On January 12, 1998 (day 377 since January 1, 1997), the HST secondary mirror was moved to refocus the telescope's Optical Telescope Assembly (OTA) such that NIC3 would be in focus for the first NIC 3 campaign which lasted 3 weeks.
As seen from the focus history Table and plot , the goal of refocussing was successfully achieved. The focus position for camera 3 was found to be -9.76 +/- 0.24 mm in PAM space. This is well within the NIC3 campaign focus tolerance of 1 mm at the nominal PAM setting of -9.5 mm. The focus measurements showed that the focus shift resulting from the OTA refocussing was consistent with the expected 2.5 mm PAM change.
Note also that, in the plot, focus during NIC3 campaigns is corrected for the Secondary Mirror move while the Table contains the actual uncorrected values.
Note that the secondary mirror refocussing and the focus change induced by the FOM do not change the plate scale, and hence the "plate scale method" continues to track the internal dewar motions only. It no longer gives, in an absolute sense, the correct value of the focus position.
The default setting of the Field Offset Mirror (FOM) for NIC3 was moved to minimize vignetting in that camera. The change was made to support the NIC3 campaign. Due to the structure of the focus montitoring proposal, the same FOM offset was also applied to NIC1 and NIC2. Normal prime observations in NIC1 and NIC2 do not have the offset FOM position. Indicated in the focus plots are estimates of the FOM-corrected focus position for NIC1 and NIC2 only (NIC3 prime observations do have the new FOM position). The new FOM position gives a shift in focus position by approximately 1.5 mm, although the exact value is rather uncertain.