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Radio Astronomy and the NRAO

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Title: Radio Astronomy and the NRAO


1
Radio Astronomy and the NRAO
  • Phil Jewell
  • Assistant Director for Green Bank Operations
  • National Radio Astronomy Observatory

VIP Visit to Green Bank 8 May 2004
2
Presentation Overview
  • What is Radio Astronomy?
  • What is the National Radio Astronomy Observatory?
  • What are the differences between single dishes
    and Interferometers?
  • What makes Green Bank and the GBT special?

3
What is Radio Astronomy?
  • Radio waves are part of the electromagnetic
    spectrum, along with the infrared, visible light,
    ultraviolet, x-rays, and gamma rays.
  • Radio astronomy studies cosmic electromagnetic
    emission from about 10 MHz to 1000 GHz
    (wavelengths from about 30 meters to 0.3 mm.)

From Living with a Star NASA/UCB
4
What can we do with Radio Astronomy?
  • Radio waves have lower energy than light waves,
    x-rays, etc. Whereas light waves come from very
    hot objects such as stars, radio waves usually
    come from cooler objects. Astronomers use the
    whole electromagnetic spectrum to probe the
    universe.
  • Because radio waves have long wavelengths, they
    pass through dust that obscure light waves. This
    allows radio astronomers to probe to the heart of
    star forming regions, to disks around the nuclei
    of galaxies, and possibly to the event horizons
    of black holes.

5
What sorts of things do Radio Astronomers Observe?
  • Solar System objects
  • The Sun, planets, asteroids, and comets
  • From their own emission, or in some cases, from
    radar echoes
  • The structure of the Milky Way and other
    galaxies.
  • Can trace the gas and dust through the emission
    of atomic and molecular spectral lines
  • The star formation process
  • Can use molecular spectral line and dust emission
    to trace the collapse of interstellar clouds
  • Neutron stars
  • Extremely dense, collapsed stars that are
    rotating as fast as 1000 times a second.
  • An incredible laboratory for basic physics that
    cannot be replicated on Earth.

6
What sorts of things do Radio Astronomers
Observe? contd
  • Black hole physics
  • Incredible energy engines in the nuclei of
    galaxies and quasars
  • The origin and structure of the universe and
    galaxies within it
  • Astrochemistry
  • Almost 130 molecules some quite complex -- are
    known in the interstellar medium
  • Some are of biological significance and may be a
    key to the formation of life on the early Earth

7
What is the National Radio Astronomy Observatory?
  • Since 1956, the NRAO has been the premier radio
    astronomy observatory in the U.S.
  • Its primary mission is to operate and develop
    unique radio astronomical telescopes and related
    instruments for use by researchers at
    universities and institutes in the U.S. and
    around the world.
  • The NRAO is headquartered in Charlottesville,
    Virginia and presently operates three main
    instruments
  • The Robert C. Byrd Green Bank Telescope (GBT)
  • Green Bank, West Virginia
  • The Very Large Array (VLA)
  • Near Socorro, New Mexico
  • The Very Long Baseline Array (VLBA)
  • 10 telescopes in an array from St. Croix, US
    Virgin Islands to Mauna Kea, Hawaii
  • The NRAO is also in charge of the North American
    contribution to the construction and operation of
    the Atacama Large Millimeter Array (ALMA), an
    international project being built in Chile.
  • The ALMA North American Science Center will be in
    Charlottesville.

8
The Robert C. Byrd Green Bank Telescope
9
The Very Large Array
27, 25-meter diameter telescopes located on the
Plains of San Augustin near Socorro, New
Mexico. EVLA expansion project underway which
will give x10 improvement in sensitivity.
10
The Very Long Baseline Array (VLBA)
10, 25-meter diameter telescopes for ultra-high
angular resolution

11
Atacama Large Millimeter Array
64, 12-meter diameter dishes for millimeter and
sub-millimeter- wave imaging, presently under
construction in Chile.
12
NRAO Headquarters FacilitiesCharlottesville,
Virginia
HQ NA ALMA Science Center (Edgemont Road on UVa
Campus)
NRAO Technology Center (Ivy Road)
13
Interferometers and Single Dishes
  • Radio Astronomy telescopes come in two basic
    flavors, single dishes and and arrays of dishes,
    known as interferometers

