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.
Because radio astronomy signals have Gaussian amplitude
statistics, it turns out that the ACF can be accurately
estimated even when the signal has been sampled by a
digitizer of very low precision. The extreme case is 1-bit
sampling, where only the polarity of the voltage is
measured. The advantage of coarse quantization is hardware
simplicity. The cost is an increase in averaging time. The
procedures used to convert the ACF of a coarsely sampled
voltage into the ACF of the undistorted original voltage are
generally called "Van Vleck" corrections, after J.H. Van
Vleck, who was the first to analyze the 1-bit case.
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.
Why calibration is needed
Atmospheric Opacity (briefly, as it will be covered later)
Calibration scale definitions
Antenna efficiency measurements
Temperature scale calibration techniques
Baseline/Flux calibration
Error Budget
Reference Papers & Resources
The tutorial will provide a census of the components of the package (GUI,
plotter, logs, history, etc), an overview of the available reduction
tools (flagging, averaging, regridding, imaging, etc), and worked examples of
different experiments from start to finish. The interface, both GUI and
command line, will be discussed in detail, including ways to expand
the tool-kit through the use of Glish scripts.
Documentation is available via:
http://www.nrao.edu/daily/docs/aips++.html
CDs with the software will also be provided.
As radio waves "do not know" any national boundaries, such management
issues have to be agreed upon globally. The mechanisms that have been
set up for this purpose, with their global and national counterparts,
will be described in this talk. How, as a minority, the interests of
radio astronomers may be best served, both now and in future, will also
be discussed.
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.
Basic Principles of Single-Dish Radio Telescopes
Paul F. Goldsmith (NAIC)
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
Donald B. Campbell (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.
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.
Backends (part 1)
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.
Noise Statistics and Spectral Analysis
Jon Hagen (NAIC)
The output of a radio astronomy receiver is a time varying,
random valued voltage with a mean value of zero. This
voltage can be partially described by its power, the mean
square value. The voltage can be completely described by its
autocorrelation function, R(tau), defined as the average
value of the product of pairs of voltage samples taken at
times separated by tau seconds. Note that the power is just
R(0). Special purpose back-end processors form the averages
to estimate hundreds or thousands of points on the
autocorrelation function (ACF). This estimate of the ACF is
then Fourier transformed to produce an estimate of the power
spectral density function (PSD) wherein emission from a
molecular line will show up as an obvious peak.
Alternatively, the PSD can be estimated directly by Fourier
transforming successive blocks of voltage samples and then
averaging the magnitudes of the respective Fourier
components.
Stray Radiation and How to Deal with It
Felix J. Lockman (NRAO, Green Bank)
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.
Continuum 1: 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
Continuum 2: Specific Applications
Chris Salter (NAIC)
(This talk will compliment "Continuum 1" by covering more specific
observing/analysis techniques.)
o Effects of RFI (broadband impulsive and narrowband CW), use
of spectrometer to remove narrowband RFI.
o Effects of atmosphere (why does continuum need to position switch much
more rapidly than spectral-line observing)
o Total-power receiver (short-comings), Dicke switching (e.g. load
switching, sky-horn switching, double Dicke switching, Ryle-Vonberg
system, nutating subreflectors, wobblers, etc.), correlation receiver,
software beam switching.
o Mapping: Scanning, basket weaving, Emerson, Klein, & Haslam algorithm
and other developments. Array detectors.
o Comparison of continuum images (spectral index distributions,
rotation measure distributions).
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
John Dickey (University of Minnesota)
We will discuss spectral line mapping, and especially the interpretation
of maps and spectral cubes. I will concentrate on three areas,
-- observational techniques for making spectral line maps,
-- the spectral line cube and its moments,
-- interpretation of patterns in the cube, especially for rotating disks.
By the end of the class, I hope students will be ready to make
and interpret maps of the HI emission from nearby galaxies, HVC's,
or Galactic ISM structures for their practicum.
Observational techniques for spectral line mapping include the
simple questions of sensitivity, bandwidth, resolution, and
integration time, plus more problematic questions such as
bandpass calibration, continuum subtraction, gridding, and
absolute brightness and flux calibration. I will show some
examples of how we calibrate "on-the-fly" mapping data from
the Parkes and Arecibo telescopes. Briefly I will discuss
combining maps from interferometer and single dish telescopes.
Understanding spectral line cubes is the main goal of my class.
I will illustrate structures in cubes with several real data sets
of both Galactic and extra-galactic HI maps. I hope to use the
KARMA package for this, with the students working directly with
the data in small groups, to do some quick exercises. Then I will
formally define the moments of the cube and illustrate them
with data. We will discuss the best ways to compute moments
(robust against noise and interference) and what the moments mean
physically.
