Showing papers in "Methods in Experimental Physics in 1976"
TL;DR: In this article, the spectral-line absorption coefficient and radiative transfer expressions appropriate for calculating absorption and emission by atmospheric gases are formulated, and problems associated with the calculation of absorption by these two molecules are discussed.
Abstract: Publisher Summary This chapter focuses on absorption and emission by atmospheric gases. At centimeter and shorter wavelengths, absorption and emission by atmospheric gases can significantly affect the propagation of electromagnetic radiation through the atmosphere. At frequencies from 1 to 300 GHz, referred to as “microwave frequencies,” absorption by atmospheric gases is dominated by water vapor lines at 22 and 183 GHz, oxygen lines near 60 GHz and at 118 GHz, and relatively narrow and weaker ozone lines above 100 GHz. Non-resonant absorption by water vapor and oxygen has significant effects in the window regions away from the dominant lines. The radiative transfer expressions appropriate for calculating absorption and emission by atmospheric gases are formulated. The general expressions for the spectral-line absorption coefficient are provided, followed by specific expressions for calculating absorption by water vapor and oxygen. Problems associated with the calculation of absorption by these two molecules are discussed. The absorption by microwave lines of ozone and other minor constituents is considered. Finally, the results of calculations of atmospheric absorption and emission, as well as the measured values are presented in the chapter.
213 citations
TL;DR: In this paper, very long baseline interferometry (VLBI) is used for estimating the angular frequency derivative of the fringe phase (differenced group delays or group delay).
Abstract: Publisher Summary This chapter deals with estimation of astrometric and geodetic parameters. The technique of very long baseline interferometry (VLBI) has enormous potential for important applications in astrometry and geodesy. Considerable progress has been made in the exploitation of this technique for these applications, but the ultimate accuracy inherent in the method has not been approached. VLBI is in a stage of rapid improvement. Interferometric observations of a point source of continuum radio radiation can yield fringe phase and fringe amplitude as functions of time and frequency. Because the observations are usually broken up into relatively short time intervals and relatively narrow frequency intervals, it is convenient to consider two additional quantities: the time derivative of the fringe phase (fringe rate), and the angular frequency derivative of the fringe phase (differenced group delays or group delay). A discussion on information content of the VLBI observables and deduction of the relevant astrometric and geodetic quantities from this content is presented in the chapter. Astrometric and geodetic parameters are also elaborated in detail.
30 citations
TL;DR: In this article, the authors present an overview of very long baseline interferometric (VLBI) observations and data reduction, where the received signals are recorded, undetected, on magnetic tape under the control of atomic frequency standards and reproduced at a later time and processed to give interference fringes.
Abstract: Publisher Summary This chapter presents an overview of very long baseline interferometric (VLBI) observations and data reduction. VLBI is similar to conventional interferometry except that the receiving elements are not connected in “real time.” The received signals are recorded, undetected, on magnetic tape under the control of atomic frequency standards and reproduced at a later time and processed to give interference fringes. There are important differences between VLBI and conventional interferometry. The telescopes used in VLBI can be very far apart owing to their independence, and the measurable quantities of phase, delay, and phase rate take on much larger values and play different roles in the data interpretation. Five topics of importance to VLBI observations and data reduction are discussed in the chapter. The measurable quantities are defined by tracing the received signals through the VLBI system. The problem of estimating the fringe amplitude of weak signals in the presence of phase noise is treated, and expressions for the signal-to-noise ratio for so- “broken-coherence” averaging are derived. The probability of misidentifying the interference fringes is given. The interpretation of the data in terms of simple models of brightness distribution is discussed. Some operational problems encountered in acquiring data are also described.
21 citations
TL;DR: In this article, the authors provide an overview of very long baseline interferometer (VLBI) systems, where the undetected signals received at remote, unconnected radio telescopes are stored in such a way that they can be recovered at a later time and processed in a fashion similar to conventional interferometry.
