NAIC -- NRAO School on Single - Dish Radio Astronomy: Techniques and Applications

School Abstracts:

A Discussion of the Radio Sky

Jim Condon (NRAO)

TBD

Basic Radio Astronomy Techniques

Frank Ghigo (NRAO

TBD

The Major Single Dish Radio Telescopes

Phil Jewell (NRAO) & Chris Salter (NAIC)

TBD

History of the Green Bank Observatory

Jay Lockman (NRAO)

TBD

Introduction to Observing at the 40' Teaching Telescope

Sue Ann Heatherly (NRAO)

TBD

Basic Principles of Single-Dish Radio Telescopes

Paul F. Goldsmith (Cornell University)

This lecture covers the basics of how a radio telescope collects radiation from the "radio sky". We begin with a definition of key radio astronomical terminology, including the brightness distribution, brightness temperature, point-like and extended sources, flux density, and antenna temperature. In order to analyze radio telescope performance, we introduce the feed system which couples the receiver to the antenna. We continue by developing the concepts of the antenna as a phase transformer and the aperture plane field distribution, invoking the reciprocity principle to replace the receiver with a transmitter and treating the radiation pattern that would be produced by the resulting system. Using the distribution of the electric field in the aperture plane, we calculate the sensitivity of the radio telescope to a point source. This discussion is based on the antenna aperture efficiency and includes its dependence on the characteristics of the feed system, on the blockage, on systematic errors due to feed or secondary reflector defocus, and on random errors. The aperture plane field distribution is the starting point for using the Fourier Transform relationship to compute the far-field response, or power pattern, of the antenna. We analyze the effects of the telescope characteristics on the beam width and sidelobe level of the antenna's power pattern, and on the coupling efficiency to an extended source. Understanding this basic behavior allows us to determine what measurements are needed to calibrate the antenna, and to determine whether the system is performing properly.

The Measurement Process with Fully-Filled Apertures

Chris Salter (NAIC)

This lecture will concentrate on how a single dish is used to obtain information about the sky brightness distribution including point and extended sources. There will be discussions of: 1) the limitations on the information that can be obtained due to the angular size of the antenna far field response or power pattern; the convolution of the sky brightness distribution with the power pattern and the concept of spatial Fourier components of the sky brightness distribution and antennas as filters of these Fourier components; 2) sources of noise, the concept of system temperature and the detectability of weak signals in the presence of noise; 3) the detectability of weak sources in the presence of "confusion" from other sources; 4) simple receiver systems, detectors and recording techniques; 4) digitization of analogue signals and the Nyquist sampling rate.

Why Single-Dishes?

Darrel Emerson (NRAO)

o Single dishes (SDs) give full spatial frequency coverage up to D/lambda. Large area surveys of very broadly distributed celestial components, e.g. Galactic HI, background continuum, etc.

o The different spatial frequency responses of SDs and interferometers, (especially in the overlap region). Picking "the right instrument for the right job".

o SDs can provide the data to enhance short spatial frequency coverage for wide-field synthesis imaging. ALMA, the first instrument designed to make single-dish measurements as well as interferometry.

o The ability to use a large collecting area on transient phenomena (e.g. pulsar single pulses, solar bursts, flare stars, SETI, etc.)

o For any frequency band, an SD needs only a single receiver; as this is a single device it can be made the very best possible. SDs can achieve the maximum radio frequency coverage with a small suite of appropriate receivers, limited only at the high-frequency end by the dish rms.

o For spectral-line observing, SDs usually have only a single data stream to process, so very large numbers of frequency channels (i.e. high frequency resolution) can be simply and economically applied on that data stream.

o SDs in finder surveys for high-resolution (interferometry) follow ups (e.g. HI in galaxies, megamasers, OH/IR star catalogs).

o Flexibility of SDs means that it is easy to make real time decisions while observing.

o Big SDs can be co-opted on occasion into high sensitivity VLBI arrays, (i.e. EVN & Global Arrays, ad-hoc arrays.)

o SDs and planetary radar.

