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:

• Divide the cal deflection by the source deflection and then multiply by the source strength in mJy.
• Multiply by the za gain curve.

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

• Fig 1: The za versus az tracks for the sources. Each source is coded in a different color. The dashed blue lines are when the azimuth points at a tiedown. The dashed yellow lines are when the azimuth is opposite a tiedown cable. The tiedown cables can lose tension and affect the pointing error when:
• the azimuth points at the tiedown and the dome is at high za.
• the azimuth points 180 degrees from the tiedown and the dome is at low za.
This happens during warm weather (not as often at night). In the plot you might expect to lose tension for 0409+22 (red), B1442+10 (purple).
• Fig 2: Cal size versus various parameters: Each source is coded in a different color.
• Plot1: cal value versus day number of year. The variation increases starting near day number 230.
• Plot2: cal value versus hour of day. The largest deviations occur at 6 am and 17 pm.
• Plot3: cal value versus za:
• Plot4: cal value versus az: The dashed lines show when the az is aligned and opposite a tiedown cable.
• Plot5: cal value versus the minimum tension in a tiedown cable. When the tension goes to zero the platform tilts causing the pointing to go off. The source 0409+22 was taken with one of the tiedown cables loose. B1442+10 also had some measurements at low tension as did 4C+35.28.

• B0409+22:  The change in cal is caused by no tension in one of the tiedowns (see bottom plot page 2).
• B1442+10:  The 3 outliers at 1100,1200 are from no tension in a tiedown (see bottom plot page 2). The other variation is not explained.
• B0304+04: This source has an offset. It's flux is probably incorrect.
• B2251+112: There are 3 outliers. I have clipped them to 1250 (the original values were: 2403,2289,1624). I see no explanation for them.

    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.
 

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