C Tuning

As described in Michael's thesis [1, Chap. 3.4.7], the measurements of the calibration constant $C$, which have been done offline in Pavel's lab, have to be adjusted to be correct relative to each other for all segments. We want to have equal response in all segments for equal incident x-ray flux. This is again done with a detector map file (usually the same as was used for the crosstalk correction).

Before you do the $C$ tuning, make sure

1. The detector map file is loaded and the crosstalk correction has been done (or the proper crosstalk matrix has been loaded)

2. The dark current correction has been done

3. Initial setting for mvolts_per_femtocoulomb have been loaded from an existing detector calibration file or a c_tune file

Then, click the "C Tuning" button. The flux distribution for the first segment will be displayed (which is a histogram of the pixel values). It should have two peaks, the left one representing the dark pixels and the right one representing the bright pixels (of the actual segments). For all segments except Segment 8, the right one will be much smaller than the left (the ratio of the peaks will be the ratio of the area of the segment to the area of the background).

You will have to click at the edges of the right peak so that the program can fit a histogram to it (see Fig. 17). Click the left mouse button to the left and the right mouse button to the right of the peak (but remember, the right peak!). Once you are happy with the fit limits you chose, click the "Accept" button and repeat the procedure for all segments. After clicking "Accept" for the last segment, the program will find the mean flux per segment by fitting a gaussian into the peaks and correct the values of mvolts_per_femtocoulomb so that each segment shows the same flux.

Figure 17: C tuning in detcal_gui. For each segment, you'll have to mark the right peak with the left and right mouse buttons.