The Receiver System - cm Regime

Roger D. Norrod (NRAO)

The receiver front-end of a radio telescope is generally considered to encompass components which amplify, filter, and frequency convert signals provided by the antenna to a level and frequency range appropriate for detection. This presentation will discuss critical parts of the centimeter wave radio astronomy front-end and factors impacting the design and performance. The feed efficiently converts propagating electromagnetic fields near a reflector antenna's focal point to a guided wave in coax or waveguide. Some types of feeds inherently detect and separate polarizations; other types require an orthomode transducer to deliver orthogonal polarizations to separate channels. Low-noise amplifiers, usually cryogenically cooled, amplify the signal and set the receiver noise level, and are followed by filters, mixers, and additional amplification. All the passive and active components add electrical noise to the signal, and models used during receiver design will be presented, explaining why loss and noise introduced in the early stages of the receiver are critical. The linear operating range of active components is limited by their power handling capacity, and how these limitations are considered will be discussed. We will also discuss stability of the receiver, and practical means to achieve the required performance. Finally, the presentation will briefly cover related subsystems such as local oscillators and cryogenics.

IF/LO Paths

Lisa Wray (NAIC)

TBD

Backends

Rick Fisher (NRAO)

The final stages of receiver electronics are designed to extract information about the intensity of cosmic signals as a function of time, frequency, and polarization. The required signal processing can be as simple as a total power detector or as complex as a pulsar search machine that looks for periodic, dispersed pulse signatures in the time and frequency domain. This mini-lecture will give a brief overview of square-law detectors, FFT spectrometers, polarimeters, and pulsar processors. I will touch on calibration issues and the synchronous control of front-end calibration signals and beam and load switches.

Fourier Transforms, Noise, and Correlators

Avinash Deshpande (NAIC)

TBD

Continuum: General Aspects

Jim Condon (NRAO)

My intention is to cover the basic principles of continuum observing, present simple equations, data, and references to help the novice continuum observer plan and reduce single-dish observations, and reference specialized papers for those wishing to pursue a topic further.
o What is a continuum source?

o Why observe continuum sources?
-- To measure source flux density, position, etc.
-- As calibrators for other types of observations.

o How to measure surface brightness and flux density.

o How to measure source positions.

o How to measure polarization (very brief, since this will have been covered by C. Heiles).

o Discrete source surveys.

o Why use a single dish instead of an interferometer?

o Ideal radiometer equation.

o Nonideal effects:
-- receiver gain fluctuations
-- atmospheric emission
-- ground radiation
-- scintillation (ionosphere, IPM, ISM)
-- confusion by discrete sources

Spectral-Line 1: General Aspects

Harvey S. Liszt (NRAO)

The use of spectral lines in radio astronomy was conceived by Oort as a means of mapping the large-scale structure of the Milky Way, free from the obscuration of interstellar dust and earthly weather. Now, as we celebrate the 50th anniversary of the discovery of the H Iline, spectral line radio astronomy has detected thousands of different transitions from hundreds of atomic and molecular species in interstellar and circumstellar gas, neutral and ionized, near and far.

The line profiles we take home from the radiotelescope originate when local processes on the scale of individual atoms cause the emission and absorption of photons. These photons propagate through the medium and do not emerge (if at all) until they have run quite a gauntlet. There is absolutely no guarantee that the emergent spectrum represents conditions deep inside the gas. Worse, the viewing geometry and our sensing of only one component of the gas motion (projected onto the line of sight) can cause signals from various parts of the medium to blend together at the same received frequency (the opposite can also occur).

Of course the good news about these sensitivities, which accrue on a range of scales differing by 30 orders of magnitude (from nm to kpc) is that we can hope to measure an astonishingly wide range of properties, if only we can interpret our data. In the hope of providing even the barest introduction to this field, I'll pick a few representative combinations of source medium, line species, viewing geometry, earthly observing conditions and receiving equipment, and analyze them at varying levels of depth and sophistication.

