Session 10 - NICMOS.
Display session, Wednesday, January 07
Exhibit Hall,

## [10.03] NICMOS Foci

A. Storrs, A. A. Suchkov, L. Bergeron (STScI), G. G. Galas (Greater Homewood Community, Inc., Baltimore)

The three NICMOS cameras installed on Hubble Space Telescope were designed to share a common focus whose position can be adjusted using the Pupil Alignment Mechanism (PAM), by moving PAM's mirror within \pm10 mm. Unfortunately, the unforeseen deformation of the NICMOS dewar has caused mechanical distortions within NICMOS, which resulted in loss of a common focus and unstable focus position. This necessitates checking the NICMOS camera foci on a regular basis so as to ensure timely adjustments of PAM nominal settings if required. We present the results of the NICMOS focus monitoring program and discuss long term focus variations due to ongoing deformation processes within NICMOS as well as short term variations due to HST breathing'' effects. The data suggest that since the beginning of June 1997, there is no substantial trend in NICMOS foci, with focus position fluctuating typically within \pm 0.5 mm or less (in PAM space'') about the average on the time scale of about two weeks. The fluctuations due to breathing'' on the orbital time scale are typically within \pm 0.2 mm. Along with temporal variations, the NICMOS foci have been found to vary spatially across detectors' field of view. For camera 2, the focus position difference between the top and the bottom of the detector corresponds to \sim 1.5 mm of PAM space'', which is a non-negligible amount in terms of the impact of focus abberations on photometry. In camera 3, the amplitude of focus field variation is \sim 1 mm, somewhat smaller abberations have been found for camera 1 as well. We have quantified the focus field abberations and present the results which can be used for better assessment of the accuracy of field photometry.