Abstract: Publisher Summary This chapter provides an overview of very long baseline interferometer (VLBI) systems. In VLBI, the undetected signals received at remote, unconnected radio telescopes are stored in such a way that they can be recovered at a later time and processed in a fashion similar to conventional interferometry. Several essential components are required to make two remote telescopes into an interferometer which include: a stable local-oscillator system for signal frequency conversion; a video converter for conversion of the intermediate frequency (i.f.) signal to a video band; a means of synchronizing the clocks at the stations; a device for storing the received signal and precise timing information; and a playback system. The antennas and front-end amplifiers used in other applications of radio astronomy are generally adequate for VLBI measurements. There are no special tracking requirements for the antennas. Local oscillators in single-telescope continuum studies can be free running and, in spectral-line studies, need only be frequency locked, those in VLBI applications must be phase locked to high-performance frequency standards. A general block diagram of a VLBI system is provided in the chapter along with an overview of single-sideband video converter. Details of IBM-compatible, digital-recording system are also presented.
17 citations
TL;DR: In this article, the authors provide details of computer programs for radio astronomy, focusing on radial-velocity corrections for Earth motion, and present a program called DOP which can be used to calculate the velocity component of the observer with respect to the LSR as projected onto a line specified by right ascension and declination for the epoch of date with time specified.
Abstract: Publisher Summary This chapter provides details of computer programs for radio astronomy. It focuses on radial-velocity corrections for Earth motion. The calculation of the radial velocity between a position on the Earth's surface and a distant astronomical object involves a number of velocity terms. How many terms need to be included is determined by the accuracy required. A program has been written to calculate the velocity of a telescope with respect to the Local Standard of Rest (LSR). This program omits the motion of the sun around its barycenter. The computer program called DOP may be used to calculate the velocity component of the observer with respect to the LSR as projected onto a line specified by the right ascension and declination for the epoch of date with time specified. The location of the observer is specified by the latitude (ALAT), the geodetic longitude in degrees (OLONG), and the elevation in meters above sea level (ELEV). The chapter also discusses about special relativity, conventional tabulation of redshifts, the aberration of light, and velocity reference frames.
13 citations
TL;DR: In this paper, the authors discuss the theory of two-element interferometers and arrays, with a special focus on theory of the two-dimensional Fourier transform relation between brightness and visibility.
Abstract: Publisher Summary This chapter discusses interferometers and arrays, with a special focus on theory of two-element interferometers. The basic observables in radio interferometry are the correlated amplitude and relative phase of waves from a common source at two points on the wavefront. The vector between the two points on the wavefront is known as the interferometer baseline. Optical interferometers bring the two “signals ” together using mirrors and superpose them onto a detector so that they either reinforce or cancel, depending on their relative phases. The interference of the signals in this manner produces interferometer “fringes.” In radio interferometry, the “signals” at the ends of the baseline can be translated to a lower frequency, transmitted through cables, added and detected or cross correlated and Fourier transformed, or even recorded on magnetic tape for post real-time detection. The chapter illustrates that the effective interferometer aperture is the geometric mean of the two apertures, and the effective system temperature is the geometric mean of the system temperatures. The signal analysis is performed for a point radio source and a two-dimensional Fourier transform relation between brightness and visibility is analyzed.
12 citations
TL;DR: This chapter discusses application of fast Fourier transform (FFT) in radio astronomy and it is shown how this algorithm is programmed on a digital computer.
Abstract: Publisher Summary This chapter discusses application of fast Fourier transform (FFT) in radio astronomy. The Fourier transform is a particularly useful computational technique in radio astronomy. The essence of the FFT technique is that it is possible to treat the one-dimensional DFT as though it were a pseudo-two-dimensional one, and then reduce the running time by performing the inner and outer summations separately. The basic idea behind the FFT is discussed and it is shown how this algorithm is programmed on a digital computer. Because of the requirement for computational speed, a number of programs are given. These include short, moderately efficient subroutines for the transform of one-dimensional, complex data (FOURG and FOURI). With the addition of a subroutine (FXRLI) to either of the above routines, real, one-dimensional data may be transformed in half the time with half the memory storage. Additional subroutines (CFFT2, RFFT2, and HFFT2) permit the transform of two-dimensional data. A program is also given for transforming real, symmetric data for which only the cosine (or sine) transform is desired (FORSI).
12 citations
TL;DR: In this paper, the authors provide an overview of the use of the occultation technique for radio sources with continuous spectra and the time scale of a point source's occultation curve.
