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Fundamentals of the GBT and Single-Dish Radio Telescopes
Dr. Ron Maddalena
National Radio Astronomy Observatory
Green Bank, WV March 2016 ©Associated Universities, Inc., 2016
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National Radio Astronomy Observatory
National Laboratory Founded in 1954 Funded by the National Science Foundation
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Telescope Structure and Optics
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Telescope Structure and Optics
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Telescope Structure and Optics
•Large 100-m Diameter: •High Sensitivity •High Angular Resolution – wavelength / Diameter
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Telescope Structure and Optics
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GBT Telescope Optics
110 m x 100 m of a 208 m parent paraboloid • Effective diameter: 100 m • Off axis - Clear/Unblocked Aperture
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Telescope Optics
High Dynamic Range High Fidelity Images
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Telescope Optics
Blockage
Spillover
Stray Radiation
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Telescope Optics
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Telescope Optics
Prime Focus: Retractable boom Gregorian Focus: 8-m subreflector - 6-degrees of freedom
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Telescope Optics
Rotating Turret with 8 receiver bays
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Telescope Structure
Fully Steerable Elevation Limit: 5° Can observe 85% of the entire Celestial Sphere
Slew Rates: Azimuth - 40°/min; Elevation - 20°/min
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National Radio Quiet Zone
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National Radio Quiet Zone
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Atmosphere
• Index of Refraction
– Weather (i.e., time) and frequency dependent
– Real Part: Bends the light path
– Imaginary part: Opacity
• http://www.gb.nrao.edu/~rmaddale/Weather/
• Winds
– Wind-induced pointing errors
– Safety
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• Opacity – Calibration
– System performance – Tsys
– Observing techniques
– Hardware design
• Refraction – Pointing
– Air Mass • Calibration
• Interferometer & VLB phase errors
– Aperture phase errors
• Cloud Cover – Continuum performance
– Calibration
• Winds – Pointing
– Safety
• Telescope Scheduling – Proportion of proposals
that should be accepted
– Telescope productivity
The Influence of the Atmosphere and Weather at cm- and mm-wavelengths
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Weather Forecasts for Radio Astronomy
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Weather Forecasts for Radio Astronomy
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Telescope Structure
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GBT active surface system
• Surface has 2004 panels – average panel rms: 68 µm
• 2209 precision actuators
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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
Surface Panel Actuators
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31
Finite Element Model Predictions
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Mechanical adjustment of the panels
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Image quality and efficiency
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Image quality, efficiency, resolution
mm) (3 GHz 115at 6.5"
cm) (21 MHz 1420at '9
m) (1 300MHzat '40
2.1
DHPBW
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Image quality and efficiency
71.0PowerIncident
Power Detected Efficiency Aperture A
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Holography
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Holography
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Surface accuracy (rms) = 240 µm
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Aperture Efficiency
2)/4(7.0 eA
= rms surface error
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Telescope Structure
Blind Pointing: (1 point/focus) Offset Pointing: (90 min) Continuous Tracking: (30 min)
mmfocus
arc
5.2)(
sec52
mmfocus
arc
5.1)(
sec7.22
sec12 arc
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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 4—8.0 GHz Recently upgraded
X Band 8—12.0 GHz Commissioned
Ku Band 12—15 GHz Commissioned
K Band Array 18—27 GHz Commissioned
Ka Band 26—40 GHz Commissioned
Q Band 40—50 GHz Commissioned
W Band 68—92 GHz Commissioned
Mustang Bolometer 86—94 GHz Being upgraded
ARGUS 80—115 GHz Being commissioned
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Receiver Room
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Typical Receiver
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Receiver Feeds
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Typical Receiver
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Typical Components
Amplifiers
Mixers
Attenuators
Power Detectors
Synthesizers
Splitters
Couplers
Filters
Switches
Multipliers
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Types of Filters
Edges are smoother than illustrated
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Types of Mixers
f fIF
fLO
• n and m are positive or negative integers, usually 1 or -1
• Up Conversion : fIF > f • Down Conversion : fIF < f
• Lower Side Band : fLO > f
- Sense of frequency flips • Upper Side Band : fLO < f
fIF = n*fLO + m*f
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40-Ft System
Determine values for the first LO for the 40-ft when Observing HI at 1420 MHz
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GBT – Astrid program does all the hard work for you…..
