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Fundamentals of the GBT andSingle-Dish Radio TelescopesMarch 2016 Associated Universities, Inc., 2016Dr. Ron MaddalenaNational Radio Astronomy ObservatoryGreen Bank, WV

National Radio Astronomy Observatory National Laboratory Founded in 1954 Funded by the National ScienceFoundation

Telescope Structure and Optics

Telescope Structure and Optics

Telescope Structure and Optics Large 100-m Diameter: High Sensitivity High Angular Resolution – wavelength / Diameter

Telescope Structure and Optics

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 OpticsStrayRadiationBlockageSpillover

Telescope Optics

Telescope OpticsPrime Focus: Retractable boomGregorian Focus: 8-m subreflector - 6-degrees of freedom

Telescope OpticsRotating Turret with 8 receiver bays

Telescope Structure Fully Steerable Elevation Limit: 5 Can observe 85% of the entire Celestial Sphere Slew Rates: Azimuth - 40 /min; Elevation - 20 /min

National Radio Quiet Zone

National Radio Quiet Zone

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

The Influence of the Atmosphere andWeather at cm- and mm-wavelengths Opacity––––CalibrationSystem performance – TsysObserving techniquesHardware design Refraction– Pointing– Air Mass Calibration Interferometer & VLB phaseerrors– Aperture phase errors Cloud Cover– Continuum performance– Calibration Winds– Pointing– Safety Telescope Scheduling– Proportion of proposalsthat should be accepted– Telescope productivity

Weather Forecasts for Radio Astronomy

Weather Forecasts for Radio Astronomy

Telescope Structure

GBT active surface system Surface has 2004 panels– average panel rms: 68 µm 2209 precision actuators29

Surface Panel ActuatorsOne of 2209 actuators. Actuators are located undereach set of surface panelcorners30Actuator Control Room 26,508 control and supply wiresterminated in this room

Finite Element Model Predictions31

Mechanical adjustment of the panels32

Image quality and efficiency33

Image quality, efficiency, resolution HPBW 1.2 D 40' at 300M Hz (1 m) 9' at 1420 M Hz (21 cm) 6.5" at 115 GHz (3 mm)34

Image quality and efficiencyDetected PowerAperture Efficiency A 0.71Incident Power35

Holography36

Holography37

Surface accuracy (rms) 240 µm

Aperture Efficiency A 0.7e rms surface error ( 4 / ) 2

Telescope StructureBlind Pointing:(1 point/focus) 2 5 arc sec ( focus ) 2.5 mmOffset Pointing:(90 min) 2 2.7 arc sec ( focus ) 1.5 mmContinuous Tracking:(30 min) 2 1 arc sec

ReceiversReceiverOperating RangeStatusPrime Focus 10.29—0.92 GHzCommissionedPrime Focus 20.910—1.23 GHzCommissionedL Band1.15—1.73 GHzCommissionedS Band1.73—2.60 GHzCommissionedC Band4—8.0 GHzRecently upgradedX Band8—12.0 GHzCommissionedKu Band12—15 GHzCommissionedK Band Array18—27 GHzCommissionedKa Band26—40 GHzCommissionedQ Band40—50 GHzCommissionedW Band68—92 GHzCommissionedMustang Bolometer86—94 GHzBeing upgradedARGUS80—115 GHzBeing commissioned

Receiver Room

Typical Receiver

Receiver Feeds

Typical Receiver

Typical torsPower DetectorsSynthesizersFiltersSwitchesMultipliers

Types of FiltersEdges are smoother than illustrated

Types of MixersffIFfLO n and m are positive or negativeintegers, usually 1 or -1 Up Conversion : fIF f Down Conversion : fIF ffIF n*fLO m*f Lower Side Band : fLO f- Sense of frequency flips Upper Side Band : fLO f

40-Ft SystemDetermine values for the first LO for the 40-ftwhen Observing HI at 1420 MHz

GBT – Astrid program does all the hardwork for you .configLine """receiver "Rcvr1 2"beam “B1"obstype "Spectroscopy"backend "Spectrometer"nwin 1restfreq 1420.4058deltafreq 0bandwidth 12.5swmode "tp"swtype "none"swper 1.0swfreq 0.0, 0.0tint 30vlow 0vhigh 0vframe "lsrk"vdef "Radio"noisecal "lo"pol "Linear"nchan "low"spect.levels 3"""

Power Balancing/Leveling and NonLinearity

Spectral-line observationsRaw DataReducedData – HighQualityReducedData –Problematic

Reference observations Difference a signal observation with a referenceobservation 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

Model Receiver

Out-Of-Band Frequency Switching

On-Off ObservingNoise DiodeSignalSignalDetector

Nodding with dual-beam receivers Telescope motion Optical aberrationsDifference in spillover/ground pickupRemoves any ‘fast’ gain/bandpass changesOverhead from moving the telescope. All the time is spent on source

Nodding with dual-beam receivers Subreflector motion Optical aberrationsDifference in spillover/ground pickupRemoves any ‘fast’ gain/bandpass changesLow overhead. All the time is spent on source

Intrinsic Power P (Watts)Distance R (meters)Aperture A (sq.m.)Flux Power Received/AreaFlux Density (S) Power Received/Area/bandwidthBandwidth (BW)A “Jansky” is a unit of flux density10 26Watts / m2 / Hz10 26 PS 4 R 2 BW2kTA S A Ag e AirMassGain TA S A Ag 2761Gain 2.84 A for GBTGain 2.0 for GBT at low frequencie s

System TemperatureTSYS TRcvr (1 l ) TSpill l TCMB TA TBackground e Airmass TATM (1 e Airmass ) Radiometer Equation1 G TSYS BW t G 2

40-Ft System

System TemperatureTSYS TRcvr (1 l ) TSpill l TCMB TA TBackground e Airmass TATM (1 e Airmass ) TSYS TRcvr TNoiseDiode (1 l ) TSpill l TCMB TA TBackground e Airmass TATM (1 e Airmass )

System TemperatureV GElectronics TSYSDiodeOffTSYS TRcvr (1 l ) TSpill l TCMB TA TBackground e Airmass TATM (1 e Airmass ) DiodeOnTSYS TRcvr TNoiseDiode (1 l ) TSpill l TCMB TA TBackground e Airmass TATM (1 e Airmass ) VCalOnOff GElectronics TSYS GElectronics TNoiseDiode GElectronics VCalOnOffTNoiseDiode

On-Off ObservingNoise DiodeSignalSignal 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 secDetector

Continuum - Point SourcesOn-Off ObservingTNoiseDiode 3KOn SourceOff SourceOff SourceDiode On

Source Antenna Temperature VSigRef GElectronics TSYS GElectronics TA VCalOnOff GElectronics TSYS GElectronics TNoiseDiode GElectronics VCalOnOffTNoiseDiodeTA VSigRef TNoiseDiode VCalOnOffTSYS VRefCalOff TNoiseDiode VCalOnOff

Continuum - Point SourcesOn-Off ObservingOff SourceOff SourceDiode OnTNoiseDiode 3KTA 6KOn SourceTSYS 20K

Converting TA to Scientifically UsefulValuesTA ( K ) A Area e Airmass2kS (W m 2 Hz 1 ) Src e Airmass TB ( K )Point SourceExtended source; Src depends uponsource size e Airmass TB ( K )Source HPBW2 Src Airmass e TB ( K ) Source HPBW but not point source HPBW MB e Airmass TMB ( K )Equivalent to a uniformsource that fills just the main beam

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 . National Radio