The Green Bank Telescope
Ronald Maddalena
National Radio Astronomy Observatory
National Radio Astronomy Observatory
National Laboratory
Founded in 1954
Funded by the National Science
Foundation
Telescope Structure and Optics
Telescope Structure and Optics
•Large 100-m Diameter:
•High Sensitivity
•High Angular Resolution – wavelength / Diameter
GBT Telescope Optics
110 m x 100 m of a 208 m parent paraboloid
• Effective diameter: 100 m
• Off axis - Clear/Unblocked Aperture
Telescope Optics
High Dynamic Range
High Fidelity Images
Telescope Optics
Telescope Optics
Telescope Optics
Prime Focus: Retractable boom
Gregorian Focus: 8-m subreflector - 6-degrees of freedom
Telescope Optics
Rotating Turret with 8 receiver bays
Telescope Structure
Fully Steerable
Elevation Limit: 5º
Can observe 85% of the entire Celestial Sphere
Slew Rates: Azimuth - 40º/min; Elevation - 20º/min
Telescope Structure
Blind Pointing: 2 5 arc sec
(1 point/focus) ( focus) 2.5 mm
Offset Pointing: 2 2.7 arc sec
(90 min) ( focus) 1.5 mm
Continuous Tracking: 2 1 arc sec
(30 min)
Telescope Structure
Active Surface
Surface Deformations from Finite Element Model
Active Surface
Active Surface
Main Reflector: 2209 actuated panels with 68 μm rms.
•Total surface: rms 400 μm
Receivers
Receiver Operating Range Status
Prime Focus 1 0.29—0.92 GHz Commissioned
Prime Focus 2 0.910—1.23 GHz Commissioned
L Band 1.15—1.73 GHz Commissioned
S Band 1.73—2.60 GHz Commissioned
C Band 3.95—5.85 GHz Commissioned
X Band 8.2—10.0 GHz Commissioned
Ku Band 12.4—15.4 GHz Commissioned
K Band 18—26.5 GHz Commissioned
Ka Band 26—40 GHz Partially Commissioned
Q Band 40—50 GHz Commissioned
W Band 68—92 GHz Under Construction
Penn Array 86—94 GHz Under Construction
Backends
National Radio Quiet Zone
National Radio Quiet Zone
Science with the GBT
Current Science Projects
Milky Way
Our Home Galaxy
Projected image on the night sky is the Milky Way
Dust in the Interstellar Medium obstructs our optical view.
Need Radio observations to peer through the dust
Our perspective is from a star in the outer Milky Way.
Serves as a nearby example of the 100 billion other galaxies
Interstellar Medium
The Material Between the Stars
• Constituents
• Gases
•Hydrogen (92% by number)
•Helium (8%)
•Oxygen, Carbon, etc. (0.1 %)
• Dust Particles
•1% of the mass of the ISM
• Average Density: 1 H atom / cm3
• Place where stars & planets form
• The byproduct of the death of stars
Interstellar Medium
Properties
State of Temperature Densities Percent
Hydrogen (H/cm3) Volume
HII Regions Ionized 5000 K 0.5 300 10%
Clouds
Scientific Results - Imaging
Scientific Results - Imaging
Scientific Results - Imaging
Diffuse ISM – Galactic Center
Diffuse ISM – Galactic Center
Atomic Hydrogen
Spectral-Line Radiation
•Discovered by Ewen and Purcell in 1951.
•Found in regions where H is atomic.
•300 K, 30 H/cm3
•Spin-flip (hyperfine) transition
•Electron & protons have “spin”
•In a H atoms, spins of proton and electron may be
aligned or anti-aligned.
•Aligned state has more energy.
•Difference in Energy = h v
•v = 1420 MHz
•An aligned H atom will take 11 million years to flip
the spin of the electron.
•But, 1067 atoms in Milky Way so 1052 H atoms per
second emit at 1420 MHz
Spectral-Line Radiation- What do they tell us?
• Width of line Motion of gas within the region
• Height of the line Maybe temperature of the gas
• Area under the line Maybe number of atoms in that direction.
Doppler Affect
Frequency Observed = Frequency Emitted / (1 + V/c)
Spectral-Line Radiation
Milky Way Rotation and Mass
• For any cloud
• Observed velocity = difference
between projected Sun’s motion
and projected cloud motion.
• For cloud B
• The highest observed velocity
along the line of site
• VRotation = Vobserved + Vsun*sin(L)
• R = RSun * sin(L)
• Repeat for a different angle L and
cloud B
• Determine VRotation(R)
• From Newton’s law, derive M(R)
from V(R)
Scientific Results – Milky Way Gas
Scientific Results – Milky Way Gas
Scientific Results – Milky Way
Interstellar Molecules
•Hydroxyl (OH) first molecule found with radio
telescopes (1964).
•Molecule Formation:
•Need high densities
•Lots of dust needed to protect molecules for
stellar UV
•But, optically obscured – need radio telescopes
•Low temperatures (< 100 K)
•Some molecules (e.g., H2) form on dust grains
•Most form via ion-molecular gas-phase
reactions
Interstellar Molecules
Ion-molecular gas-phase reactions
•Starts with a cosmic ray
that ionizes a H atom
•All exothermic reactions
•Charge transfer
•Two-body interactions
Interstellar Molecules
•About 90% of the over 129 interstellar molecules
discovered with radio telescopes.
•Rotational (electric dipole) Transitions
•Up to thirteen atoms
•Many carbon-based (organic)
•Many cannot exist in normal laboratories (e.g., OH)
•H2 most common molecule:
•No dipole moment so no rotational transition at radio
wavelengths.
•Only observable in UV (rotational) or Infrared
(vibrational) transitions from space.
•Use CO, the second most common molecule, as a
tracer for H2
Interstellar Molecules
A few molecules (OH, H2O, …) maser
Scientific Results - Molecules
Molecular Clouds
Discovered 1970 by
Penzias, Jefferts, & Wilson
and others.
Coldest (5-30 K), densest
(100 –106 H atoms/cm3) parts
of the ISM.
Where stars are formed
50% of the ISM mass
A few percent of the
Galaxy’s volume.
Concentrated in spiral
arms
Dust Clouds = Molecular
Clouds
Molecular Clouds
Discovered 1970 by
Penzias, Jefferts, & Wilson
and others.
Coldest (5-30 K), densest
(100 –106 H atoms/cm3) parts
of the ISM.
Where stars are formed
50% of the ISM mass
A few percent of the
Galaxy’s volume.
Concentrated in spiral
arms
Dust Clouds = Molecular
Clouds
Scientific Results – Lunar Radar
Scientific Results – Galaxy Formation
Scientific Results - Pulsars
Scientific Results - Pulsars
• Fastest Pulsar