Interferometric Gain Calibration for ALMA

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Interferometric Gain Calibration for ALMA

• Jim Gibson and Jack Welch
• UC Berkeley RAL

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Major Features

• Accuracy comes from (a) a well understood
  standard horn gain calculation elt1 (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 gt 40cm
• Use the system correlator as the power meter
• Use the two load cal system for the gain transfer
  antenna

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Single Antenna
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Interferometer Pair
with
form the fraction
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Gain Determination
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(No Transcript)
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Jovian Spectrumnear ? 1 cm
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Tk R
10 1.42
15 2.00
20 2.38
25 2.63
30 2.83
35 2.99
40 3.09
45 3.18
50 3.25
CO Line Ratio vs. Kinetic Temperature
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Derived Tk from different Brightness ratio
accuracies

• A. 20 R2.5 range 1.75 3.25
• 13K Tk 51K
• B. 10 R2.5 range 2.15 2.85
• 17K Tk 32K
• C. 5 R2.5 range 2.3 2.7
• 20K Tk 27K

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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

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