A golf training aid having an alignment rod rotatably mounted to an elongated rail. The flow velocity control structure includes, in the direction intersecting with the flow direction, plural flow velocity control members provided alternately with spaces through which the fluid passes. A calibration target spanning measurement fields of the measurement units may be used. In other embodiments, the number of crystals can be increased by a process that includes annealing a region in which the crystals are present, etching the region to remove crystals having an orientation other than the selected orientation, andor growing the crystals having the selected orientation. The compensation module is operably coupled to the 1st mixing stage andor the 2nd mixing stage to modify the operational characteristics of the 1st andor 2nd mixing stages based on a control signal.