Transcript The Green Bank Telescope Richard Prestage NASA/NRAO Joint Institute 19th July 2006 Outline of talk • Overview of the GBT, and how it came to be.

The Green Bank
Telescope
Richard Prestage
NASA/NRAO Joint Institute
19th July 2006
Outline of talk
• Overview of the GBT, and how it
came to be built
• Brief outline of unique active control
systems
• A few science highlights
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Chronology of the GBT
• A next-generation, large single dish had been desired for
many years and recommended by 1980 Radio Decade
Review panel
• Study group on a new, large dish, was at work at NRAO
during 1988
• 300 Foot collapsed unexpectedly on 15 November 1988
while in routine use
• Workshop on design of a new telescope held on 2-3
December 1988
• Senator Byrd (WVa) offers to help in 1989
• $75M appropriated for new telescope in June 1989
• Contract awarded in December 1990 to Radiation Systems,
Inc. for $55M ($20M for NRAO systems)
• Dedicated: August 25, 2000; Commissioning: 2001/2002;
Fully Operational: 2003
Original Schedule - 1989
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Groundbreaking
May 1, 1991
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Track foundation, Oct. 1991
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Track sections - 1992
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Alidade - Oct 1994
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Hoisting the elevation axle - May 1995
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Elevation gear assembly – March 1996
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Dish backup structure – June 1996
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Horizontal Feedarm – May 1997
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Horizontal arm and backup structure –
Dec 1997
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Final sections of backup structure – June
1998
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Subreflector and
tip of feed arm –
April 1999
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Installing the surface panels – Nov 1999
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Dedication – August 2000
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What makes the GBT
special?
• Size
• Unblocked main aperture
• Precision Control System
– Active Surface
– Metrology
• Frequency coverage
• National Radio Quiet
Zone location
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The GBT is large…..
• Largest fully-steerable telescope in the
world
• At 17.3 Million Pounds (7856 metric tons),
probably the largest moving structure on
land.
• Despite size and mass, built to extremely
high precision
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Unique design – unblocked
aperture
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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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Advantages of an
unblocked
aperture
Reduces systematic responses, that are often the ultimate
limitation in sensitivity:
• No blockage of incident signal
• Reduced scattering sidelobes
• Reduced spectral standing waves
• Less RFI pickup
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GBT
Precision Control System
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Performance Metrics
• Telescope performance can be quantified by two
main quantities:
• 1. Image quality / efficiency:
– PSF / Strehl ratio (optical)
– Beam shape / aperture efficiency (radio)
• 2. Ability to point it in the right direction
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Image quality - optical
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Image quality/efficiency radio
Aperture efficiency
η = Ae/A
Ruze formula
η = η0 exp[(-4πε/λ)2]
ε = rms surface error
“acceptable” performance:
ε = λ/4π
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Image quality/performance
requirements for GBT
• total wavefront error ~1/15th of the wavelength of observations:
– L-band (21cm) – 1cm or ~ ½ inch
– W-band (3mm) – 200µm, the thickness of a human hair!
• telescope pointed on the sky to ~1/10th of its beamwidth to avoid
lost sensitivity or inaccurate results.
– L-band (21cm) – 1 arcmin (diameter of Venus)
– W-band (3mm) – 1 arcsec (small telescope stellar image)
– 8m optical telescope performance!
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Scientific Requirements
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Small telescopes….
• Small optical telescopes (λ << D)
– geometrical optics/aberration theory.
– Two-mirror telescopes ~ perfect images given atmosphere
• Small radio telescopes (satellite dishes/Direct TV)
<< D)
(λ not
– Need to use diffractive optics
– Beam pattern in focal plane becomes an Airy disk
– Can still build ~perfect implementations of chosen optical
design
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Challenges for large
telescopes
• Manufacturing to required tolerances (100m
diameter primary accurate to 1 part in a million)
• Accurate alignment
• Gravitational deformations
• Thermal deformations
• “non-repeatable” effects – wind, servo errors, etc.
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Challenges for large
telescopes
The Astronomical Journal, February 1967
Solutions…
• innovative design/construction
• Calibration measurements
• Real-time monitoring/dynamic adjustments
(Potential alternative: use laser rangefinders to
measure absolute position of all optical elements
and correct appropriately. Not yet demonstrated.)
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Telescope Construction
The Astronomical Journal, February 1967
Homologous design
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Homologous design
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GBT active surface
system
• Surface has 2004 panels
– average panel rms: 68 m
• 2209 precision actuators
Designed to operate in:
• open loop from
look-up table
• closed loop from
laser metrology
system
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Mechanical adjustment of the panels.
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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
Current FEM Model
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FEM corrections work well to
20GHz
rms after active surface correction < 0.5mm at 50 deg.