Finally as an important example of using moment maps, we will
consider the case of a gaseous disk in differential rotation, as
in a spiral galaxy. We will discuss the shapes of rotation
curves, the significance of the velocity dispersion, and the
column density distribution as measured with the 21-cm line
for spiral galaxies. We will also consider how best to fit
the several parameters of a model for a rotating disk, and the
characteristic signatures of errors in the parameters which appear
in the residual map.
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.
Pulsar Observations: Coherent Dedispersion, Polarization, and Timing
Ingrid Stairs (NRAO)
Pre-detection, or "coherent," dedispersion is a powerful technique which
completely eliminates the effects of dispersive smearing on pulsar
profiles, greatly increasing the precision of timing and polarization
observations. Various software and hardware implementations of this
method are described. Polarization observations with filterbanks and
with coherent dedispersion are discussed, including calibration and
compensation for instrumental effects. Finally, the technique of pulsar
timing is presented, with descriptions of the method for obtaining
pulse times of arrival and the software used to fit models of the
pulsar's spin-down behavior, and a broad overview of scientific goals.
Planetary Radar
Greg Black (NRAO)
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.
Fundamentals of Polarization Measurement
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.
Calibration techniques at radio wavelengths
Karen O'Neil (NAIC)
o System temperature
- How to measure
o Flux Density conversion
- Rayleigh-Jeans approximation
o Aperture efficiency
- definition, measurement techniques
o Beam efficiency
o Coma/sidelobes
o Pointing/Focus Issues
o Pulsed noise cal
o Load cal
o Position/Frequency Switching
o Flux density standards
o Gain curves
o Standard sources - line/continuum
o Kraus
o Baars paper
o etc.
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.
Reduction and Analysis Techniques
R. 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.
AIPS++ DISH tutorial
Joe McMullin (NRAO)
DISH is a package within AIPS++ that provides a streamlined environment for
working with single dish radio astronomy data. The priority in the development
of DISH has been to produce traditional spectral line analysis tools with the
flexibility and extensibility that is a key attribute of the AIPS++
package.
The Effects of the Atmosphere and Ionosphere
Luca Olmi (University of Massachusetts)
In this lecture I will mainly discuss the effects of the neutral
atmosphere, and to a lesser extent of the ionosphere, on radio astronomical
observations carried out with
single-dish telescopes from the surface of the Earth. We are concerned
with three types of effects: large-scale refractive effects,
absorption, and scattering by the turbulent structure in the media. The
phenomenon of scattering results in "seeing", and creates refractive
index variations which limit the resolution and sensitivity of
observations of astronomical sources. In the troposphere, water vapor
plays a fundamental role in radio propagation, the refractivity of
water vapor being about 20 times greater in the radio range than in the
near-infrared or optical regimes. As a consequence, phase fluctuations
at frequencies higher than about 1 GHz are predominantly caused by
fluctuations in the distribution of water vapor, and I will thus
concentrate this lecture on tropospheric-induced radio seeing.
On filled-aperture telescopes, radio seeing shows up as an anomalous
refraction (AR), i.e. an apparent displacement of a radio source from
its true position. The magnitude of this effect, as a fraction of the
beam width, is bigger on larger telescopes, and thus its impact on
pointing is likely to become critically important in the next
generation of electrically large, filled-aperture radio telescopes. I
will thus present the results of recent, systematic AR measurements and
discuss a model study of AR effects, obtained via numerical
simulations of two-dimensional phase screens. I will finally discuss
the basic concept and requirements of a tip-tilt compensation system at
millimeter wavelengths, and will also present a proposed design based
on a scanning microwave radiometer as a wave front sensing device.
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.
Spectrum Management
Tapasi Ghosh (NAIC)
Life without the use of radio frequencies is unimaginable today. The
frequency range involved, and the mode of its usage, is diverse and
ever-changing with the developments of the latest technology. In the
midst of all this, there is a minority of spectrum users who are
labeled "passive". Radio astronomers and remote sensing groups come
under this heading, having no control over the signals that they
attempt to receive. It is of the utmost importance that all spectrum
users may operate and evolve in a manner of peaceful coexistence. This
is the goal of spectrum management.
Short-Spacings Correction from the Single-Dish Point of View
Snezana Stanimirovic (NAIC)
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.
Focal Plane Arrays
John Payne (NRAO)
Over the past few years, the sensitivity of mm/sub-mm receivers has
improved dramatically. We are now at the point of the atmosphere and
other inevitable sources of noise being equal to, or perhaps greater than,
the noise contributed by the receiver itself. Under such conditions, the
observing efficiency (for mapping extended regions) may be improved by
adding receivers in the focal plane in the manner of the CCDs used in
optical astronomy. Details of the principles involved and brief
descriptions of existing systems will be given.
Bolometers for Millimeter-Wave Astronomy
Wayne S. Holland & William D. Duncan (ATC)
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.
Last modified: 3-April-2001
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