Figure vsg.3 : Period Variation (dp/p) Trends. Changes in grating dp/p within the "active" region is shown versus time. The dashed lines represent the dp/p value 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 as a per cent change relative to this dp/p value. The dotted lines represent dp/p changes of +/- 30% from the 9611 dp/p value. See the "Period Variation (dp/p) Plot Anomalies" section of the text for a list and discussion of anomalies in these trend plots.