1. Brief Introduction to Radio Telescopes Frank Ghigo, NRAO-Green Bank July 2015 Terms and Concepts 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 Jansky Bandwidth Resolution Antenna power pattern Half-power beamwidth Side lobes Beam solid angle Main beam efficiency Effective aperture

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

3. 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

4. Subreflector and receiver room

5. On the receiver turret

6. Basic Radio Telescope Verschuur, 1985. 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 Kraus, 1966. Fig.6-1, p. 153. P n = normalized power pattern

11. dB ?? P1/P2Δp(dB) 10 23 10 10020 100030

12. Convolution relation for observed brightness distribution Thompson, Moran, Swenson, 2001. Fig 2.5, p. 58.

13. Smoothing by the beam Kraus, 1966. Fig. 3-6. p. 70; Fig. 3-5, p. 69.

14. Some definitions and relations Main beam efficiency,  M Antenna theorem

15. 1 Aperture efficiency,  ap Effective aperture, A e Geometric aperture, A g

16. another Basic Radio Telescope Kraus, 1966. Fig.1-6, p. 14.

17. Aperture Illumination Function  Beam Pattern A gaussian aperture illumination gives a gaussian beam: Kraus, 1966. Fig.6-9, p. 168.

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

19. Detected power (W, watts) from a resistor R at temperature T (kelvin) over bandwidth  (Hz) Power W A detected in a radio telescope Due to a source of flux density S power as equivalent temperature. Antenna Temperature T A Effective Aperture A e

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

21. Gain(K/Jy) for the GBT Including atmospheric absorption:

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

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

24. More Calibration : GBT Convert counts to T

25. Position switching