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As previously mentioned, the timetool trace edge is found and reported in pixels along the OPAL camera (e.g. arbitrary spatial units), and must be converted into a time delay (in femtoseconds). Because the TT response is a function of geometry, and that geometry can change even during an experiment due to thermal expansion, changing laser alignment, different TT targets, etc, frequent calibration is recommended. A good baseline recommendation is to do it once per shift, and then again if something affecting the TT changes.

To calibrate, we drive change the timetool laser delay stage (for which affects the white light) while keeping the nominal delay between the x-rays and laser stage for the TT constant. This causes the edge to transverse the camera, going from one end to the other, as the white light delay changes due to the changing propagation length. Because we know the speed of light, we can figure out what the change in time delay was, and use that known value to calibrate how much the edge moves (in pixels) for a given time delay change.

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