Fundamentals of the GBT and Single-Dish Radio Telescopes Dr. Ron Maddalena

Transcription

1 Fundamentals of the GB and Single-Dish Radio elescopes Dr. Ron Maddalena March 2016 Associated Universities, Inc., 2016 National Radio Astronomy Observatory Green Bank, WV

2 National Radio Astronomy Observatory National Laboratory Founded in 1954 Funded by the National Science Foundation

3 elescope Structure and Optics

4 elescope Structure and Optics

5 elescope Structure and Optics Large 100-m Diameter: High Sensitivity High Angular Resolution wavelength / Diameter

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9 elescope Structure and Optics

10 GB elescope Optics 110 m x 100 m of a 208 m parent paraboloid Effective diameter: 100 m Off axis - Clear/Unblocked Aperture

11 High Dynamic Range High Fidelity Images elescope Optics

12 elescope Optics Stray Radiation Blockage Spillover

13 elescope Optics

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15 elescope Optics Prime Focus: Retractable boom Gregorian Focus: 8-m subreflector - 6-degrees of freedom

16 elescope Optics Rotating urret with 8 receiver bays

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

18 National Radio Quiet Zone

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21 National Radio Quiet Zone

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23 Index of Refraction Atmosphere Weather (i.e., time) and frequency dependent Real Part: Bends the light path Imaginary part: Opacity Winds Wind-induced pointing errors Safety

24 he Influence of the Atmosphere and Weather at cm- and mm-wavelengths Opacity Calibration System performance sys Observing techniques Hardware design Refraction Pointing Air Mass Calibration Interferometer & VLB phase errors Aperture phase errors Cloud Cover Continuum performance Calibration Winds Pointing Safety elescope Scheduling Proportion of proposals that should be accepted elescope productivity

25 Weather Forecasts for Radio Astronomy

26 Weather Forecasts for Radio Astronomy

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28 elescope Structure

29 GB active surface system Surface has 2004 panels average panel rms: 68 µm 2209 precision actuators 29

30 Surface Panel Actuators One of 2209 actuators. Actuators are located under each set of surface panel corners 30 Actuator Control Room 26,508 control and supply wires terminated in this room

31 31 Finite Element Model Predictions

32 Mechanical adjustment of the panels 32

33 33 Image quality and efficiency

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

35 Image quality and efficiency Aperture Efficiency A Detected Incident Power Power

36 36 Holography

37 37 Holography

38 Surface accuracy (rms) = 240 µm

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

40 elescope Structure Blind Pointing: (1 point/focus) 2 ( 5 arcsec focus ) 2.5 mm Offset Pointing: (90 min) 2 ( 2.7 arcsec focus ) 1.5 mm Continuous racking: (30 min) 2 1 arcsec

41 Receivers Receiver Operating Range Status Prime Focus GHz Commissioned Prime Focus GHz Commissioned L Band GHz Commissioned S Band GHz Commissioned C Band GHz Recently upgraded X Band GHz Commissioned Ku Band GHz Commissioned K Band Array GHz Commissioned Ka Band GHz Commissioned Q Band GHz Commissioned W Band GHz Commissioned Mustang Bolometer GHz Being upgraded ARGUS GHz Being commissioned

42 Receiver Room

43 ypical Receiver

44 Receiver Feeds

45 ypical Receiver

46 ypical Components Amplifiers Splitters Mixers Couplers Attenuators Power Detectors Filters Synthesizers Switches Multipliers

47 ypes of Filters Edges are smoother than illustrated

48 ypes of Mixers f f IF n and m are positive or negative integers, usually 1 or -1 f LO f IF = n*f LO + m*f Up Conversion : f IF > f Down Conversion : f IF < f Lower Side Band : f LO > f - Sense of frequency flips Upper Side Band : f LO < f

49 40-Ft System Determine values for the first LO for the 40-ft when Observing HI at 1420 MHz

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53 GB Astrid program does all the hard work for you.. configline = """ receiver = "Rcvr1_2" beam = B1" obstype = "Spectroscopy" backend = "Spectrometer" nwin = 1 restfreq = 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 """

54 Power Balancing/Leveling and Non- Linearity

55 Spectral-line observations Raw Data Reduced Data High Quality Reduced Data Problematic

56 Reference observations Difference a signal observation with a reference observation ypes 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

57 Model Receiver

58 Out-Of-Band Frequency Switching

59 On-Off Observing Noise Diode Signal Signal Detector

60 Nodding with dual-beam receivers - elescope 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

61 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

62 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 Watts / m / Hz P S 2 4R BW 2k S A A A g e AirMass Gain Gain Gain A S 2.84 A A A g 2761 for GB 2.0 for GB at low frequencies

63 ) (1 ) (1 Airmass AM Airmass Background A CMB l Spill l Rcvr SYS e e 2 1 G G t BW SYS System emperature Radiometer Equation

64 40-Ft System

65 ) (1 ) (1 Airmass AM Airmass Background A CMB l Spill l Rcvr SYS e e System emperature ) (1 ) (1 Airmass AM Airmass Background A CMB l Spill l NoiseDiode Rcvr SYS e e

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67 SYS s G Electronic V System emperature NoiseDiode CalOnOff Electronics NoiseDiode Electronics SYS Electronics CalOnOff V G G G V ) (1 ) (1 Airmass AM Airmass Background A CMB l Spill l Rcvr DiodeOff SYS e e ) (1 ) (1 Airmass AM Airmass Background A CMB l Spill l NoiseDiode Rcvr DiodeOn SYS e e

68 On-Off Observing Noise Diode Signal Signal 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 Detector

69 Continuum - Point Sources On-Off Observing NoiseDiode =3K On Source Off Source Diode On Off Source

70 A Electronics SYS Electronics SigRef G G V Source Antenna emperature NoiseDiode CalOnOff Electronics NoiseDiode Electronics SYS Electronics CalOnOff V G G G V CalOnOff NoiseDiode RefCalOff SYS CalOnOff NoiseDiode SigRef A V V V V

71 A =6K NoiseDiode =3K Continuum - Point Sources On-Off Observing On Source Off Source Diode On Off Source SYS =20K

72 Converting A to Scientifically Useful Values A ( K) A Area e 2k Airmass S( W m 2 Hz 1 ) Point Source e Src MB e Airmass Src HPBW e Airmass 2 e B Airmass B Airmass ( K) Extended MB B source size ( K) Source ( K) Source source; HPBW HPBW depends upon but not point source ( K) Equivalent to a uniform source that fills just themain beam Src