Information about VLA

VLA C-configuration (2012 Feb)

Calibration of VLA data (https://science.nrao.edu/facilities/vla/docs/manuals/obsguide/calibration)

  • The steps in the VLA calibration are done by observatory staff, observer and data analyser
    Observatory Staff Observer Data Analyser
    Antenna (receiver frequencies, receiver system temperatures, optics),
    antenna positions, timing, and correlator visibilities    		 Instrumental delay, instrumental polarization, spectral bandpass response,
    absolute flux density scale, and other possible properties antenna pointing, delay, attenuator and requantizer settings, and other possible
    properties assumed to be only slowly varying during the observation
    should be performed by the observer    		 Calibration of antenna gains, atmospheric phase fluctuations, and other possible
    properties expected to vary more rapidly with observing conditions and geometry during the observation should be
    performed more frequently than the time scale over which the property changes. Calibration of the position of a source with respect to another source, calibration of a frequency
    to a line-of-sight velocity, calibration of a polarization angle to a reference angle, calibration of the flux density scale of
    a single source in one observation to another observation of the same source, etc.
  • Typical intervals for complex gain (amplitude and phase) calibration are dependent on the weather conditions and baseline length. On longer baselines (i.e., on longer uv-distances and therefore more important for high frequency observing), the phase change on the interferometer will be more rapid and requires more frequent calibration than at the lower observing frequencies.
  • The gain calibration uses the assumption that the calibration toward the sky, in which the calibrator source is observed, can be interpolated over time and viewing angle and resembles the same atmospheric conditions toward the sky of the target source.
  • Dynamic Scheduling of VLA helps in flexibly change calibrator observation duration to count for weather changes. You may want to make longer calibrator observation in bad weather.
  • Requantizer gains need to be redetermined after a change of tuning, including when returning to the original observational setup.
  • Calibration intervals for antenna pointing are largely dependent on the geometry of the observation. As a rule-of-thumb, after tracking for about an hour, the direction of the solutions of the antenna pointing calibration will have changed significantly from where the target position is now—on the order of 20°—warranting a new pointing solution.