1. Introduction to Using Radio Telescopes
Frank Ghigo, NRAO-Green Bank
The Fifth NAIC-NRAO
School on Single-Dish Radio Astronomy
July 2009

2. 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
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

3. Pioneers of Radio Astronomy
Grote Reber
1938
Karl Jansky
1932
Jansky’s antenna: designed for specific frequency, 20 MHz. Reber: general purpose parabolic dish.

4. 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

5. Subreflector and receiver room

6. On the receiver turret

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

8. Signal paths

9. 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
Power P Isotropic radiator; in general power may be directive.

10. Antenna Beam Pattern (power pattern)
Beam solid angle
(steradians)
Main Beam
Solid angle
Note Pn is normalized to maximum = 1;
Like optical PSF
Need to change equation to ~ not =
Pn = normalized power pattern
Kraus, Fig.6-1, p. 153.

12. Some definitions and relations
Directivity or Directive Gain
Main beam efficiency, M
Antenna theorem
Aperture efficiency, ap
Effective aperture, Ae
Geometric aperture, Ag
Add directivivity or directive gain D = 4piAe/lambda^2

13. Directive gains for GBT, Arecibo
For  = 21cm, e=0.7
(Gregorian)

14. Aperture feed pattern, or illumination pattern.
Note: aperture illumination function, and spillover
Kraus, Fig.1-6, p. 14.

15. Aperture Illumination Function and Beam Pattern are Fourier transforms of each other
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.

16. Surface efficiency -- Ruze formula
 = rms surface error
Effect of surface efficiency
John Ruze of MIT -- Proc. IEEE vol 54, no. 4, p.633, April 1966.

17. 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.

18. Gain (or sensitivity) (K/Jy)
GBT:
Arecibo:
(Gregorian:)
Atmospheric absorption has been left out of this equation
Correct for atmospheric absorption:

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

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

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

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

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

24. More Calibration : GBT
Convert counts to T

25. Position switching

27. 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

28. 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

29. Surface efficiency -- Ruze formula
 = rms surface error
Effect of surface efficiency
John Ruze of MIT -- Proc. IEEE vol 54, no. 4, p.633, April 1966.

30. Improving the surface for High-Frequency Performance:
Mechanical adjustments
Photogrammetry
FEM (finite element model)
OOF (“out of focus” holography) model - global
AutoOOF - correct thermal errors short term
“Traditional” holography

31. Mechanical adjustment of the panels.

32. OOF: out of focus “holography”
Zernike polynomials

33. Auto-OOF corrections

34. Auto-OOF scan type

35. “Traditional Holography”

36. Holography results

37. 20 GHz Gain Curves

38. 43 GHz Gain Curves
Admit that these curves are poly fits to data that is much noisier than this seems to imply.