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