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Introduction to Radio Telescopes. Frank Ghigo, NRAO-Green Bank. The Fourth NAIC-NRAO School on Single-Dish Radio Astronomy July 2007. Terms and Concepts. Jansky Bandwidth Resolution Antenna power pattern Half-power beamwidth Side lobes Beam solid angle Main beam efficiency

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introduction to radio telescopes
Introduction to Radio Telescopes

Frank Ghigo, NRAO-Green Bank

The Fourth NAIC-NRAO

School on Single-Dish Radio Astronomy

July 2007

Terms and Concepts




Antenna power pattern

Half-power beamwidth

Side lobes

Beam solid angle

Main beam efficiency

Effective aperture

Parabolic reflector




Feed horn

Local oscillator


Noise Cal

Flux density

Aperture efficiency

Antenna Temperature

Aperture illumination function



System temperature

Receiver temperature


pioneers of radio astronomy
Pioneers of radio astronomy

Grote Reber


Karl Jansky


unblocked aperture
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
basic radio telescope
Basic Radio Telescope

Verschuur, 1985. Slide set produced by the Astronomical Society of the Pacific, slide #1.

intrinsic power p watts distance r meters aperture a sq m
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

antenna beam pattern power pattern
Antenna Beam Pattern (power pattern)

Beam solid angle


Main Beam

Solid angle

Pn = normalized power pattern

Kraus, 1966. Fig.6-1, p. 153.

some definitions and relations
Some definitions and relations

Main beam efficiency, M

Antenna theorem

Aperture efficiency, ap

Effective aperture, Ae

Geometric aperture, Ag

detected power w watts from a resistor r at temperature t kelvin over bandwidth hz
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

another basic radio telescope
another Basic Radio Telescope

Kraus, 1966. Fig.1-6, p. 14.

aperture illumination function beam pattern
Aperture Illumination FunctionBeam Pattern

A gaussian aperture illumination

gives a gaussian beam:

Kraus, 1966. Fig.6-9, p. 168.

gain k jy for the gbt
Gain(K/Jy) for the GBT

Including atmospheric absorption:

Effect of surface efficiency

system temperature
System Temperature

= total noise power detected, a result of many contributions

Thermal noise T

= minimum detectable signal

For GBT spectroscopy

convolution relation for observed brightness distribution
Convolution relationfor observed brightness distribution

Thompson, Moran, Swenson, 2001. Fig 2.5, p. 58.

smoothing by the beam
Smoothing by the beam

Kraus, 1966. Fig. 3-6. p. 70; Fig. 3-5, p. 69.

physical temperature vs antenna temperature
Physical temperature vs antenna temperature

For an extended object with source solid angle s,

And physical temperature Ts, then



In general :

calibration scan of cass a with the 40 foot
Calibration: Scan of Cass A with the 40-Foot.



Tant = Tcal * (peak-baseline)/(cal – baseline)

(Tcal is known)

more calibration gbt
More Calibration : GBT

Convert counts to T