Advanced Spectral Line Topics

Al Wootten (NRAO)

TBD

A Hueristic Introduction to Radioastronomical Observations

Carl Heiles (University of California Berkeley)

The polarization of electromagnetic radiation, no matter what wavelength, is best specified by Stokes parameters because they are independent, can be arithmetically manipulated, and the noise statistics are Gaussian. However, as happens in all kinds of measurements, there are instrumental effects that must be accounted for.

Instrumental effects in polarization measurement, no matter what wavelength, break down into several types. One is the relationship between the measured Stokes parameters and the true ones; this is best specified by the 4 X 4 element Mueller matrix, which describes the transfer function of the instrumentation. Another is the stability of these matrix parameters, which translates directly into the accuracy of polarization measurement. Another is the variation of the matrix parameters within the telescope beam or field of view, which affects directly the accuracy with which polarization of extended sources having angular structure can be measured.

We will discuss the fundamentals behind these issues using simple examples and phenomenological illustrations, with a minimum of mathematical emphasis. Radio astronomy is unique in allowing all four Stokes parameters to be measured simultaneously with no loss in signal/noise by using cross correlation, if the relevant instrumentation is available. We will describe the practical issues involved and offer recommended techniques for specific cases of polarization measurement. Perhaps the most important application is accurate measurement of the total intensity (the Stokes I parameter), which is the sum of two orthogonal polarizations, and we will offer our perspective on why current commonly-used techniques are not always optimum.

Calibration techniques at radio wavelengths

Karen O'Neil (NAIC)

Why calibration is needed

Atmospheric Opacity (briefly, as it will be covered later)

Calibration scale definitions
o System temperature
- How to measure
o Flux Density conversion
- Rayleigh-Jeans approximation

Antenna efficiency measurements
o Aperture efficiency
- definition, measurement techniques
o Beam efficiency
o Coma/sidelobes
o Pointing/Focus Issues

Temperature scale calibration techniques
o Pulsed noise cal
o Load cal

Baseline/Flux calibration
o Position/Frequency Switching
o Flux density standards
o Gain curves
o Standard sources - line/continuum

Error Budget

Reference Papers & Resources
o Kraus
o Baars paper
o etc.

Reduction and Analysis Techniques

Ron Maddalena (NRAO)

Single-dish observations can be made in a myriad of ways with different observing technique almost always requiring different kinds of data analysis. We will cover in this class the standard and basic continuum and spectral line analysis algorithms common to all single-dish data analysis packages. However, we will not cover the very specialized fields of polarimetry, pulsar, or radar data reduction. In the case of continuum observations the student will learn the steps used to derive the flux of a point source as well as the more common data analysis techniques for generating and analyzing maps of extended sources. For spectral line data, we will discuss how the analysis of an observation will depend upon the backend type (filter-bank, autocorrelation, or AOS) and observing technique (frequency-, position-, or beam-switched). We will concentrate on the analysis algorithms usually applied to single spectra (bandpass and velocity calibration, data averaging and smoothing, baseline fitting, component fitting, ...) and how to produce and analyze spectral-line data cubes.

Short-Spacings Correction from the Single-Dish Point of View

Snezana Stanimirovic (University of California, Berkely)

While, in general, interferometers provide high spatial resolution for imaging small-scale structure (corresponding to high spatial frequencies in the Fourier plane), single-dishes can be used to image the largest spatial scales (corresponding to the lowest spatial frequencies), including the total power (corresponding to zero spatial frequency). For many astrophysical studies, it is essential to bring `both worlds' together by combining information over a wide range of spatial frequencies. This lecture will demonstrate the effects of missing short-spacings, and concentrate on two main issues: (a) how to provide missing short-spacings to interferometric data, and (b) how to combine short-spacing single-dish data with that from an interferometer.