14
Interferometers
  • Interferometers arrays of antennas that work
    together -- provide
  • very high angular resolution
  • imaging through earth rotation synthesis
  • Their angular resolution is set by the farthest
    distance between antennas
  • Modern interferometers have extraordinary high
    resolution imaging capability. Examples
  • NRAO Very Large Array -gt EVLA
  • NRAO Very Long Baseline Array
  • Atacama Large Millimeter Array (ALMA)

15
Single Dishes
  • Single dishes are limited in their angular
    resolution by the size of the dish, but
  • They are very sensitive to large scale emission
    that may be missed by interferometers, and
  • They have very high sensitivity to weak, extended
    emission
  • In addition, they allow
  • Easy use of innovative instrumentation
  • Comparatively easy to build an instrument for one
    dish rather than 27 or 64
  • So single dishes and interferometers provide
    complementary information and capabilities, which
    is why the NRAO has both types of telescope.

16
Single Dishes and Interferometers -- Sensitivity
to Structures on Different Angular Scales
  • Example
  • The large scale structure of the GBT image of the
    Omega Nebula (M17) would not be detected by most
    interferometers.
  • And conversely, an interferometer might be able
    to image a tiny site of star formation in this
    cloud that would be seen only as a point by a
    single dish.

17
Why is there an observatory in Green Bank?
  • The National Radio Astronomy Observatory was
    founded in 1956
  • Green Bank was the first site of the Observatory
    and served as its first headquarters until 1967
    when it was moved to Charlottesville.
  • Green Bank was chosen for its sheltered location
    and natural protection from radio frequency
    interference, yet proximity to the population
    centers on the east coast.
  • In the late 1950s, the region around Green Bank
    was given special protection by the Federal
    Communications Commission and became the National
    Radio Quiet Zone.

18
National Radio Quiet Zone
Owing to the sensitivity of radio astronomy
observations, the NRQZ is critical to the
continued success of radio astronomy in Green
Bank, and the GBT in particular.
19
The story of the GBT The 300 Foot
20
The 300 Foot in collapse Nov. 1988
21
What makes the GBT special?
  • Size
  • Unblocked main aperture
  • Precision Control System
  • Active Surface
  • Sensing systems
  • Frequency coverage
  • National Radio Quiet Zone location

22
GBT Size
  • Largest fully-steerable telescope in the world
  • At 16.7 Million Pounds (7600 metric tons),
    probably the largest moving structure on land.
  • Despite size and mass, built to extremely high
    precision
  • Why is the GBT so big? Sensitivity

23
Conventional optics with symmetric (blocked) feed
supports
Effelsberg 100 m Telescope
NRAO 140 Foot Telescope
24
Unblocked aperture
  • 100 x 110 m section of a parent parabola 208 m in
    diameter
  • Cantilevered feed arm is at focus of the parent
    parabola

25
GBT Pointing and Surface Compensation Systems
  • To overcome distortions in its reflecting surface
    that result from gravity and thermal changes, the
    GBT is equipped with a fully active surface
    (motor actuated, computer controlled)
  • Various temperature and position sensing systems
    and computer models will be used to point the
    telescope and command the surface actuators
  • Ultimately, this will allow the GBT to work to
    115 GHz or 2.6 mm wavelength

26
GBT active surface system
  • Surface has 2004 panels
  • average panel rms 68 ?m
  • 2209 precision actuators

27
Surface Panel Actuators
  • One of 2209 actuators.
  • Actuators are located under each set of surface
    panel corners
  • Actuator Control Room
  • 26,508 control and supply wires terminated in
    this room

28
Discrete HI Clouds in the Galactic Halo
Artists rendition of the Milky Way with
actual GBT data in the inset Lockman (2002).
29
Continuum Images of the Rosette Nebula
Ghigo Maddalena (2003)
30
GBT / Arecibo radar image of the Moon
B. Campbell et al. (2004)
31
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