For those projects which require a high accuracy of the circularly polarized intensity, notably HI Zeeman splitting observations, observers should be aware of the following feature. There is an option in the IF system to send the right hand circularly polarized signal, e.g. from IF A, through the path normally reserved for the left hand signal, IF C, and vice versa2.7. This is useful to calibrate out subtle differences between the signal path taken by each polarization. It is accomplished by the transfer switch. This switch can be steered by specifying a non-standard SYSxROT file in observe where x stands for the band used (e.g., SYSLROT for L-Band) and which contains sub-reflector and front-end parameters. Any changes need to be entered by the VLA Operator. Use of this option should be discussed well in advance of your run with your VLA Staff Contact.
When using the transfer switch, one has to observe one's target alternatingly with the switch in the default and the ``crossed signal'' position; the calibrators should also be observed in both modes.