Exploration of the environments of nearby stars with the NICMOS coronagraph - instrumental performance considerations

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS), installed into the Hubble Space Telescope (HST) in February 1997, incorporates a coronagraphic imaging capability. The coronagraph is comprised of two optical elements. The camera 2 field divider mirror, upon which the I-IST f/24 input beam is imaged, includes a 170 mu m diameter hole which contains similar to 93% of the encircled energy from a stellar Point Spread Function (PSF) at a wavelength of 1.6 mu m. The coronagraphic hole lowers both the diffracted energy in the surrounding region by reducing the high spatial frequency components of the occulted core of the PSF, and down stream scattering. The geometrical radius of this occulting spot, when re-imaged through the camera 2 f/45 optics, is similar to 4 pixels (or 0.3 ") at the detector focal plane. An oversized cold pupil-plane mask (maintained at similar to 100K), with radial structures co-aligned with the HST secondary mirror spider, acts over the whole 19.1 " x19.2 " field to further reduce the diffracted energy in the direction of the spider vanes. The absolute performance levels of the coronagraph were ascertained during the Servicing Mission Observatory Verification (SMOV) program. Using a differential imaging strategy (by rolling the spacecraft) we expect to achieve statistically significant detections of sub-stellar companions at 1.6 mu m with a Delta H of similar to 10 and separations as close as 0.5 " (corresponding to 2.5AU at 5pc). The NICMOS Environments of Nearby Stars (EONS) programs is exploiting this capability in systematic surveys of nearby, and young stars searching for brown dwarfs and giant planets, and protoplanetary disks around main-sequence stars.