Calibrating visibility amplitudes

Before calibrating the visibility data, we need to plot it and flag the blue channels 21 to 32 because of their low signal-to-noise ratio. Select VisSq, the uncalibrated raw (squared) visibility amplitude, and plot it versus channel. Select all channels to be plotted, and to avoid getting a plot for each of them separately, choose Option|All in 1. (Alternatively, the plot slice can be set to 'ch'. For a data selection involving one or more indices with a range of more than one element, e.g. all channels and all scans, the slice parameter determines which index is NOT to be used as a loop index.) The two figures show the visibility on the EW baseline before and after editing.

Note the way the y-axis is labeled, with each colored label corresponding to a plot of the same color, all of them in the same frame. Also note that data from channel 11 is absent, since the back ground data for this channel had been flagged. The hierarchical editing scheme employed by OYSTER ensures that data dependent on the type being flagged is also flagged. For example, if count rates are flagged, visibilities for the same channel are unlikely to be unaffected and have to be removed too. On the other hand, if visibility amplitudes are flagged, the photon rate of the same channel is left unchanged because the reasons leading to visibility corruption are not necessarily related to detector performance.

A very simple but fairly decent way to calibrate visibilities of a program star with a nearby unresolved calibrator (star) is to plot them side by side versus time, smooth the calibrator visibility variations with time and apply the resulting normalization factors to the program star. The following figure shows two nearby calibrators and the program star (a binary, FKV0123). The calibrators are well correlated, and based on the assumption that any difference between them and the program star is due to actual difference in the program star's structure, the calibration scheme is justified.

The calibration widget, Calibrate|Scans|CALIBRATE, allows you to select the calibrators (here FKV0139, FKV0151), and to select the so-called indicators. They are variables which have shown in the past, and not only for NPOI but also for the Mark III interferometer, correlations with the visibility. NOT ALL of those are applicable to a specific night! Usually it is just one or two of them. But they are all listed because of our past experience with other interferometers. Here we selected Time, and from a small widget which is created for each indicator, a function to be used to model the correlation. In this example, I chose S_20, which is a smoothing function of 20 minutes length. The base function terms in the left column are polynomial terms, in the middle column Legendre polynomials. These can be used simultaneously with other indicators.

In the plot widget, we select the stars to which the calibration should be applied. Do not forget to include the calibrators themselves in this selection! The data selection defines which data is to be used to compute the correlation parameters, with the exception that if the Option|Loop parameter is selected in the calibration widget, the channels are calibrated independently, doing every selected channel. As the variable to be calibrated, we selected VisSq in the calibration widget. (The variable selected in the plot widget is for plotting purposes only!) It is actually VisSq c/e, initially set to VisSq/e, which is used for the determination of the parameters. (By doing this we can stack several calibrations on top of each other.) Click on Calibrate to compute the parameters and apply the results to the selected stars. The following figure shows the results for 4 channels.

Note that the two calibrator star's visibilities are around unity, whereas the binary star's amplitudes are significantly lower. Also look at the rms information printed in the message window.

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