Z Alignment

To align the detector in Z, you have two options. Note that you probably won't find the "perfect" position, but that's ok.

1. Scan XDET or YDET vs. ZDET. Since the big X detector stage doesn't move very smoothly, you might be better off with YDET. Again, there should be no sample inserted, or you should be inside a pinhole. Scan, say, $100 \times 200$ pixels with a pixel size of 3 microns in X and 5 microns in Z. Display the signal from segment 8 and use the "Stern Special" colortable again (see Fig. 21). The ideal Z position is around the tip of the black inverted triangle. Also look at the horizontal profile through the image - at the correct Z position, the profile should just lift off from zero at the center.

   Figure 21: YDET vs. ZDET scan for detector alignment. Only the signal from segment 8 (the outer ring) is displayed, and the Stern Special colortable is used. Move the detector to the tip of the black triangle, where the horizontal profile just touches zero.

   

   The file x1ais_20051028_0039.sm is an example for such a scan.

2. Insert a pinhole as sample and focus. Go to the center of the pinhole and then scan XPZT or YPZT vs. ZDET. Look at the signal from segment 8 again (see Fig. 22). The two vertical lines are more or less constant in intensity, that's scattered light from the pinhole edges. Outside the pinhole should be constantly dark. Inside the pinhole, the intensity should go from high (detector too far) to low (detector too close). The ideal Z position is where the intensity inside the pinhole (look at the Y profile) just starts to lift up from the constant base.

   Figure 22: XPZT vs. YDET scan with a 10 micron pinhole installed. Only the signal from segment 8 (the outer ring) is displayed. The correct Z position for the detector is just where you start getting some signal inside the pinhole.

   
   The file x1ais_20051028_0040.sm is an example for such a scan.

Note that the Z alignment is really not that critical. For an 80micron diameter zone plate with a focal length of 1mm, the ideal detector position is $600/80 \times 1 \mbox{mm} = 7.5 \mbox{mm}$ from the focus. If you are off by 0.5mm in Z (which is a lot), the diameter of the beam cone changes by $0.5/7.5 \times 600 = 40 \mbox{microns}$ in diameter or 20microns in radius. That's 20 percent of the width of the quadrant segments. And with the methods described above, you should find the best Z position better than 0.5mm.