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NAIC-NRAOSchool on Single-Dish Radio AstronomyJuly
10-17, 2005, Arecibo, Puerto Rico
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NAIC-NRAOSchool on Single-Dish Radio AstronomyJuly
10-17, 2005, Arecibo, Puerto Rico
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Historical Introduction to Radio Astronomy
R. Brown (NAIC)
The entire history of Radio Astronomy, from Karl Jansky's discovery of cosmic
radio emission to the present, encompasses barely 70 years. This makes the
telling of the intellectual development of radio astronomical research
comparatively concise. On the other hand, it is a history that from its origins
has been the story of the development of technology. Many of the fundamental
research achievements of radio astronomy were spawned by technological
innovations thus leading to new astrophysical ideas and insight. Unlike the
case with optical astronomy where often ideas motivated observations that in
turn led to technological innovations, in radio astronomy the intellectual
progression occurred in exactly the opposite sense. In this talk we will
review the research capabilities that are the foundation of modern radio
astronomy and examine how, and why, they came to be.
Historical Introduction to the Arecibo Observatory
D. Altschuler (NAIC)
Introduction to the GBT
P. Jewell (NRAO)
Basic Principles of Single Dish Radio Telescopes
P. F. Goldsmith (NAIC)
Astronomy at radio wavelengths is carried out with individual antennas,
or antennas combined into arrays. We here focus on "single dish"
systems and define the basic terminology and parameters that determine
their performance. At all but the longest wavelengths, radio telescopes
start with a feed system, which a feed illuminates the antenna aperture
with a well-defined amplitude distribution. The primary antenna acts as
a phase transformer, and from its output we can calculate the far-field
radiation pattern. With this very general and powerful model, we
analyze in some detail the behavior of different types of antennas, and
how their performance impacts the radio astronomical data that they
produce.
Theory of Measurements with Fully-Filled Apertures
C. Salter (NAIC)
This lecture covers the fundamental principles of radio-astronomical
observations with a fully-filled aperture, simple receiver systems and
measurement limitations imposed by noise, etc. The telescope as a
Fourier filter of the spatial frequency content of the celestial
brightness distribution will be introduced, and constraints on sampling
the sky brightness distribution are described.
Why Single Dishes?
D. Emerson (NRAO)
Many new radio telescopes under construction operate as interferometers,
rather than as stand-alone single dish instruments. Interferometers have
many advantages over single dish telescopes, most notably in providing much
superior angular resolution.
However, single dish instruments still provide some unique capabilities
not available with purely interferometric instruments - in particular
better sensitivity to large scale structure and greater flexibility.
Some scientific projects require BOTH single dish observations and
interferometric data; some new instruments already integrate both
capabilities, so in future the distinction between "single dish" and
"interferometer" observing may become blurred. It is important to
choose the right tools for a given job.
Microwave Receiver Systems, including IF/LO
J. B. Hagen (NAIC)
This presentation will explain what comprises a receiver, the
functions of a receiver, and the distinctions between "front ends"
and "back ends" and single polarization and dual polarization
receivers. Receiver noise and dynamic range will be discussed, as well
as an overview of receiver technology.
Backends
R. Fisher (NRAO) / J. Hagen (NAIC)
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.
Continuum Observing
J. Condon (NRAO)
Single dishes are used to make continuum images of extended fields and
low-brightness sources, and they provide missing short-spacing data
for interferometers. Continuum calibrators are monitored by all
single-dish observers to measure and optimize telescope performance.
The consequences of pointing errors, receiver noise and gain changes,
atmospheric emission and absorption, and source confusion are
described, along with observing techniques that minimize or correct
for them. References are given to data sets useful for planning and
calibrating single-dish observations.
Spectral Line Theory and Observations
H. Liszt (NRAO)
This will be a really snappy introduction designed to give an
intuitive feeling for the 21cm H I line profiles which students
will encounter during the Summer School. It will range (some
might say veer) widely (or perhaps wildly) from the internal
physics of atoms -- the material of the printed version of this
lecture -- to astrophysics of the interstellar medium (which
was originally covered by John Dickey in a separate lecture).
Along the way I'll note the ways in which our instruments and
observing practices shape our view of what is really "out there".
Calibration Techniques at Radio Wavelengths
K. O'Neil (NRAO)
I will give an overview to calibration techniques at centimeter wavelengths.
The talk will include a description of why calibration is needed, calibration
scale defnitions, antenna efficiency measurements, temperature measurements, and
baseline calibration.
Single Dish Data Reduction and Analysis Techniques
R. Maddalena (NRAO)
Single-dish observations can be made in a myriad of ways with
different observing techniques 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.
Techniques for Spectral Baseline Fitting
C. Heiles (UCB)
Any observed spectrum of a line (the ON spectrum), e.g. the
21-cm line, is always contaminated by instrumental effects. These
effects are produced at either the radio frequencies of the line (before
the first mixer) or intermediate frequencies of the system (after the
first mixer). The conventional way to eliminate them is by obtaining a
comparison (OFF) spectrum, taken either off the line frequency or off of
the source position, and subtracting it and/or dividing it into the ON
spectrum. These OFF spectra are taken under different conditions than
the ON, so the instrumental effects don't cancel perfectly. We describe
a new technique, Least-Squares Frequency Switching, which explicitly
derives the instrumental effects so they can be eliminated without using
OFF spectra. The GALFA group uses this to obtain accurate 21-cm line
maps with the multifeed ALFA system.
