National Radio
Astronomy Observatory

4.3.2 Antenna Gain-Elevation Curve Calibration

Large antennas (such as the 25-meter antennas used in the JVLA and VLBA) have a forward gain and efficiency that changes with elevation. Gain curve calibration involves compensating for the effects of elevation on the amplitude of the received signals at each antenna. Antennas are not absolutely rigid, and so their effective collecting area and net surface accuracy vary with elevation as gravity deforms the surface. This calibration is especially important at higher frequencies where the deformations represent a greater fraction of the observing wavelength. By design, this effect is usually minimized (i.e., gain maximized) for elevations between 45 and 60 degrees, with the gain decreasing at higher and lower elevations. Gain curves are most often described as 2nd- or 3rd-order polynomials in zenith angle.

Gain curve calibration has been implemented in CASA for the JVLA and VLA (only), with gain curve polynomial coefficients available directly from the CASA data repository. To make gain curve corrections for JVLA and VLA data, set gaincurve=True for any of the calibration tasks.

ALERT: The gaincurve parameter must be supplied to any calibration task that allows pre-application of the prior calibration (e.g. bandpass, gaincal, applycal). This should be done consistently through the calibration process. In future updates we will add the capability to gencal (§ 4.3.5) to create a calibration table for this.

For example, to pre-apply the gaincurve during gain calibration:

NOTE: Set gaincurve=False if you are not using E/VLA data.

The gain curve will be calculated per timestamp. Upon execution of a calibration task (e.g., gaincal, bandpass, applycal, etc.), the gain curve data appropriate to the observing frequencies will be automatically retrieved from the data repository and applied.

ALERT: Currently, gain-curves for E/VLA are built into the CASA system and this is what is applied when gaincurve=True. Therefore, the application of the gain-curves, if gaincurve=True, is allowed only if the E/VLA is set as the telescope of observation in the MS, otherwise an error will be generated. Set gaincurve=False if you are not using VLA data. A general mechanism for incorporating gaincurve information for other arrays will be made available in future releases. The gain-curve information available for the E/VLA is time-dependent (on timescales of months to years, at least for the higher frequencies), and CASA will automatically select the date-appropriate gain curve information. Note, however, that the time-dependence was poorly sampled prior to 2001, and so gain curve corrections prior to this time should be considered with caution.

More information about CASA may be found at the CASA web page

Copyright © 2010 Associated Universities Inc., Washington, D.C.

This code is available under the terms of the GNU General Public Lincense