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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 1026 Watts / m2 / Hz P 1026 4R 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 4 D A Aperture efficiency, ap Effective aperture, Ae Geometric aperture, Ag ap Ae Ag Antenna theorem A 2 Ae ap pat surf block ohmic Directive gains for GBT, Arecibo D Ag (GBT) 7854m2 4 ap Ag 2 Ag (Arecibo) 70686m2 For = 21cm, e=0.7 D(GBT ) 1.57 10 6 61.9dB D(Arecibo ) 1.4110 7 71.5dB Ag (Arecibo) 39650m2 (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