Since the focus data reduction and analysis techniques are being constantly improved, and the results from the early data sets are being re-computed each time an improvement is made, the reader may find slight differences between the currrent and previous versions of the focus history plot/table.
Phase-retrieval and Encircled Energy measurements give results in absolute PAM units. As the cold-well moves forward and back relative to the cold-mask, the effective plate-scale of the detector changes. The change in plate-scale between epochs is proportional to the motion of the cold-well. The scale changes are related to absolute PAM space by scaling them to measures from phase-retrieval at a few of the epochs.
Note that, in the plot, focus during NIC3 campaigns is corrected for the Secondary Mirror move while the Table contains the actual uncorrected values.
Focus History Table
Focus measurements during the two NIC3 campaigns are uncorrected for the Secondary Mirror move in the table.
Focus History Plot
This plot shows the "best focus" position of NICMOS cameras as a function of time. The abscissa is the day of observation (day number starting January 1st, 1997). The ordinate gives the PAM position which would bring the detector into focus (implied PAM position, often referred to as "the focus position in the PAM space").
The flat dash-dot line is the current nominal PAM setting. The dash-dot line at -9.5 mm for camera 3 is the current NIC3 nominal PAM setting, and the end of the useable PAM focus range. The different methods used to derive the focus position are indicated by different symbols as shown in the plot legend.
Focus measurements during the two NIC3 campaigns are corrected for the Secondary Mirror move in the plot.