Mopra has a hybrid polariser which goes at the subreflector and it should be very good. Note that the 2 circular polarisation should have the same Tsys. If they don't it maybe that the polarisation will need tweeking until they are equal.
Polarisation was setup correctly a week earlier. However, the 3cm Rx is having problems and the levels between polars A and B are changing. Hence, the hybrid setting goes wrong and the polarisation purity is terrible!! There is a lot of signal in the cross polars LR and RL even in the fringe fitting. Probably polarisation isolation not better than 10db.
This also makes the Tsys values very asymmetric. For Calibration perhaps an average for Ta and Tb may be the best approximation.
13:00 UT Recording on ATNF 006B
15:59:50 UT Time check. All ok.