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

We are conducting a search for very low mass (M) companions of stars within 10 pc M ! 0.2 , 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.