Instrumental phase

At the precision needed for 3-mm observing, the ATCA antennas are not identical in terms of their physical construction. In particular the offset between the elevation and azimuth axis will differ between antennas. VLBI astronomers, who have used heterogeneous arrays of antennas, have coped with this issue for years. For the ATCA at 3mm, what is presumed to be a difference in this offset introduces a instrumental phase error that needs to be corrected.