Abstract: Publisher Summary This chapter provides an overview of lunar occultation measurements. The occultation technique is concerned only with sources that contain angular structure much smaller than the size of the Moon. The variation in received power when a source passes behind the Moon depends on the size of the source relative to the size of a Fresnel zone at the Moon's distance, which, over the range of frequencies of interest, ranges from a few seconds of arc to a maximum of about 20 arc sec. Provided the source size is much greater than the size of the Fresnel zone, diffraction effects can be neglected, and the shape of the occultation curve represents the strip integral of the brightness distribution in a direction perpendicular to the limb at the point of occultation. The true strip brightness distribution can then be recovered simply by differentiating the observed occultation curve. A discussion on the use of occultation technique in measurement of the positions and structures of small angular size radio sources with continuous spectra is presented and time scale of the occultation curve of a point source is also analyzed in the chapter.
11 citations
Abstract: Publisher Summary This chapter discusses concepts related to frequency stability and the performance of various high-stability oscillators. There are two ways of looking at frequency stability— in the frequency domain and in the time domain. Spectral measurements tell the behavior of phase (or frequency) fluctuations of an oscillator. The spectral density of phase fluctuations can be visualized as the result of looking with a spectrum analyzer at the output from a perfect phase detector. The spectral density of frequency fluctuations is the result of analyzing the output from a perfect frequency discriminator. Spectral densities give the best clues about the inside workings of an oscillator because it is possible to relate the spectral behavior with noise processes in various parts of the oscillator, leading to certain models for the behavior of the oscillator. From these, it is possible to often find physical processes that cause the noise. Frequency-domain and time-domain measures of frequency stability and their relationship are explored in the chapter. Spectral-density models are analyzed, and phase and time prediction is elaborated. Normalized phase departure versus observation time for various frequency standards is also illustrated.
10 citations
TL;DR: This chapter presents an overview of autocorrelation spectrometers and Sampling and quantizing considerations are discussed in the chapter and degradation factors for quantized correlators are elaborated.
Abstract: Publisher Summary This chapter presents an overview of autocorrelation spectrometers. The digital correlation approach makes use of the Fourier transform relationship that exists between the autocorrelation function of a signal and its power spectrum and becomes relatively simple to implement when one-bit quantization of the input signal is employed. Although the correlation approach requires ready access to a computer to carry out a Fourier transformation, a small computer is now no more expensive than the multichannel analyzer that is normally used at the “back end” of filter-type spectrometers. The autocorrelation spectrometer has been demonstrated to have superior stability for long integrations. By locking the intermediate frequency (i.f.) conversion oscillators and the digital clock system to an atomic standard, the frequency calibration of the spectrum can also have atomic accuracy, whereas in a filter-type spectrometer, expensive crystal filters are needed to guarantee freedom from frequency drifts. Sampling and quantizing considerations are discussed in the chapter and degradation factors for quantized correlators are elaborated. A discussion on prefilters and video converters is presented and a block diagram of one-bit correlator is displayed. An overview of zero-crossing discriminator is also presented along with some examples of correlation spectrometers.
7 citations
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TL;DR: A review of the properties of the ionospheric layers, their origin, their effect on the propagation of electromagnetic waves, and the methods available for observing their properties so that corrective measures can be taken can be found in this article.
Abstract: Publisher Summary This chapter provides a review of the properties of the ionospheric layers, their origin, their effect on the propagation of electromagnetic waves, and the methods available for observing their properties so that corrective measures can be taken. The ionosphere is that part of the upper atmosphere where free electrons and ions exist in quantities, such that they can substantially affect the propagation of radio waves. The part of the ionosphere below an altitude of 90 km is referred to as the “ D region.” This region primarily affects the radio waves of interest in radio astronomy through absorption effects because the neutral density is so high that the electron-neutral collision frequency is substantial. The ionosphere causes radio waves to be attenuated through absorption or through refraction effects. Phase and group delays may be introduced and may result in serious changes both in angular position and in polarization in many cases. A number of observations made to provide information on ionic parameters are summarized in this chapter.
TL;DR: In this article, the authors provide an overview of multichannel-filter spectrometers and a discussion of detectors, integrators, and special operating features, as well as a comparison spectrum between the signal input obtained from the antenna and a flat comparison spectrum.
