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).
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):
The plots explain some of the cal variations:
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
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).
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
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.
change in cal is caused by no tension in one of the tiedowns (see bottom
plot page 2).
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