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Antenna gain considerations and flux calibration

One characteristic of the ATCA dishes is that they are big by 3-mm standards. Indeed van Hoerner (1967) has considered the limits to the size of an antenna before various effects become significant. For construction styles like the ATCA dishes, he suggests that 11m is the maximum dish size for 3-mm observations before thermal effects become significant. As the ATCA dishes are twice this, it is no surprise that the ATCA antenna gain and primary beam response varies as a result of thermal distortions. Changes in temperature, the position of the sun, cloudiness and shadows will all affect the gain of the ATCA antennas. Measurements to date tend to bear this out, with changes in antenna gain of 25% noted that appear to be related to thermal effects. Distortion of the dishes by gravity is also significant (eg Subrahmanyan 2005). A gain/elevation correction will solve this - to first order at least. Because of the way the ATCA panels are set, the antenna gain peaks at about 60 degrees elevation.

Note that gravitational and thermal distortions are coupled - a perfect correction for these effects is never possible.

To first order, the gain change will be calibrated out provided the phase calibrator is very close to the target source. Applying the standard gain/elevation curve to the data as the first step in the data reduction will also help. However these two steps will not account for the change in the beamshape. This might be an important effect for widefield imaging and mosaicing.

The best way to avoid thermally-induced gain variations is by observing in the pre-dawn hours or on a calm cloudy day! The best way to avoid large elevation changes in a synthesis is to observe in a hybrid array - hybrid arrays allow a synthesis to be performed without the need to track the source from horizon to horizon.

The gain variations must be considered when attempting to correct the flux density scale. The best approach to ensuring a good flux scale is to ensure that the phase and the flux density calibrators are observed nearly simultaneously and at the same elevation. Although this may at first sound difficult if not impossible, generally it is straightforward. Generally there should be a time when the phase and the flux density calibrators are at the same elevation. It may be that one is rising while the other is setting, and so they are at significantly different parts of the sky (ie significantly different azimuths). Unless the sky is cloudy, being at different azimuths is not important.