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Interferometric Gain Calibration for ALMA

This paper discusses innovative methods for interferometric gain calibration at ALMA, highlighting key accuracy features derived from a well-defined standard horn gain calculation and precise two-load Tsys calibration. We outline the use of a simple ambient temperature calibrator, wideband natural sources, and the system correlator as the power meter. The study also explores the fractional gain determination from a single antenna interferometer pair and the implications for Jovian spectrum analysis. Calibration effectiveness is examined through various brightness ratio accuracies with temperature variations derived from Saturn as a transfer standard.

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Interferometric Gain Calibration for ALMA

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

  2. 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

  3. ki Single Antenna

  4. Interferometer Pair with form the fraction

  5. Gain Determination

  6. Jovian Spectrumnear λ 1 cm

  7. CO Line Ratio vs. Kinetic Temperature

  8. Derived Tk from different Brightness ratio accuracies • A. 20% : R=2.5: range: 1.75 – 3.25 • 13K ≤ Tk ≤ 51K • B. 10% : R=2.5: range :2.15 – 2.85 • 17K ≤ Tk ≤ 32K • C. 5%: R=2.5: range: 2.3 – 2.7 • 20K ≤ Tk ≤ 27K

  9. 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 =0.2m2 • S/N = 150 in 1000 seconds with four antenna pairs

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

  11. Issues/Alternatives • Mount cal receiver on an ACA antenna • Use an STI 65K cooler on cal receiver for better Tsys. 5000$, 106 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 Tsys to cal horn

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