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Introduction to Using Radio Telescopes
Frank Ghigo, NRAO-Green Bank The Fifth NAIC-NRAO School on Single-Dish Radio Astronomy July 2009
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Introduction to Using Radio Telescopes
Frank Ghigo, NRAO-Green Bank The Fifth NAIC-NRAO School on Single-Dish Radio Astronomy July 2009 Terms and Concepts Jansky Bandwidth Resolution Antenna power pattern Half-power beamwidth Side lobes Beam solid angle Main beam efficiency Effective aperture Parabolic reflector Blocked/unblocked Subreflector Frontend/backend Feed horn Local oscillator Mixer Noise Cal Flux density Aperture efficiency Antenna Temperature Aperture illumination function Spillover Gain System temperature Receiver temperature convolution
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Pioneers of Radio Astronomy
Grote Reber 1938 Karl Jansky 1932 Jansky’s antenna: designed for specific frequency, 20 MHz. Reber: general purpose parabolic dish.
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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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Subreflector and receiver room
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On the receiver turret
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Basic Radio Telescope Verschuur, Slide set produced by the Astronomical Society of the Pacific, slide #1.
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Signal paths
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Intrinsic Power P (Watts) Distance R (meters) Aperture A (sq.m.)
Flux = Power/Area Flux Density (S) = Power/Area/bandwidth Bandwidth () A “Jansky” is a unit of flux density Power P Isotropic radiator; in general power may be directive.
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Antenna Beam Pattern (power pattern)
Beam solid angle (steradians) Main Beam Solid angle Note Pn is normalized to maximum = 1; Like optical PSF Need to change equation to ~ not = Pn = normalized power pattern Kraus, Fig.6-1, p. 153.
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Some definitions and relations
Directivity or Directive Gain Main beam efficiency, M Antenna theorem Aperture efficiency, ap Effective aperture, Ae Geometric aperture, Ag Add directivivity or directive gain D = 4piAe/lambda^2
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Directive gains for GBT, Arecibo
For = 21cm, e=0.7 (Gregorian)
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Aperture feed pattern, or illumination pattern.
Note: aperture illumination function, and spillover Kraus, Fig.1-6, p. 14.
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Aperture Illumination Function and Beam Pattern are Fourier transforms of each other
A gaussian aperture illumination gives a gaussian beam: Trade off low side lobes (high main beam efficiency) for lower aperture efficiency. Kraus, Fig.6-9, p. 168.
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Surface efficiency -- Ruze formula
= rms surface error Effect of surface efficiency John Ruze of MIT -- Proc. IEEE vol 54, no. 4, p.633, April 1966.
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Power WA detected in a radio telescope
Detected power (W, watts) from a resistor R at temperature T (kelvin) over bandwidth (Hz) Power WA detected in a radio telescope Due to a source of flux density S power as equivalent temperature. Antenna Temperature TA Effective Aperture Ae Factor of one-half because detector is only sensitive to one polarization.
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Gain (or sensitivity) (K/Jy)
GBT: Arecibo: (Gregorian:) Atmospheric absorption has been left out of this equation Correct for atmospheric absorption:
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System Temperature Thermal noise T
= total noise power detected, a result of many contributions Thermal noise T = minimum detectable signal For GBT spectroscopy
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Convolution relation for observed brightness distribution
Thompson, Moran, Swenson, Fig 2.5, p. 58.
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Smoothing by the beam Kraus, Fig p. 70; Fig. 3-5, p. 69.
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Physical temperature vs antenna temperature
For an extended object with source solid angle s, And physical temperature Ts, then for for In general :
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Calibration: Scan of Cass A with the 40-Foot.
peak baseline Tant = Tcal * (peak-baseline)/(cal – baseline) (Tcal is known)
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More Calibration : GBT Convert counts to T
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Position switching
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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
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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
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Surface efficiency -- Ruze formula
= rms surface error Effect of surface efficiency John Ruze of MIT -- Proc. IEEE vol 54, no. 4, p.633, April 1966.
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Improving the surface for High-Frequency Performance:
Mechanical adjustments Photogrammetry FEM (finite element model) OOF (“out of focus” holography) model - global AutoOOF - correct thermal errors short term “Traditional” holography
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Mechanical adjustment of the panels.
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OOF: out of focus “holography”
Zernike polynomials
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Auto-OOF corrections
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Auto-OOF scan type
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“Traditional Holography”
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Holography results
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20 GHz Gain Curves
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43 GHz Gain Curves Admit that these curves are poly fits to data that is much noisier than this seems to imply.
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