Summary A new type of interferometer for measuring the diameter of discrete radio sources is described and its mathematical theory is given. The principle of the instrument is based upon the correlation between the rectified outputs of two independent receivers at each end of a baseline, and it is shown that the cross-correlation coefficient between these outputs is proportional to the square of the amplitude of the Fourier transform of the intensity distribution across the source. The analysis shows that it should be possible to operate the new instrument with extremely long baselines and that it should be almost unaffected by ionospheric irregularities.

A New type of interferometer for use in radio astronomy

We describe the Navy Prototype Optical Interferometer (NPOI), a joint project of the Naval Research Laboratory (NRL) and the US Naval Observatory (USNO) in cooperation with Lowell Observatory. The NPOI has recently begun operations at the Lowell Observatory site near Flagstaff, Arizona, obtaining its first images, of a binary star, in 1996 May and June and its first limb-darkening observations during 1996 November to 1997 February. This paper gives an overview of the NPOI, including the characteristics of optical interferometry that affect its design. The NPOI includes subarrays for imaging and for astrometry. The imaging subarray consists of six moveable 50 cm siderostats feeding 12 cm apertures, with baseline lengths from 2.0 to 437 m. The astrometric subarray consists of four fixed 50 cm siderostats feeding 12 cm apertures (35 cm apertures to be installed in 1998), with baseline lengths from 19 m to 38 m. The shared back end covers 450-850 nm in 32 channels. The NPOI features vacuum feed and delay systems, active group-delay fringe tracking, and a high degree of automation. The astrometric subarray also includes an extensive site laser metrology system to measure the motions of the siderostats with respect to one another and to the bedrock. For imaging stellar surfaces, arrays with equal spacing between elements are superior to arrays that have been laid out to optimize (u, v) coverage and that therefore have unequal spacing. The imaging subarray of the NPOI provides a number of equally spaced configurations with linear scales at ratios of ≈ 1.64. Unequally spaced configurations are available for a variety of other imaging programs. Coherence across either type of imaging configuration is maintained by "phase bootstrapping": the phases on the longest baselines, on which fringes may be too weak to track, are stabilized by tracking fringes on the shortest baselines. In principle, the four elements of the astrometric subarray provide enough independent baselines to solve for stellar positions and the array geometry simultaneously while observing each of 11 stars only once. The anticipated magnitude limit is 7 mag or better with 12 cm apertures and average seeing; with 35 cm apertures, we expect the limit to be one or more magnitudes fainter. The anticipated wide-angle astrometric precision of the NPOI is ≈ 2 mas. The best angular resolution of the imaging subarray will be ≈ 200 μas. Our experience with the Mark III interferometer suggests that we will be able to measure stellar diameters as small as 200 μas with 1% precision and binary star separations as small as ρ ≈ 65 μas (for Δm ≈ 0 mag) or ρ ≈ 200 μas (for Δm ≈ 3-4 mag). With its large bandwidth and phase bootstrapping, the imaging subarray should be able to make images 10 resolution elements across the disks of nearby late-type stars.

https://iopscience.iop.org/article/10.1086/305365/pdf

The Navy Prototype Optical Interferometer

A new type of radio interferometer has been developed which has a number of important advantages over earlier systems. Its use enables the radiation from a weak ‘point’ source such as a radio star to be recorded independently of the radiation of much greater intensity from an extended source. It is therefore possible to use a very much greater recorder sensitivity than with earlier methods. It is, in addition, possible to use pre-amplifiers at the aerials, and the resolving power which may be used is therefore not restricted by attenuation in the aerial cables.

A New Radio Interferometer and Its Application to the Observation of Weak Radio Stars

A general description of the nature of radio astronomy, its differences from optical astronomy, a review of the earliest beginnings of galactic and solar radio astronomy, and a listing of other important observational discoveries is given. A nearly complete bibliography of these early publications and of the principal review books and papers on Radio Astronomy is given. Some practical aspects and instrumental developments of possible interest to radio engineers are pointed out. The papers in this issue of the PROCEEDINGS on Radio Astronomy are discussed generally and individually. A brief description of some new results on solar-burst spectra obtained at the Radio Astronomy Observatory at the University of Michigan is presented.

Introduction to Radio Astronomy

The Naval Research Laboratory and the National Radio Astronomy Observatory completed implementation of a low-frequency capability on the Very Large Array at 73.8 MHz in 1998. This frequency band offers unprecedented sensitivity (~25 mJy beam-1) and resolution for low-frequency observations. The longest baselines in the VLA itself provide 25'' resolution; the system has recently been extended to the nearby Pie Town antenna of the Very Long Baseline Array, which provides resolutions as high as 12''. This paper reviews the hardware, the calibration, and imaging strategies of this relatively new system. Ionospheric phase fluctuations pose the major difficulty in calibrating the array, and they influence the choice of calibration strategy. Over restricted fields of view (e.g., when imaging a strong source) or at times of extremely quiescent ionospheric "weather" (when the ionospheric isoplanatic patch size is larger than the field of view), an angle-invariant calibration strategy can be used. In this approach a single phase correction is devised for each antenna, typically via self-calibration; this approach is similar to that used at higher frequencies. Over larger fields of view or at times of more normal ionospheric weather when the ionospheric isoplanatic patch size is smaller than the field of view, we adopt a field-based strategy in which the phase correction depends on location within the field of view. In practice, we have implemented this second calibration strategy by modeling the ionosphere above the array using Zernike polynomials. Images of 3C sources of moderate strength are provided as examples of routine, angle-invariant calibration and imaging. Flux density measurements of a subsample of these sources with previously well-determined low-frequency spectra indicate that the 74 MHz flux scale at the Very Large Array is stable to a few percent and that flux densities tied to the Baars et al. value of Cygnus A are reliable to at least 5%. We also present an example of a wide-field image, devoid of bright objects and containing hundreds of weaker sources, constructed from the field-based calibration. The paper also reviews other practical aspects of low-frequency observations, in so far as they differ from those encountered at higher frequencies, including aspects of interference excision and wide-field imaging. We close with a summary of lessons that the 74 MHz system offers as a model for new and developing low-frequency telescopes.

https://www.ursi.org/Proceedings/ProcGA02/papers/p1554.pdf

The 74 MHz System on the Very Large Array

Experimental studies of solar radiation on a frequency of 200 Mcyc./sec. are described. This radiation has characteristics similar to those of thermal radiation but is always hundreds of times greater than the thermal radiation anticipated from the photosphere and sometimes greater by a factor of 104.

/pdf/solar-radiation-at-radio-frequencies-and-its-relation-to-5eq1c0f239.pdf

Joseph Lade Pawsey

Papers

Solar radiation at radio frequencies and its relation to sunspots.