Calibrating visibility phases

To do the closure phases (or triple phases), essentially use the same procedure, except in this case only the correlation with time makes sense. (To deselect an indicator, simply click on it in the list widget, and it will remove its base function widget.) I have plotted the uncalibrated phase versus time for all calibrators, using the Cal directive in the plot widget. This causes OYSTER to check a flag (bflag) for every star in the startable to see whether the star is listed as a calibrator (based on the file /home/cah/catalogs/npoi/diameter.bsc). All calibrators show a very nice and tight correlation/drift with time for the channel displayed.

Look at another 'goodie' in the above figure: a listing with information on a star. This is procedure list_star, which has an argument specifying the star, e.g. FKV0123 (so type: list_star,'FKV0123'). It lists part of the stellar information contained in the startable on this particular star, which was collected from various catalogs when you loaded the startable at the beginning of this OYSTER session.

Before you go off and calibrate all channels make sure there are no phase wraps. Plot the calibrated phase for all channels versus channel number, as shown in the following figure. You notice that the phases are close to -180 degrees, and two data points which actually wrapped. Using Calibrate|Scans|Triple|ManualUnwrap, define a box over the data points to be unwrapped, and enter the positive or negative multiple of 360 degrees to be applied. For the example here, check the message screen at the lower left. Currently, you have to do this step BEFORE calibration of the phases, and using TriplePhaseC, since OYSTER won't allow you to change the raw phases. I might change this someday, depending on what you, the users, tell me they would prefer. The unwrapped data points will appear at their new location in a red color.

This is the result when using all four available terms for the Legendre polynomials, and using all calibrators together. I have plotted only channels 1 through 15, since for some reason, the higher channels show considerable noise. As an aside, this was the night after we had the tour of the NPOI site as part of the Lowell workshop on interferometry in October 1998. Not bad, the system stability!

This result is obtained if S_5 is used for the calibration coefficients.

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