1. 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
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

2. Pioneers of radio astronomy
Grote Reber
1938
Karl Jansky
1932

3. 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

4. Subreflector and receiver room

5. On the receiver turret

6. Basic Radio Telescope
Verschuur, Slide set produced by the Astronomical Society of the Pacific, slide #1.

7. Signal paths

8. 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

9. Antenna Beam Pattern (power pattern)
Beam solid angle
(steradians)
Main Beam
Solid angle
Note Pn is normalized to maximum = 1;
Like optical PSF
Pn = normalized power pattern
Kraus, Fig.6-1, p. 153.

10. Some definitions and relations
Main beam efficiency, M
Antenna theorem
Aperture efficiency, ap
Effective aperture, Ae
Geometric aperture, Ag

11. 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.

12. another Basic Radio Telescope
Kraus, Fig.1-6, p. 14.

13. Aperture Illumination Function  Beam Pattern
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.

14. Effect of surface efficiency
Gain(K/Jy) for the GBT
Including atmospheric absorption:
Atmospheric absorption has been left out of this equation
Effect of surface efficiency

15. System Temperature Thermal noise T
= total noise power detected, a result of many contributions
Thermal noise T
= minimum detectable signal
For GBT spectroscopy

16. Convolution relation for observed brightness distribution
Thompson, Moran, Swenson, Fig 2.5, p. 58.

17. Smoothing by the beam
Kraus, Fig p. 70; Fig. 3-5, p. 69.

18. Physical temperature vs antenna temperature
For an extended object with source solid angle s,
And physical temperature Ts, then
for
for
In general :

19. Calibration: Scan of Cass A with the 40-Foot.
peak
baseline
Tant = Tcal * (peak-baseline)/(cal – baseline)
(Tcal is known)

20. More Calibration : GBT
Convert counts to T