Figure vsg.2 : Period Trends. Changes in the grating period in the "active" region is shown versus time. The dashed lines represent the period derived from the 9611 set of VSG retests, comparable to the final LR measurements on the Flight gratings. Measurements made at other times are then shown normalized to this period. The dotted lines represent period variations of +/- 50 ppm from the 9611 value. See the "Period Plot Anomalies" section of the text for a list and discussion of anomalies in these trend plots.