Field Variations ( FFV ) and Focus Centering ±

NICMOS foci are known to vary across detector's field of view. These variations must be taken into account when determining ''best'' focus from a particular stellar image. Focus field variations (FFV) may introduce substantial positional dependence in aperture corrections , thus they may also be important for high precision stellar photometry. In this report, we present empirical results of a study of time dependence as well as PAM setting dependence of FFV for the NICMOS cameras. Although the results are not easy to inter-prete because of substantial errors, they still suggest that some amount of both time and PAM position dependence of FFV is consistent with the cameras 2 and 3 data. There is a preliminary understanding of why FFV should depend on PAM setting and may change with time. However a detailed interpretation of the actual FFV time and PAM dependence is beyond the scope of this report and is left for future analysis. As a practical result, we present the coefficients of the second order surface fit to the FFV data averaged over time and PAM positions. They are recommended for use in the NICMOS focus monitoring program to reduce focus to the detector center (focus " centering ").