Abstract. Analysis of NICMOS images indicates that the instrument’s optics provide high quality imaging over the wavelength range and field of view of the cameras (assuming that NICMOS 3 can be placed in-focus during campaign modes). Optical misalignments and low-level field- and focus-dependent aberrations have slight effects on the imaging performance. Variations of the point-spread functions (PSFs) with wavelength may be important when comparing images through different filters, especially in NICMOS 1 due to its high resolution. The NICMOS 1 and 2 camera cold masks are shifted with respect to the telescope obscurations, causing elliptical diffraction rings and alteration of the diffraction spike patterns. The mask shift varies with time, altering the diffraction structure mostly in the wings and spiders. It is not currently known whether the NICMOS 3 mask is similarly shifted. The effects of these alignment and optical surface errors can be studied using the Tiny Tim PSF modeling software. The NICMOS field and focus dependent aberrations and obscuration misalignments derived from the image measurements are included in the simulated PSFs, along with variations due to the filter passbands. The model PSFs match the observed ones well. 1
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