TinyTim modeling of NICMOS PSF subtraction: effects of focus breathing, cold mask wiggling, and color variations

We conduct TinyTim modeling of NICMOS camera 2 PSFs to assess limits to the accuracy of PSF subtraction imposed by color dependence, OTA focus breathing, and cold mask wiggling. The results suggest that effects of both cold mask wiggling and focus breathing can be described as a noise in a PSF-subtracted image, well above 20% of the PSF signal in narrow band filters, with a spatial scale of a few pixels. The main effect of the PSF color dependence is adding a systemic component to the PSF subtracted image. The effect is quite large if the color of the PSF used for subtraction is very different from the image PSF color. It is pronounced in blue, wide filters, like F110W, while in filters like F187W or redder/narrower it is essentially negligible.