Pulsar Observations: Propagation Effects, Searching, Distance Estimates, Scintillations and VLBI

Jim Cordes (Cornell University)

The role of propagation effects (dispersive propagation, scattering, Faraday rotation and HI absorption) will be discussed, particularly as they affect sensitivities of pulsar searches. We will summarize dedispersion techniques that manipulate "postdetection" signals from analog and digital filter banks. Search data-acquisition and algorithms for detecting isolated and binary pulsars will be discussed. Interstellar scintillation, caused by multipath scattering in the interstellar medium (ISM), influences search sensitivities but also provides unique information about turbulence in the ISM. Data acquisition and analysis of scintillations will be summarized. Dispersion and scattering provide important input to models for the electron density in the Galaxy. The most recent electron density model will be presented. Finally, the scientific importance of very-long-baseline-interferometry of pulsars, along with pulsar-specific VLBI techniques, will be outlined.

RFI and How to Deal with It

Rick Fisher (NRAO)

Astronomers share the radio spectrum with a multitude of other users who transmit useful signals with a wide variety of spectral and temporal characteristics. Even portions of the spectrum allocated for exclusive use by radio astronomy are subject to contamination by incidental radiators such as computers, digital cameras, and observatory test equipment. Much of the spectrum that is not allocated for radio astronomy is also available to us, but it can take some careful planning to obtain useful data. Important observing parameters include time of day, receiver and spectrometer dynamic range, and temporal resolution. This lecture will discuss a number of software tools have been and are being developed to help recognize and remove interference from astronomical data. I will also mention a few signal processing techniques that are being developed to remove interference coherently or with time resolutions much greater than can be realized in software.

International Spectrum Management

Darrel Emerson

TBD

Stray Radiation and How to Deal with It

Jay Lockman (NRAO)

An antenna has response in all directions, not just the direction that it is ``pointing''. When using an antenna for radio astronomy, some signals which enter the receiver and are detected are ``stray'' in that they come through a sidelobe and not the main beam. Terrestrial interference is one example, but the term ``stray'' is usually used for signals of natural origin which might be confused with the object under study. A large fraction of the stray radiation can be eliminated in the standard observational techniques used to calibrate continuum or line intensities. In some experiments, however, the stray signals constitute the fundamental limitation on the accuracy of the measurements, and can result in errors of more than 100%.

Every single-dish measurement suffers to some extent from stray radiation, so it is important to recognize the phenomenon and understand the circumstances in which it becomes important. This talk will describe the general phenomenon of stray radiation, illustrate some specific circumstances where it must be considered, and consider some of the techniques, often rather onerous, that have be used to reduce its effects.

Planetary Radar

Don Campbell (NAIC)

Radar is a powerful tool for studying the Solar System, with its reach limited only by the transmitter power available. It has been used to observe targets ranging in size from the rings of Saturn down to house-sized asteroids. Since an observer has control of the illumination source, a radar experiment provides information not available from passive observing methods. On centimeter to meter scales it is a sensitive probe of surface characteristics such as dielectric constant and roughness, and on larger scales can map topography and determine shapes of irregular objects at resolutions finer than other ground-based methods. This lecture will cover the basic techniques of planetary radar astronomy, give an overview of the scientific questions that can be addressed, and survey some recent results. Key points of the lecture will be: the radar equation; principles and benefits of modulating the transmitted signal; data processing; and an outline of current radar systems' parameters.

The Receiver System - mm Regime

John Payne (NRAO)

The millimeter wavelength radio astronomy band is now generally taken to include frequencies of approximately 60 GHz to around 300 GHz, and the so-called sub-mm band extends this up to frequencies of around 1000 GHz (1 THz). Unlike centimeter-wave, ground-based radio astronomy, the mm/sub-mm frequency range is limited by the properties of the atmosphere which are briefly described.