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Next steps - holography
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Traditional (phase-reference)
holography
• Dedicated receiver to look at (usually) a terrestrial
transmitter (at low elevation) or geostationary
satellite.
• Second dish (or reference antenna) provides
phase reference.
• Measure amplitude and phase of (near or far)-field
beam pattern.
• Fourier transform to determine amplitude and
phase of aperture illumination.
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Alternative – “phase-retreival”
holography
• There are many advantages to traditional holography,
but also some disadvantages:
– Needs extra instrumentation
– Reference antenna needs to be close by so that
atmospheric phase fluctuations are not a problem
– S/N ratio required limits sources to geostationary
satellites, which are at limited elevation ranges for the
GBT (35-45)
• Alternative: measure power (instead of phase and
amplitude) only, recover phase by modeling.
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Zernike polynomials
z2: phase gradient
(pointing shift)
z5: astigmatism
z8: coma
aperture plane
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Example Ka-band OOF maps. Weighted rms = 370 microns.
best Ka-band OOF map (prev map applied) WRMS = 80 microns
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Surface Accuracy
•
Large scale gravitational
errors corrected by “OOF”
holography.
•
Benign night-time rms
~ 350µm
•
Efficiencies:
43 GHz: ηS = 0.67 ηA =
0.47
90 GHz: ηS = 0.2
0.15
•
ηA =
Now dominated by panelpanel errors (night-time),
thermal gradients (daytime)
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Science Highlights - Pulsars
Double pulsar system
J0737-3039
• Most stringent test of GR in
the strong-field limit so far
(Kramer et al.)
Globular cluster pulsars
•
•
•
The GBT has found at least 57
new globular cluster pulsars since it
has been in operation
More in only 3 years than any of the
other radio telescopes in the world
have uncovered in their entire
lifetimes
Terzan5ad - fastest millisecond pulsar
yet discovered: 1.39ms (716 Hz)
(Hessels, Ransom et al.)
Mass-mass diagram summarizing observational
constraints on the masses of the neutron staff in the
double pulsar system. (Kramer et al.)
Science Highlights - HI
• HI (neutral hydrogen) halo of
the Milky Way near the
Scutum spiral arm
• 7 kpc from the Sun and 4
kpc from the Galactic center
• Total mass ~ 1M solar
masses
• energy powering outflow ~
100 supernova explosions
• 10-30 million years old
Yurii Pidopryhora, Jay Lockman &
Joe Shields
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super-bubble in our own back
yard
Dark Energy and H0
“While models with ΩDE=0 are not
disfavored by the WMAP data only, the
combination of WMAP data plus
measurements of the Hubble constant
strongly constrain the geometry and
composition of the universe”
Spergel et al. 2006
•“The single most important complement to the
CMB for measuring the DE equation of state at
z ~ 0.5 is a determination of the Hubble
constant to better than a few percent.”
Hu 2005
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Distances to H2O Megamasers
Requires:
•
Detect the best candidates (GBT surveys)
•
Measure accelerations (GBT Monitoring)
•
Assess VLBI calibrators (VLA snapshots)
•
VLBI imaging (VLBA + GBT + Eff)
•
Modeling
•
10+ distances to obtain H0 with better than
3% uncertainty
Goal:
NGC 4258
NGC 6323
V = 7772km/s, D ~ 110Mpc for H0 = 70 (km/s)/Mpc
(Work by Braatz et al.)
The future – imaging arrays
• Bolometer array – under construction now in collaboration
with University of Pennsylvania.
– 90GHz, 64 pixels, 8” spacing (3mm physical size)
– TES (transition-edge superconductor) sensors
– SQUID (Superconducting QUantum Interference Devices)
readouts
• MMIC (monolithic microwave integrated circuit) heterodyne
arrays – proposed future collaboration
• L-band and Ka-band “beam-forming” arrays – future NRAO
development.
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Bolometer array
•A 4x4 rendering of the GBT array.
•Each 3x3 mm absorber is suspended by
four legs.
•The TES for each sensor is shown in red.
•The detectors are spaced at 3.3 mm = 0.5 f λ.
•The design must provide uniform performance over a
34 mm diameter focal plane.
•The illumination of the primary is determined by a cold
Lyot stop placed at the primary's image.
•The array, the last lens and the Lyot stop must be kept
in a cavity that has a temperature less than 3 K to
reduce the load on the detectors
A cross-section of the planar
array showing the major
components.
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Conventional single-pixel receiver
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●
●
focal plane array: 4×4 pattern.
currently mounted on the FCRAO
14m telescope
●
Will be moved to the LMT
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fixed tuning => best performance at all frequencies
●
being expanded to 32 elements
●
InP MMIC pre-amplifiers: 35-40 dB gain band
●
(Tsys=50 – 80 K)
●
instantaneous bandwidth: 15 GHz
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