configLine = """ receiver = "Rcvr1_2" beam = “B1" obstype = "Spectroscopy" backend = "Spectrometer" nwin = 1 restfreq = 1420.4058 deltafreq = 0 bandwidth = 12.5 swmode = "tp" swtype = "none" swper = 1.0 swfreq = 0.0, 0.0 tint = 30
vlow = 0 vhigh = 0 vframe = "lsrk" vdef = "Radio" noisecal = "lo" pol = "Linear" nchan = "low" spect.levels = 3 """
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Power Balancing/Leveling and Non-Linearity
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Spectral-line observations
Raw Data
Reduced Data – High Quality
Reduced Data – Problematic
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Reference observations
• Difference a signal observation with a reference observation
• Types of reference observations – Frequency Switching
• In or Out-of-band
– Position Switching – Beam Switching
• Move Subreflector • Receiver beam-switch
– Dual-Beam Nodding • Move telescope • Move Subreflector
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Model Receiver
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Out-Of-Band Frequency Switching
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On-Off Observing
Noise Diode
Detector
SignalSignal
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Nodding with dual-beam receivers - Telescope motion
• Optical aberrations • Difference in spillover/ground pickup • Removes any ‘fast’ gain/bandpass changes • Overhead from moving the telescope. All the time is spent on source
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Nodding with dual-beam receivers - Subreflector motion
• Optical aberrations • Difference in spillover/ground pickup • Removes any ‘fast’ gain/bandpass changes • Low overhead. All the time is spent on source
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Intrinsic Power P (Watts) Distance R (meters) Aperture A (sq.m.)
Flux = Power Received/Area Flux Density (S) = Power Received/Area/bandwidth Bandwidth (BW) A “Jansky” is a unit of flux density HzmWatts //10 226
sfrequencie lowat GBTfor 0.2
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Gain
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PS
A
gAA
AirMass
gAA
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)1(
)1(
Airmass
ATM
Airmass
BackgroundACMBl
SpilllRcvrSYS
eTeTTT
TTT
21
G
G
tBWTSYS
System Temperature
Radiometer Equation
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40-Ft System
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)1(
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Airmass
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Airmass
BackgroundACMBl
SpilllRcvrSYS
eTeTTT
TTT
System Temperature
)1(
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Airmass
ATM
Airmass
BackgroundACMBl
SpilllNoiseDiodeRcvrSYS
eTeTTT
TTTT
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SYSsElectronic TGV
System Temperature
NoiseDiode
CalOnOff
sElectronic
NoiseDiodesElectronicSYSsElectronicCalOnOff
T
VG
TGTGV
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SYS
eTeTTT
TTT
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Airmass
ATM
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SpilllNoiseDiodeRcvr
DiodeOn
SYS
eTeTTT
TTTT
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On-Off Observing
Noise Diode
Detector
SignalSignal
•Observe blank sky for 10 sec
•Move telescope to object & observe for 10 sec
•Move to blank sky & observe for 10 sec
•Fire noise diode & observe for 10 sec
•Observe blank sky for 10 sec
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Continuum - Point Sources On-Off Observing
On Source
Off Source
Off Source Diode On
TNoiseDiode=3K
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AsElectronicSYSsElectronicSigRef TGTGV
Source Antenna Temperature
NoiseDiode
CalOnOff
sElectronic
NoiseDiodesElectronicSYSsElectronicCalOnOff
T
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TGTGV
CalOnOffNoiseDiodeRefCalOffSYS
CalOnOffNoiseDiodeSigRefA
VTVT
VTVT
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Continuum - Point Sources On-Off Observing
On Source
Off Source
Off Source Diode On
T No
iseD
iod
e=3
K
T A=6
K
TSYS=20K
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beammain just the fills that source
uniform a toEquivalent )(
sourcepoint not but Source )(
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Converting TA to Scientifically Useful Values