The Pupil Alignment Mechanism (PAM) consists of an adjustable
mirror in the NICMOS optical train that can be moved to make small corrections to the NICMOS focus and serves to properly position the pupil image of the telescope primary mirror onto the corrective optic. The motion of the PAM is limited to ±
10mm from its zero position. The NICMOS cameras were designed to share a common focus with the PAM close to its zero position. In the current state of the dewar, NIC1 and NIC2 can each be focused within the range of the PAM. NIC3, however, cannot be entirely focused by motions of the PAM alone and remains slightly out of focus although still scientifically usable (see next section).
The focus positions of all three NICMOS cameras have changed since
launch due to motion in the dewar. The positions are measured by observations of stars over a range of focus settings on a frequent basis. The focus history since shortly after launch is shown in Figure 4.6
. The focus position is given for the detector center. The focus variation across the camera’s field of view is ~1.5 mm in PAM space in NIC2, and about half of that in NIC1. The two largest focus excursions, on January 12 and June 4, 1998, are due to the secondary mirror reset used to place NIC3 in focus during NIC3 campaigns. A noticeable improvement in NIC3 focus occurred after December 17, 1997, when the FOM had been tilted by 16 arcsec to reduce vignetting in that camera.
After installation of the NCS the PAM has been positioned to give the
best focus for NIC1 and NIC2. This position has proven to be quite stable for NIC1 and NIC2. No additional special focus campaigns for NIC3 are planned. For more information, please see the NICMOS Focus Web page
Starting in Cycle 17 there will be an available observing mode called
DEFOCUS for de-focused observations. This mode is classified as "available but unsupported", which means that observers can request it, but the mode is not supported through a full calibration program, other than basic verification that the mechanism successfully moves to the commanded de-focused position. When in use, the PAM mechanism will move to a pre-determined position, leading to a decrease in the count rate. The amount of decrease depends on the filter in use. The de-focus mode can help prevent saturation when short exposures are necessitated by bright objects, but will suffer from a degraded image quality. To implement this mode, the observer should contact the Program Coordinator.