The focus history plot displays, for camera 3, the regular phase retrieval and plate scale measurements. The results of phase retrieval corrected for the new FOM setting (in from day 351 through day 397) and the secondary mirror move on January 12, 1998 (day 377) are presented. This is convenient for comparison of the focus behavior before and after day 377 in terms of detector motion. The plot shows that there were no dramatic changes in the camera 3 focus over the last few months except for those caused by the new FOM tilt and the secondary mirror move.
The large error in the phase retrieval result for camera 1 is explained by the fact that there were only parallel observations with this camera, which provided only one star suitable for phase retrieval. That star was very faint and yielded less reliable phase retrieval results than usual. The encircled energy measurements however were adequate. Proposal 7901 is designed in such a way that camera 1 and camera 2 alternate as prime cameras every other visit.
We have slightly improved the coefficients used to reduce focus to the detector center and recomputed focus from phase retrieval for all visits since day 160. Comparison with the previous results shows that the change caused by the new focus centering is quite small.