A Heuristic Introduction to Radio Astronomical Polarization
C. Heiles (UCB)
Radio sources are often polarized, so even this basic
measurement requires a basic understanding of polarization. Accurate
measurement of simply the flux density of a radio source requires a
basic understanding of polarization and its measurement techniques. We
provide an introductory, heuristic discussion of these matters with an
emphasis on practical application and avoiding pitfalls.
Short Spacings Correction from the Single Dish Perspective
S. Stanimirovic (UCB)
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.
Pulsars I. The Whys and Hows of Searching for Exotic Pulsars
J. Cordes (NAIC)
This first talk on pulsars will describe why we wish to conduct
deep surveys for new pulsars. The basic answer is that we want to
find rare objects that provide opportunities for studying matter
in extreme states (millisecond pulsars and magnetars) and serve as laboratories
for gravitation (pulsars in compact binaries with neutron star or
black hole companions). Along the way to finding these, we also discover
large numbers of "ordinary" pulsars; these are useful for understanding
the overall Galactic population of pulsars and the nature of the runaway
velocities of the pulsar population, and for using pulsars to model
the magnetic field and ionized gas of the Galaxy.
We will discuss key aspects of pulsar surveys, including
compensation for the dispersive nature of the interstellar medium
(dedispersion techniques), compensation for binary motion, and
algorithms that exploit the periodicity that is typical of many pulsar
signals and also single-pulse algorithms for pulsars that are heavily
modulated. Characterization of pulsar surveys through simulations will
be described. Lastly, we will cover the ongoing, massive Galactic
plane survey using the Arecibo L-band Feed Array, including a summary
of its goals, data management issues, and preliminary results.
Pulsars II. How to understand what exotic pulsars are telling us
P. Freire (NAIC)
This second talk on pulsars will describe in more detail how we define
what is an exotic pulsar. The basic concepts and techniques of pulsar
timing are presented: how we determine the period and the period
derivative and the sky position, how the age and the magnetic field of
a pulsar are estimated. We then present the P-Pdot diagram, where we
can see the basic differences between "normal" (and "young") pulsars,
millisecond pulsars and magnetars. We describe briefly the
evolutionary paths that lead to the formation of these objects, in
particular the formation of pulsar in binary systems. We then describe
how to time such binary pulsars, and how this leads to estimates of
the orbital parameters that are thousands or millions of times more
precise than what is possible by measuring radial velocities. We then
explain how this unique feature of binary pulsars has been used to
test Einstein's theory of general relativity, and is now being used to
probe the basic properties of nuclear matter at densities greatly
exceeding that of the atomic nucleus.
Stray Radiation and How to Deal with it
F. J. Lockman (NRAO)
Radio telescopes respond to signals from all directions, not just
in the direction of their main beam, and thus observations can be
contaminated by 'stray' radiation. For some experiments this is
the most significant limit on the dynamic range of the results. Thinking
about stray radiation is also useful because it makes us consider the
properties of radio telescopes more deeply than we might otherwise.
In this talk I will discuss the origin and characteristics of telescope
sidelobes and resulting stray radiation, and will present ways to
minimize or mitigate its effects.
Planetary Radar: Theory and Observation
J. Harmon (NAIC)
In this talk I will review the basic principles
of radar astronomy, discuss observation and
analysis methods, and provide illustrative examples
from observations made with the Arecibo S-band radar.
The discussion of elementary radar principles will be
presented using the radar and radiometer equations
as the basis. This will be followed by a discussion of
radar scattering theory, including quasispecular and
diffuse scattering, coherent volume backscatter, and
polarization effects. I will then discuss radar system
components and outline methods used in CW and delay-Doppler
observations. Examples will be given from observations
of Mercury, Mars, the Moon, asteroids, comets. etc.
RFI and How to Deal with it
R. Fisher (NRAO) / B. Lewis (NAIC)
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.
Large Area Sky Surveys
C. Heiles (UCB)
Large-area sky maps at any wavelength cannot stand alone, but
need comparison with other datasets to extract meaningful science. We
summarize the basic multiwavelength maps currently available. These
include the 21-cm line (LDS), diffuse IR emission (IRAS, DIRBE), H alpha
line (WHAM + others), short-wavelength radio maps (WMAP), X-rays
(ROSAT), cm-wave radio maps (various), gamma ray (CGRO). The website
SKYVIEW is very handy for accessing these databases. Having the data is
one thing; presenting large swaths on the sky on a flat sheet of paper
is another, requiring choice of a projection. We describe several
projections with mutually inconsistent properties (e.g., minimal
geometric distortion versus equal area pixels).
Writing Effective Telescope Proposals
C. Salter (NAIC)
Some general points to consider when preparing a telescope proposal
will be presented. These have been distilled from the speaker's
experience as an astronomer, a sometime proposal referee for a major
national facility, and a member of the Arecibo Scheduling Advisory
Committee. He cautions that his own proposals do not always meet with
outstanding success!
A Pulsar Renaissance: Recent Events in the World of Pulsars
S. Ransom (NRAO)
In this talk I'll briefly mention three very exciting
new developments in pulsar astronomy over the past couple years
and how they are allowing (or will allow) us new and unique
probes into a wide variety of physics: The double-pulsar binary
J0737-3039 (a "Holy Grail" at last!), the millisecond pulsar
jackpot in the globular cluster Terzan5, and the recently begun
Pulsar-ALFA survey here at Arecibo.