Abstract: Publisher Summary This chapter provides an overview of multichannel-filter spectrometers. The need for improved frequency resolution and better efficiency in obtaining detailed spectral-line profiles led to the development of multichannel receivers in which a number of adjacent or closely spaced frequency bands are processed simultaneously. A typical multichannel spectral line receiver utilizes a superheterodyne configuration with the intermediate frequency (i.f.) portion of the receiver being split into the separate bandpass filter channels. A switched multichannel-filter receiver provides a set of values representing the difference spectrum between the signal input obtained from the antenna and a “flat” comparison spectrum. This comparison spectrum is normally obtained either by switching the receiver input to a matched source such as a sky horn, off-axis feed, or cold load or by switching the frequency of the receiver so that the filters are placed in a “flat” spectral region adjacent to the region containing the signal spectrum. In principle, a multichannel-filter receiver can be operated in an unswitched or total power mode. The chapter presents an overview of the filter characteristics and a discussion of detectors, integrators, and special operating features.
TL;DR: In this paper, the authors deal with two applications of phase-measuring interferometers: the accurate measurement of position (astrometry), and the determination of source structure, in particular, mapping observations.
Abstract: Publisher Summary This chapter deals with two applications of phase-measuring interferometers: the accurate measurement of position (astrometry), and the determination of source structure, in particular, mapping observations. In both cases, the determination of the difference in phase between incoming signals (the interferometer phase) is crucial. Essential features for the interpretation of measurements of interferometer phase are a known baseline and stable electrical lengths for the various local oscillator and signal paths. The effective baseline may be varied, either by tracking the source over a range of hour angles so that the orientation and projected length of the baseline change, or by using movable elements in the interferometer. Both methods may be employed in the same instrument. In a tracking interferometer, the difference in electrical paths from the source to the two antennas changes continuously. The variation in path difference across the field of view of the interferometer is illustrated in the chapter. The disturbing effects of tropospheric irregularities under calm and disturbed conditions are discussed and an overview of Green Bank three-element interferometer is also presented.
TL;DR: An overview of radio astronomy can be found in this article, where a radio telescope is defined as an antenna that selectively collects radiation from a small region of sky, a radiometric receiver, referred to as a radiometer, that amplifies a restricted frequency band from the output of the antenna, and an indicator that registers the radiometer output so that it may be recorded by the observer.
Abstract: Publisher Summary This chapter presents an overview of radio telescopes. Radio astronomy covers a frequency range extending from a few megahertz up to a frequency of about 300 GHz or equivalently a wavelength range from roughly a hundred meters down to 1 mm. A radio telescope in its simplest form consists of three elements: an antenna that selectively collects radiation from a small region of sky, a radiometric receiver, referred to as a “radiometer,” that amplifies a restricted frequency band from the output of the antenna, and an indicator that registers the radiometer output so that it may be recorded by the observer. Radio emission from astronomical sources is noise-like in character and the signal must be measured in the presence of several kinds of extraneous noise, such as thermal emission from the surroundings and noise generated within the radiometer itself. Radiometers are designed to be linear in power so that the received signal can be calibrated in terms of a reference noise source. The reference or calibration signal is superimposed on the signal from the antenna terminals and is usually injected while the antenna points toward the comparison region.
TL;DR: In this article, it is suggested that the detection system should be linear over the usually large dynamic range of the interferogram to prevent distortions of the spectrum, and it is shown that the shape of the grooves determines the distribution of the radiation diffracted by a grating over the different orders.
Abstract: Publisher Summary This chapter elaborates the far-infrared and submillimeter-wave regions. The far infrared forms the transition between the infrared and the microwave regions. The point where the infrared becomes far is not well defined, and it may be taken somewhere around 20-pm wavelength. A frequently used method to produce submillimeter waves consists in generating higher harmonics of the radiation from a standard millimeter-wave klystron. A nonlinear element, usually a point-contact diode, is placed in a waveguide containing the fundamental wave, and the resulting components of higher frequency are coupled into a smaller waveguide. The shape of the grooves determines the distribution of the radiation diffracted by a grating over the different orders. Gratings used in the far infrared are always of the echelette type. No noise enhancement because of thebecause of the sampling that occurs in the step-and-integrate mode or in the continuous mode when the signal is integrated over the sampling interval. It is suggested that the detection system should be linear over the usually large dynamic range of the interferogram to prevent distortions of the spectrum.