The difficulties of constructing highly sensitive receivers for these high frequencies are described along with the commonly adopted solutions. Particular emphasis is given to the modern receivers such as those being planned for the ALMA interferometric array destined for installation on a 5000-meter-altitude site in the Atacama desert in Chile. These receivers use super conducting devices which require temperatures of around 4k to operate satisfactorily. Brief descriptions of the various sub-systems needed to construct such a receiver will be given.

Calibration Techniques at Millimeter Wavelengths

P. R. Jewell (NRAO)

Calibration practices for millimeter wavelengths are somewhat different than that for meter and centimeter wavelengths. There are technological, atmospheric, and historical reasons for this. This lecture will review the specific techniques used for millimeter wave calibration and will highlight the differences between these and the techniques used at longer wavelengths. Cases for which calibration techniques used at different wavelengths might be merged or rationalized will be discussed. The importance of the atmosphere at centimeter, millimeter, and submillimeter wavelengths will be discussed in detail. Topics will include specific calibration techniques such as the hot/sky chopper wheel method, variations such as hot/cold/sky schemes, sky tipping calibration, and the possibilities of subreflector based calibration sources. Calibration loss factors including rear and forward spillover and error beam losses will be described and illustrated. The TA*, TR*, and TMB temperature scales will be defined. Techniques for absolute calibration, and the effects of double sideband versus single sideband operation will also be discussed.

Bolometers for Millimeter-Wave Astronomy

Wayne Holland (United Kingdom Astronomy Technology Centre)

During the past decade considerable progress has been made in the development of bolometric detectors for submillimetre and millimetre-wave astronomy. With the introduction of imaging arrays these wavebands have been undergoing a major revolution with many new and exciting discoveries being made in almost all disciplines of astronomy.

This lecture will focus on the need for deep, wide-field continuum imaging in the submm/mm regime. The basic principles of bolometry will be explained, together with the "figures of merit" that characterise bolometer performance. Particular examples will focus on the problems of using wide-bandwidth devices in conditions where the atmospheric background can change on short timescales.

Practical devices - from single pixels to the current imaging arrays - will be discussed, with particular emphasis on the need for ultra-low temperature operation for sky-background limited performance. Examples will be given of how current instrument perform on ground-based telescopes, including the issue of optimum coupling of the small detector element to a large millimetre-wave telescope.

The development of integrated filled arrays of many thousand pixel cameras bodes well for the future. The new generation of both ground-based and space-borne instruments will also be described, as well as other exciting prospects for the future.

Rolling your own Data Reduction Software

Rick Fisher (NRAO - GB)

The common single-dish spectral line and continuum data reduction packages let observers get on with the business of doing science with their chosen instruments. However, there are plenty of reasons why some of us need or simply prefer to use our own software tools to analyze data drawn from intermediate stages in the data stream or from the original data files. This lecture outlines the operations that are performed on the raw data, such as van Vleck corrections, power transfer function linearization, and synchronization of data with antenna position, and suggests strategies for accessing the low-level data files. From there we progress to calibration extraction, difference spectra calculation, baseline fitting, and so forth through data display. Specific examples of a few unusual things you might want to do with GBT and Arecibo data will be presented.

Heterodyne Arrays

Mark Heyer (University of Massachusetts)

Heterodyne focal plane arrays have greatly expanded our capability to study astronomical phenomena at radio, mm/submm wavelengths. One can now rapidly image line emission over wide fields with high sensitivity with state of the art imaging systems. The spectral and spatial information gathered by focal plane arrays provide new insights and perspectives to the complex processes that operate within galaxies. I will discuss various observational modes that exploit the capabilities of multi-pixel systems and illustrate subtle image fidelity features unique to arrays and these data collection methods. Of course, the challenge to the astronomer is not the collection of the data at the telescope or the required processing steps to construct a final spectroscopic data cube but rather, the efficient use of the available information to address a particular astronomical question. I will present new ways to inspect and analyze the vast amount of data with particular emphasis on multi-variate statistical tools.

The Future of Single Dish Radio Astronomy

Bob Brown (NAIC)

TBD

 

Last modified: 3-April-2001
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