Characterization and Subtraction of Well‐Exposed HST/NICMOS Camera 2 Point‐Spread Functions for a Survey of Very Low Mass Companions to Nearby Stars

Abstract

ABSTRACTWe are conducting a search for very low mass (M<0.2 M⊙) companions of stars within 10 pc of the Sun using the NICMOS infrared camera on the Hubble Space Telescope. The highly exposed images of each target star are subtracted from the NICMOS Camera 2 (NIC2) field using previously observed stars so that faint companions may be detected. We have characterized the NIC2 point‐spread function (PSF) at large angles and measured encircled energies useful for point‐source aperture photometry corrections. The Camera 2 PSFs have elliptical diffraction rings and asymmetric diffraction spike banding patterns that appear to be caused by a misalignment of the NICMOS cold mask relative to the telescope obscurations. The mask's position appears to vary about a general offset, creating a time‐dependent diffraction pattern. We have devised a procedure for selecting target star images that provide the best PSF subtractions. The ultimate detection limits of our survey are F110W ∼ 21 and F222M ∼ 19.5, roughly approximating J and K measurements, respectively.