Calibrating short-timescale tropospheric phase fluctuations seen by a radio telescope: Limits from subreflector and Cassegrain feed ring radiometer placement


  • Roger Linfield

    1. Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, California, USA
    2. Now at Ball Aerospace Corporation, Boulder, Colorado, USA.
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[1] Water vapor radiometers (WVRs) measure tropospheric brightness temperatures and use those measurements to infer path delay. Calibration of short-timescale phase fluctuations at a radio telescope requires that the WVR and radio telescope sample a similar volume of the troposphere. Using a statistical (Kolmogorov frozen flow) model of tropospheric fluctuations, the short-timescale calibration capability of two WVR configurations has been quantified. The first configuration is a WVR mounted, with its own antenna, on the back side of the main radio telescope subreflector, giving a conical beam that is coaxial with the main cylindrical near-field beam of the large telescope. The second configuration uses a Cassegrain feed ring, with the WVR and radio astronomy feeds at different positions on the ring. This second configuration gives a cylindrical calibration near-field beam, offset in angle to the main cylindrical beam. An important application of short-timescale phase calibration is improving the coherence of high-frequency interferometric observations. For two cases of current/near future interest (86 GHz very long baseline interferometry with the Very Long Baseline Array; 350 GHz observations with the Atacama Large Millimeter Array, ALMA), useful calibration could be achieved with either geometry (coaxial conical beam or offset cylindrical beam). For a coaxial conical beam, a 2° WVR beam width would allow significant coherence improvement, but a beam width ≤1° (full width at half maximum) is needed for optimum performance. For an offset cylindrical beam, the desired angular offset (on the sky) is ≤1° for 43 GHz Very Large Array observations, or ≤0.3° for 350 GHz ALMA observations.