Due to varying mechanical stresses in the dewar, the focus of the NICMOS detectors was constantly changing on-orbit. A series of programs were designed to monitor the focus for each of the three cameras. These programs used the cluster NGC 3603; each "focus sweep" consisted of up to 17 exposures taken at varying PAM positions, "sweeping" through optimum focus for NIC1 and NIC2 (NIC3 focus remained out of range of the PAM).
Program 7608 ran every 2 weeks from June 1997 through the beginning of February 1998. Each visit included, for each of the three cameras, observations at 17 PAM positions from +8 mm to -8 mm in 1 mm steps.
Program 7901 was used to determine and monitor the optimal focus for all three NICMOS cameras after the first NIC3 observing campaign. This program ran from February 1998 through Oct 1998. This proposal executed a NIC3 sweep every 28 days, and either NIC1 or NIC2 sweep, alternately, every 56 days. NIC3 sweeps consisted of observations at 10 PAM positions from -0.5 mm to -9.5 mm in 1 mm steps; NIC1 & NIC2 focus sweeps ran from +8 mm to - 8mm in 1 mm steps.
Program 7962 was used to determine and monitor the optimal focus settings for NIC2 and NIC3 during the dewar warm-up period at the end of 1998. Program 7962 ran twice weekly from November 1998 through end of cryogen in early January 1999. NIC3 sweeps consisted of 10 PAM positions from -0.5 mm to -9.5 mm in 1 mm steps. NIC2 sweeps had 17 PAM positions from +8 mm to -8 mm in 1 mm steps.