calibration jumps for lbw OH (murray's data).


    Murray lewis uses continuum sources to calibrate the cal in janskies. This is done with lbw at OH frequencies in circular polarization mode. He noticed that the size of the cal was varying for data taken after 28jul05.  The processing is:

    The data was taken between 29jul05 and 23oct05. The scatter in this data set has increased compared to previous measurements. The changes could come from a pointing error (the source deflection is not correct) or from the cal deflection (the cal value is changing).

    The plots show the cal strength for the different sources (.ps).   (.pdf):

    The plots explain some of the cal variations:

    This data has been taken with diode #2 which passes through a 90 degree hybrid giving the polA, polB cal values a 90 degree phase difference. The cals then go through the hybrid that converts linear to circular (most people use linears). This combines the two inputs with another 90 degree phase shift. The two shifts end up putting all of the cal in one circular and none in the other. In practice the phase shifts are not exactly 90 degrees so a small amount of cal ends up in 1 circular and a large amount in the other. Murray was dividing the cal deflections of each polarization into the source deflections of each polarization and then multiplying by the cal value. This will now give errors since the two cal deflections do no match the cal value used. You could hope that the errors would cancel (polA two times too big, polB two times too small) but the small cal deflection will have a large relative error, so the measurement error will increase.

    A better way to calibrate this data would be to add the two polarization together and then compute the cal deflection and source deflection.