CHARACTERIZATION AND SUBTRACTION OF WELL-EXPOSED HST NICMOS-CAMERA-2 POINT-SPREAD FUNCTIONS FOR A SURVEY OF VERY-LOW MASS COMPANIONS TO NEARBY STARS

conducting a search for very low mass (M < 0.2 M.) companions of stars within IO pc of the Sun using the NICMOS infrared camera on the Hubbl e Space Telescope. Thr highly exposed images ol each target star are s ubtracted from the NICMOS Camera 2 (NIC2) field using previously obser ved stars so that faint companions may be detected. We have characteri zed the NIC2 point-spread function (PSF) at large angles and measured encircled energies useful for point-source aperture photometry correct ions. The Camera 2 PSFs have elliptical diffraction rings and asymmetr ic diffraction spike banding patterns that appear to be caused by a mi salignment of the NICMOS cold mask relative to the telescope obscurati ons. The mask's position appears to vary about a general offset. creat ing a time-dependent diffraction pattern. We have devised a procedure for selecting target star images that provide the best PSF subtraction s. The ultimate detection limits of our survey are F110W similar to 21 and F222M similar to 19.5, roughly approximating J and K measurements , respectively.