TL;DR: The chapter discusses the contributions to total system temperature for several operational radiometer systems and factors to be applied for several representative types of input bandpasses and output filters used in radiometers.
Abstract: Publisher Summary This chapter discusses the basic concepts related to radiometers. In a radiometer system, the purpose of the receiver is to select and amplify the signal received by the antenna and to provide an output signal to a chart, digital recorder, or other display/processing unit. The accurate reproduction of the amplitude and spectral characteristics of the input signal is of prime importance. The receiver must be linear in output even when it operates over a large dynamic range of amplitude and should introduce a minimum of noise to the signal being amplified. Typical power gains of receiver systems are 120 dB. Bandwidths used in observations vary in width from a few kilohertz up to a few gigahertz. The chapter discusses the contributions to total system temperature for several operational radiometer systems. Factors to be applied for several representative types of input bandpasses and output filters used in radiometer systems are given. The basic receiver system, single-sideband systems, and double-sideband systems are discussed and the Dicke modulated receiver system is also elaborated.
TL;DR: The most common antenna used for radio astronomy is the parabolic reflector with the feed horn or dipole located at parabolic focus as discussed by the authors, which can complete the aperture synthesis for a given region of the sky in a time shorter than that required by a simple two-element telescope.
Abstract: Publisher Summary This chapter discusses different types of astronomical antennas. The pattern of the pencil-beam antenna has one main lobe or maximum with a single-output terminal pair or a few main lobes each with its own separate output. The output at a single terminal pair corresponds to one main lobe for only one sense of polarization: linear, circular, or elliptical. The most common antenna used for radio astronomy is the parabolic reflector with the feed horn or dipole located at the parabolic focus. One principal advantage of this antenna is the ease with which the receiver may be coupled to it. The input terminals are at the feed horn or dipole. Operation over a wide range of wavelengths is simple; changing from one band of wavelengths to another requires only the change of the feed. The multi-element interferometer consists of a number of two-element interferometers operating simultaneously. The outputs of all the antenna elements are combined in pairs and recorded at the same time. The result is a system that can complete the aperture synthesis for a given region of the sky in a time shorter than that required by a simple two-element telescope.
TL;DR: In this article, the authors examined the advances in the development of Beam-Foil Spectroscopy (BFS) and found that the simplest analysis of the time evolution of a foil-excited system consists in expanding the state prepared by the foil in terms of energy eigenstates of the system beyond the foil.
Abstract: Publisher Summary This chapter examines the advances in the development of Beam-Foil Spectroscopy (BFS). BFS as an area of research includes a wide range of experiments having the common feature that the radiation source is a fast beam of particles that have been excited by passage through a thin solid foil. The beam-foil source has the advantage that the nuclear mass of the radiating system can be uniquely chosen by magnetic analysis of the accelerated beam. The greatest single advantage of the beam-foil radiation source is that it allows an extremely simple method for the measurement of the lifetime of the emitting state. The strongest lines of the neutral and singly ionized systems dominate beam-foil spectra of low-energy beams of the elements. It is suggested that if the lifetimes are supplemented by independent measurements of the branching ratios from the upper state to all lower states, many oscillator strengths may be deduced from a single mean lifetime. It is found that the simplest analysis of the time evolution of a foil-excited system consists in expanding the state prepared by the foil in terms of energy eigenstates of the system beyond the foil.
TL;DR: In this article, the authors describe the various aspects of microwave spectrometers and describe the most widely used oscillators for microwave spectroscopy, including reflex klystrons.
Abstract: Publisher Summary This chapter describes the various aspects of the microwave spectrometers Reflex klystrons remain as the most widely used oscillators for microwave spectroscopy It often tends to be extremely sensitive to thermal changes and mechanical vibrations Significant improvements, in both source stability and noise, can be achieved with any reflex klystron by providing a firm mount and adequate cooling Bolometers have also been used to detect microwave radiation, and offer an entirely different set of characteristics from diode detectors A possibility for increasing sensitivity in recovering very weak signals is to amplify the signal at the microwave frequency before it is detected Several types of low noise amplifiers are used in spectrometers operating with radio telescopes Stark-effect modulation has been, by far, the most popular technique, and has been used on the most sensitive spectrometers The Stark effect depends on the fact that the relative positions of the energy levels in polar molecules can be shifted by applying an external electric field It is suggested that both the specificity and sensitivity of microwave methods can offer significant advantages in applications where specific molecules must be monitored
TL;DR: In this article, extinction by condensed water was analyzed using Mie theory and the extinction cross section per unit volume or the attenuation coefficient (specific attenuation) made using the average measured drop-size distribution reported by Laws and Parsons.
