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In rare cases, a sample may require a negative offset (Fig. 15). This is where the focal plane
of interest sits above the main reflective interface in the sample. The PureFocus850 has a
limited negative offset range that can be used if required.
By aligning the peak maximum with the pinhole centre, the required offset value has been
calculated. If the peak maximum crosses the top of the y-axis of the graph, reduce the laser
power until the peak maximum is just visible. If the peak is very small, increase the laser power
until the peak maximum is at least half way up the y-axis.
Make a note of the calculated offset value and compare to the table at the start of section 6.3.
