitude is discussed.

Interferometrics has developed a portable holographic antenna measurement system. The system is comprised of a small interferometer which uses satellite signals to measure the complex beam pattern of the antenna under test. A PC controls all aspects of data collection and analysis. The system produces maps of surface profile errors and the illumination power in reflector antennas.

New VLBI systems are under development in many countries in preparation for large dedicated arrays, including orbiting radio telescopes to be launched in the 1990s. In particular, several new recording systems and correlators are being constructed: the VLBA in the U.S., the S2 in Canada, and the K4/VSOP in Japan. This paper reviews the issues of technical compatibility among these systems in an effort to facilitate global VLBI experiments.

The International VLBI Satellite is under assessment study by the European Space Agency and the USSR Academy of Sciences. The IVS mission is conceived as a second generation space VLBI mission with the goal of providing higher sensitivity, greater maximum resolution and a wider spectral coverage compared to the precursor missions and, in addition, some single-dish capability.