Abstract: Publisher Summary This chapter analyzes extinction by condensed water. Individual hydrometeors—such as rain, hail, and snow particles—absorb as well as scatter incident radiation. The interaction between a particle and the incident field may be computed using electromagnetic scattering theory. Exact solutions are available only for simple shapes and distributions of dielectric properties within the scatterer. Rain may be reasonably modeled with spheres having the dielectric properties of water. More exact models of rain that take into account the non-spherical nature of the raindrops have been tried but are not generally used. The computations of the extinction cross section per unit volume or the attenuation coefficient (specific attenuation) made using the average measured drop-size distribution reported by Laws and Parsons are provided in the chapter. For comparison, the attenuation coefficients for liquid water clouds are presented and the effects of drop-size distributions are analyzed. A comparison of attenuation versus rain-rate models for a drop temperature of 10°C and a rate of 101.6 mm/hr is done. The single-scattering albedo is also computed for a given drop-size distribution using Mie theory.
TL;DR: In this paper, the authors describe the physical phenomena utilized in frequency tuning the individual infrared and visible laser, and types of tunable lasers, and the typical components of the tunable laser tube.
Abstract: Publisher Summary This chapter describes the physical phenomena utilized in frequency tuning the individual infrared and visible lasers, and types of tunable lasers The typical components of the tunable laser are explained The laser tube consists of a cylindrical, fused, quartz tube of about 6 mm id, and lengths that have ranged from 03 to 166 m The configuration and energetics of molecular N 2 –CO 2 , and N 2 –CO lasers bear great similarity to those of the atomic He–Ne lasers A visible glow emanating from the laser tube also indicates that excited electronic states of N 2 and CO play a role through collisions, or cascades to lower levels whereby the correct population inversion is achieved It is found that each diode-injection laser is also characterized by a threshold current density, where laser action first dominates the combined processes of spontaneous emission and internal absorption Several physical parameters, controlled individually or in combination, are used to frequency tune the radiant output from semiconductor lasers The parameters include temperature, forward bias current, magnetic induction, uniaxial pressure, and hydrostatic pressure The resonant enhancement of two-photon absorption from contrapropagating laser beams of different frequencies is also elaborated
TL;DR: In this article, the basic concepts related to antenna calibration are discussed, including the criteria for the selection of calibration sources, and lists of calibrators, both for positional and for flux scale calibration, are given.
Abstract: Publisher Summary This chapter discusses the basic concepts related to antenna calibration. Direct antenna calibrations of radio telescopes—such as the measurement of the polar pattern on a turntable—are usually not possible. Present-day radio telescopes are too large for these methods, which are generally employed in the testing of antennas for radio communication purposes. To achieve meaningful calibration by scaling, both a far-field measurement of the polar pattern of the model and near-field probing of the aperture field in the model and of the full antenna need to be combined. Consequently, very specialized and intricate measurements must be made before a successful antenna calibration is available. The criteria for the selection of calibration sources are described in the chapter, and lists of calibrators, both for positional and for flux scale calibration, are given. With a view of the discrepancies between observatories at frequencies of 408 MHz and below, discussion of present-day accuracy of the flux scale is undertaken. The techniques of calibration are described, as used in the tests of the 100-m fully steerable, paraboloidal radio telescope at Effelsberg. A description of the aims of a complete pointing theory is also presented.
TL;DR: In this article, measurements of galactic 21-cm hydrogen were performed either in emission or in absorption against a background source, and the problem of conversion of antenna temperature to brightness temperature was discussed.
