Interferometric Gain Calibration for ALMA Jim Gibson and Jack Welch UC Berkeley RAL

Major Features Accuracy comes from (a) a well understood standard horn gain calculation: e<1% (Schelkunoff and Frijs), and (b) accurate two load ALMA Tsys calibration Use simple ambient temperature cal receiver Use wideband natural source, flux need not be known Decorrelation delay path with 1GHz band > 40cm Use the system correlator as the power meter Use the two load cal system for the gain transfer antenna

Single Antenna kiki

Interferometer Pair with form the fraction

Gain Determination

Jovian Spectrum near λ 1 cm

TkTk R CO Line Ratio vs. Kinetic Temperature

Derived T k from different Brightness ratio accuracies A. 20% : R=2.5: range: 1.75 – K ≤ T k ≤ 51K B. 10% : R=2.5: range :2.15 – K ≤ T k ≤ 32K C. 5%: R=2.5: range: 2.3 – K ≤ T k ≤ 27K

Saturn as a transfer standard at 2.8 mm Assumptions: ALMA rcvr Tsys = 50k Cal rcvr Tsys = 1000k Receiver bandwidth = 1 GHz Cal antenna: 20x2.7x2.0 cm ~30db gain (exactly known) Horn loss: 2% +/-.2% Saturn flux ~ 220 Jy Saturn diameter: ~16” Geom mean Tsys = 220k Geom mean effective area: (68 x 5.4 x10 -4) 1/2 =0.2m 2 S/N = 150 in 1000 seconds with four antenna pairs

Proposed Calibration Receiver LO1 ref LO2 ref IF Mixer and IF amplifiers

Issues/Alternatives Mount cal receiver on an ACA antenna Use an STI 65K cooler on cal receiver for better T sys. 5000$, 10 6 hours life. Horn gain: Shelkunoff and Frijs: Advanced Antenna Theory; Wrixon and Welch, IEEE Trans AP, AP20, 1972; Chiu and Semplak, BSTJ, 44,,527, 1965; Gibson et al, 2005, Icarus, 173, 439. Transfer of T sys to cal horn