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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
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
Parabolic reflector
Blocked/unblocked
Subreflector
Frontend/backend
Feed horn
Local oscillator
Mixer
Noise Cal
Flux density
Jansky
Bandwidth
Resolution
Antenna power pattern
Half-power beamwidth
Side lobes
Beam solid angle
Main beam efficiency
Effective aperture
Aperture efficiency
Antenna Temperature
Aperture illumination function
Spillover
Gain
System temperature
Receiver temperature
convolution
Pioneers of Radio Astronomy
Karl Jansky
1932
Grote Reber
1938
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
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/Area
Flux Density (S) = Power/Area/bandwidth
Bandwidth ()
A “Jansky” is a unit of flux density
10 26Watts / m 2 / Hz
P  10264R 2 S
Antenna Beam Pattern (power pattern)
Beam solid angle
(steradians)
 HPBW 

Main Beam
Solid angle
 A   Pn ( ,  )d
4
M 
 P ( , )d
n
main
lobe
D
Pn = normalized power pattern
Kraus, 1966. Fig.6-1, p. 153.
Some definitions and relations
Directivity or
Directive Gain
Main beam efficiency, M
M
M

A
Aperture efficiency, ap
Effective aperture, Ae
Geometric aperture, Ag

4
D
A
 ap
Ae

Ag
Antenna theorem
A 

2
Ae
 ap   pat surf  block ohmic 

Directive gains for GBT, Arecibo
D
Ag (GBT)  7854m 2
4ap Ag

2
Ag (Arecibo)  70686m 2

For  = 21cm, e=0.7

D(GBT) 1.57 106  61.9dB

D(Arecibo) 1.41107  71.5dB
Ag (Arecibo)  39650m 2
(Gregorian)
Aperture feed pattern, or illumination pattern.
Kraus, 1966. Fig.1-6, p. 14.
Aperture Illumination Function
and
Beam Pattern
are Fourier transforms of each
other
A gaussian aperture illumination
gives a gaussian beam:
 pat  0.7
Kraus, 1966. Fig.6-9, p. 168.
Surface efficiency -- Ruze formula
surf  e
(4  /  )2
 = rms surface error
Effect of surface efficiency
 ap   pat surf   
John Ruze of MIT -- Proc. IEEE vol 54, no. 4, p.633, April 1966.
Detected power (W, watts) from a resistor R
at temperature T (kelvin) over bandwidth (Hz)
W  kT
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
WA  12 AS
2kTA
S
Ae
Gain (or sensitivity) (K/Jy)
2kTA
S
Ae
TA ap Ag ap Ag
G


S
2k
2761
GBT: G(K /Jy)  2.84  ap

Arecibo:
(Gregorian:)

G(K /Jy)  25.6  ap
G  14.4  ap

Correct for atmospheric absorption:
2kTA a
e
S 
Ae
System Temperature
= total noise power detected, a result of many contributions
a
Tsys  Tant  Trcvr  Tatm (1  e )  Tspill  TCMB     
Thermal noise T
= minimum detectable signal
For GBT spectroscopy
T  k1
Tsys
  tint
Convolution relation
for observed brightness distribution
S ( ) 
 A( ' ) I ( ' )d '
source
Thompson, Moran, Swenson, 2001. Fig 2.5, p. 58.
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
for
for
In general :
s   A
s   A
1
TA 
A
TA 
s
Ts
A
TA  Ts
 P ( , )T ( , )d
n
source
s
Calibration: Scan of Cass A with the 40-Foot.
peak
baseline
Tant = Tcal * (peak-baseline)/(cal – baseline)
(Tcal is known)
More Calibration : GBT
Convert counts to T
Tcal
G
Ccal on  Ccal off
Tsys  G  Csys
1
1
 G  (Coffsource,calon  Coffsource,caloff )  Tcal
2
2
Tant  G  Csource
Position switching
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
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
Surface efficiency -- Ruze formula
surf  e
(4  /  )2
 = rms surface error
Effect of surface efficiency
 ap   pat surf   
John Ruze of MIT -- Proc. IEEE vol 54, no. 4, p.633, April 1966.
Improving the surface
for High-Frequency Performance:
• Surface
• Mechanical adjustments
• Photogrammetry
• FEM (finite element model)
• OOF (“out of focus” holography) model - global
• AutoOOF - correct thermal errors short term
• “Traditional” holography
Mechanical adjustment of the panels.
OOF: out of focus “holography”
Zernike polynomials
Auto-OOF corrections
Auto-OOF scan type
“Traditional Holography”
Holography results
20 GHz Gain Curves
43 GHz Gain Curves