Abstract: Publisher Summary This chapter focuses on measurements of galactic 21-cm hydrogen. Measurements of atomic hydrogen at a wavelength of 21-cm are performed either in emission or in absorption against a background source. Compared to molecular or recombination lines, the astronomer's choice of technique is severely restricted when it comes to observing the 21-cm line. This is because hydrogen exists all over the sky. Therefore, it is not possible to beam switch or use a reference horn; it becomes necessary to either observe a reference load or to obtain a reference spectrum at a different frequency. However, each technique has the problem of establishing the zero level. The problem peculiar to the 21‑cm line is the conversion of antenna temperature to brightness temperature. In the case of point sources, existing absolute measurements of bright sources are simply referred as primary standards. For hydrogen, however, the brightness temperature distribution must be integrated over not only the main beam, but particularly the near-in sidelobes and the remaining contributors to the stray factor as well. Naive use of a beam efficiency factor can lead to serious errors, especially for telescopes with poor surfaces or large diffraction lobes.
TL;DR: An overview of millimeter wave masers is provided in this paper, where the basic properties of maser amplifiers are described and operational considerations are elaborated, and data for specific maser amplifiers and systems is presented.
Abstract: Publisher Summary This chapter provides an overview of MASER (microwave amplification by stimulated emission of radiation) amplifiers. The word “maser” was coined by Charles H. Townes and his co-workers at Columbia University after they successfully operated the ammonia maser oscillator, the first electronic device making direct use of stimulated emission. A similar device, the hydrogen maser oscillator, emitting at the hydrogen hyperfine-splitting frequency (1420 MHz), is in frequent use by radio astronomers as the most accurate frequency standard available. Rather than a gas, practical maser amplifiers use solid-state single crystals as the active medium, thereby taking advantage of the much higher densities of active particles available in a solid. The original ammonia maser made use of only two energy levels of the ammonia molecules, whereas practical maser amplifiers, without exception, utilize at least three discrete energy levels. The basic properties of maser amplifiers are described in the chapter. Systems and operational considerations are elaborated, and data for specific maser amplifiers and systems is presented. An overview of millimeter wave masers is also provided.
TL;DR: Observing techniques generally applicable to either filter receivers or correlators are discussed in the chapter and a discussion on baseline fitting is presented.
Abstract: Publisher Summary This chapter focuses on measurements using radio-frequency spectrometers. Almost all spectrometers currently used in radio astronomy fall into two categories: multichannel filter banks and autocorrelators. Filter receivers are older and more common, however, use of the one-bit digital autocorrelator in radio-frequency spectrometers is now well established. Correlation systems have some advantages and some disadvantages in comparison with filter systems. For various practical reasons, correlators tend to be more versatile and more tractable if a large ratio of window width to resolution is needed. One-bit correlators have poorer noise performance for the same observing mode and usually require the use of an electronic digital computer, at least to perform the Fourier transform. Filter receivers are characterized by channels (samples) in frequency space, while correlators are characterized by channels in lag space but a continuous function in frequency space. Observing techniques generally applicable to either filter receivers or correlators are discussed in the chapter and a discussion on baseline fitting is presented. Noise considerations are analyzed, and the spectral resolution in a correlator system is also studied.
TL;DR: In this article, the authors discuss concepts related to scintillation measurements and analyze diffraction by thin layers and present an overview of interplanetary scintillations and radio source structure.
Abstract: Publisher Summary This chapter discusses concepts related to scintillation measurements. Radio sources having angular diameters less than about 1 arc sec show fluctuations of intensity, having a time scale of a few seconds, when observed within 90° of the Sun. The fluctuations are produced by scattering of the radiation from irregularities of plasma density in interplanetary space, and the phenomenon is known as interplanetary scintillation. In addition, fluctuations of intensity of the radiation from pulsars, having a time scale of a number of minutes, result from a similar mechanism operating in the interstellar medium, which is known as interstellar scintillation. Observations of the intensity fluctuations have proved useful in determining the electron density variations and scales of the plasma irregularities producing the scintillation. Enhancements of interplanetary scintillation have been correlated with the presence of sector structure in the solar wind. The chapter outlines the diffraction effects, which occur when radiation traverses an extended medium containing randomly distributed phase-changing irregularities. It analyzes diffraction by thin layers and presents an overview of interplanetary scintillation and radio source structure.
