Introduction to Radio Telescopes

Frank Ghigo, NRAO-Green Bank

The Fourth NAIC-NRAOSchool on Single-Dish Radio Astronomy

July 2007

Terms and Concepts

Parabolic reflectorBlocked/unblocked

SubreflectorFrontend/backend

Feed hornLocal oscillator

MixerNoise Cal

Flux density

Aperture efficiencyAntenna Temperature

Aperture illumination functionSpillover

GainSystem temperature

Receiver temperatureconvolution

JanskyBandwidthResolution

Antenna power patternHalf-power beamwidth

Side lobesBeam solid angle

Main beam efficiencyEffective aperture

Pioneers of radio astronomy

Karl Jansky1932

Grote Reber1938

• 100 x 110 m section of a parent parabola 208 m in diameter• Cantilevered feed arm is at focus of the parent parabola

Unblocked Aperture

Subreflector and receiver room

On the receiver turret

Basic Radio Telescope

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

Signal paths

Intrinsic Power P (Watts)Distance R (meters)Aperture A (sq.m.)

Flux = Power/AreaFlux Density (S) = Power/Area/bandwidth

Bandwidth ()

A “Jansky” is a unit of flux density

HzmWatts //10 226

SRP 226410

Antenna Beam Pattern (power pattern)

4

),( dPnABeam solid angle(steradians)

lobemain

nM dP ),( Main BeamSolid angle

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

Pn = normalized power pattern

HPBW D

Some definitions and relations

Main beam efficiency,

Aperture efficiency, ap

Effective aperture, Ae

Geometric aperture, Ag

A

MM

g

eap A

A

22

7854)100(2

1)( mmGBTAg

Antenna theorem

ohmicblocksurfpatap

eA A

2

Detected power (W, watts) from a resistor R at temperature T (kelvin) over bandwidth (Hz)

kTW

Power WA detected in a radio telescope Due to a source of flux density S

ASWA 21

power as equivalent temperature.Antenna Temperature TA

Effective Aperture Aee

A

A

kTS

2

another Basic Radio Telescope

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

Aperture Illumination Function

Beam Pattern

A gaussian aperture illuminationgives a gaussian beam:

7.0pat

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

Gain(K/Jy) for the GBT

e

A

A

kTS

2 apJyKG 84.2)/(

a

e

A eA

kTS 2

Including atmospheric absorption:

Effect of surface efficiency

surfpatap

G TA

S

apAg

2k

System Temperature

Thermal noise T

= total noise power detected, a result of many contributions

= minimum detectable signal

For GBT spectroscopy

CMBspill

aatmrcvrantsys TTeTTTT )1(

int

1t

TkT sys

Convolution relationfor observed brightness distribution

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

source

dIAS ')'()'()(

Smoothing by the beam

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

Physical temperature vs antenna temperature

For an extended object with source solid angle s, And physical temperature Ts, then

sA

sA TT

As for

forAs

sA TT

dTPT s

source

nA

A ),(),(1 In general :

Calibration: Scan of Cass A with the 40-Foot.

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

(Tcal is known)

peak

baseline

More Calibration : GBT

offcaloncal

cal

CC

TG

Convert counts to T

calcaloffoffsourcecalonoffsource

syssys

TCCG

CGT

2

1)(

2

1,,

sourceant CGT