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Introduction to Radio Telescopes Frank Ghigo, NRAO-Green Bank The Fourth NAIC-NRAO School on Single-Dish Radio Astronomy July 2007 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 Main beam efficiency, M Antenna theorem M M A Aperture efficiency, ap Effective aperture, Ae Geometric aperture, Ag A ap 2 Ae Ag 1 Ag (GBT) (100m) 7854m2 2 2 Ae ap pat surf block ohmic 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 another Basic Radio Telescope Kraus, 1966. Fig.1-6, p. 14. Aperture Illumination Function Beam Pattern A gaussian aperture illumination gives a gaussian beam: pat 0.7 Kraus, 1966. Fig.6-9, p. 168. Gain(K/Jy) for the GBT 2kTA S Ae TA ap Ag G S 2k G( K / Jy) 2.84 ap Including atmospheric absorption: 2kTA a S e Ae Effect of surface efficiency ap pat surf 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