TL;DR: In this paper, the authors focus on pulsar observing techniques and present the major characteristics of pulsar radio emissions, and concepts related to intensity variations and propagation effects are discussed Sensitivity and time resolution are also described.
Abstract: Publisher Summary Pulsars, as a class of galactic radio sources, have characteristics which are totally unlike other radio sources Because of this, special observing techniques have been evolved This chapter focuses on pulsar observing techniques The primary class characteristic of pulsars is the pulsed nature of their radio emission An observation of a train of successive pulses from a typical pulsar reveals a great deal of detailed structure, which varies considerably from pulse to pulse The synchronous average of several hundred successive pulses results in a stable and repetitive “average pulse shape” This average pulse shape reflects the probability of occurrence of individual narrow pulses more than it represents any typical pulse Although emission occurs at varying phases with respect to the primary pulsation period, it is confined to the narrow and well-defined pulse window The fractional width of the pulse window is quite small, typically 5%, but ranges from 1 to 20% for the ∼70 known pulsars The major characteristics of pulsar radio emissions are presented in the chapter and concepts related to intensity variations and propagation effects are discussed Sensitivity and time resolution are also described
TL;DR: In this article, the authors examined the technique and applications of radio-frequency spectroscopy and showed that high-loss dielectrics can be matched to the characteristic impedance of the waveguide, so that the standing waves are almost completely damped out.
Abstract: Publisher Summary This chapter examines the technique and applications of radio-frequency spectroscopy. Radio-frequency spectroscopy is conducted in absorption rather than in emission, the material is exposed to coherent electromagnetic radiation, the absorption, and refraction of which are measured, often simultaneously. There are within the radio-frequency region some characteristic emissions and absorptions arising from transitions between quantum states of atoms and molecules. Electrode polarization is because of the transport of ions to the electrode surface, where they are often unable to recombine, so that they form a local space charge at the interface of specimen and electrode. It is found that efficient propagation of electromagnetic waves down a hollow metal tube is possible only when the tube dimensions are sufficiently large. High-loss dielectrics can be matched to the characteristic impedance of the waveguide, so that the standing waves are almost completely damped out. The physical basis of dielectric relaxation spectra is discussed in the chapter.
TL;DR: An overview of paraboloidal reflector systems can be found in this paper, where the dyadic Green's function over the currents induced on the reflector is estimated using geometrical optics.
Abstract: Publisher Summary This chapter presents an overview of paraboloidal-reflector systems. The rf performance of paraboloidal reflectors may be analyzed using various techniques of diffraction theory. Such techniques generally involve the integration of the dyadic Green's function over the currents induced on the reflector. These induced currents are estimated using geometrical optics. In principle, however, they may also be calculated using moment-method techniques or by applying classical results to appropriate boundary-value problems. Once the currents are determined, evaluation of the fields becomes straightforward although the computations may be lengthy and laborious. Computers are generally needed for all but a few special cases that may be integrated in closed form. The analysis of the transmitting properties of the antenna is presented in the chapter. An analysis and the geometry of the paraboloid with a prime-focus feed are also elaborated and illustrated. The aperture projection of blocked portions of a paraboloidal reflector is displayed and the classical dual-reflector antenna systems are analyzed. The calculation of radiation from dual-reflector systems by applying physical optics (PO) is also discussed.
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TL;DR: In this paper, the authors describe various types of sources, detectors, resolving instruments, and optical materials used in the infrared spectroscopy, and they find that the collision cross sections for the broadening of lines involving transitions between lower rotational states are larger than those between rotation states of higher energy.
Abstract: Publisher Summary This chapter describes the various types of sources, detectors, resolving instruments, and optical materials used in the infrared spectroscopy. The range of quantum energies encompassed by the infrared region represents energies well below those separating the lowest energy levels of most atoms. The atomic lines appearing in this region are associated with transitions between highly excited states of atoms and can usually be studied only in emission. One method of ensuring the elimination of undesired radiation involves the use of a fore prism as a more or less crude prism spectrometer in series with the grating instrument. It is found that the collision cross sections for the broadening of lines involving transitions between lower rotational states are larger than those between rotational states of higher energy. The chapter also observes that the general spectral features of water and ice have corresponding counterparts in the spectra of the liquid and solid phases of other compounds with small molecules.