Transcription
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
