NICMOS Focus: Design vs. On-Orbit Performance
The NICMOS cameras were designed to share a common focus, the position of which can be adjusted using the Pupil Alignment Mechanism, or PAM. The PAM's main component is an adjustable mirror which can be moved within +/- 10 mm about its zero position, thus allowing fine tuning of the actual location of the focus. It was hoped that whatever changes occurred to the HST optical path, the focus could always be brought back to the position of the detectors. This was not to be the case on-orbit.
Unfortunately, the unforeseen deformation of the NICMOS dewar caused large mechanical distortions within NICMOS which resulted in loss of a common focus for the three cameras. NIC1 and NIC2 were close to a common focus but NIC3 was not. NIC3 was pushed beyond the range for which the PAM could correct the focus. Soon after its installation during the Second Servicing Mission (SM2) in 1997, NIC3 required a PAM position of about -17 mm to be in focus; the PAM could only adjust the focus to +/-10 mm. The mechanical stresses in the dewar continue to vary and the ongoing deformation processes in NICMOS kept changing the location of the detectors with respect to the PAM zero position, i.e., the focus of the cameras was changing with time..