Transcript Jewell - GBT

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
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Presentation Overview
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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?
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What is Radio Astronomy?
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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
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What can we do with Radio Astronomy?
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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.
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What sorts of things do Radio
Astronomers Observe?
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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.
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What sorts of things do Radio
Astronomers Observe? – cont’d
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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
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What is the National Radio Astronomy Observatory?
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Since 1956, the NRAO has been the premier radio astronomy
observatory in the U.S.
It’s 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.
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The Robert C. Byrd Green Bank Telescope
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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.
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The Very Long Baseline Array (VLBA)
Mauna Kea
Hawaii
Owens Valley
California
Brewster
Washington
North Liberty
Iowa
Hancock
New Hampshire
Kitt Peak
Arizona
Pie Town
New Mexico
Fort Davis
Texas
Los Alamos
New Mexico
St. Croix
Virgin Islands
10, 25-meter diameter telescopes for ultra-high angular resolution
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Atacama Large Millimeter Array
64, 12-meter diameter dishes for millimeter and sub-millimeterwave imaging, presently under construction in Chile.
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NRAO Headquarters Facilities
Charlottesville, Virginia
HQ + NA ALMA Science Center
(Edgemont Road on UVa Campus)
NRAO Technology Center
(Ivy Road)
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Interferometers and Single Dishes
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Radio Astronomy telescopes come in two basic flavors,
single dishes and and arrays of dishes, known as
interferometers
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Interferometers
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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 -> EVLA
 NRAO Very Long Baseline Array
 Atacama Large Millimeter Array (ALMA)
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Single Dishes
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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.
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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.
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Why is there an observatory in Green Bank?
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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.
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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.
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The story of the GBT – The 300 Foot
The 300 Foot in collapse – Nov. 1988
What makes the GBT special?
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Size
Unblocked main aperture
Precision Control System
 Active Surface
 Sensing systems
Frequency coverage
National Radio Quiet
Zone location
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GBT Size
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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
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Conventional optics with symmetric
(blocked) feed supports
Effelsberg 100 m Telescope
NRAO 140 Foot Telescope
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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
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GBT Pointing and Surface Compensation
Systems
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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)
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Various temperature and position sensing systems and
computer models will be used to point the telescope and
command the surface actuators
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Ultimately, this will allow the GBT to work to ~115 GHz or
2.6 mm wavelength
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GBT active surface system
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Surface has 2004 panels
 average panel rms: 68 m
2209 precision actuators
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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
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Discrete HI Clouds in the Galactic Halo
Artist’s rendition of the Milky Way with actual
GBT data in the inset – Lockman (2002).
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Continuum Images of the Rosette
Nebula
Ghigo & Maddalena (2003)
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GBT / Arecibo radar image of the Moon
B. Campbell et al. (2004)
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