Coronagraphic Imaging with the Hubble Space Telescope and the Space Telescope Imaging Spectrograph

The unfiltered Space Telescope Imaging Spectrograph (STIS) CCD in tandem with focal plane wedges and a Lyot stop provides a simple white-light coronagraph with a bandpass of 0.2-1.0 mum, which has been used since late 1998 to image nebulosity around stars in the ranges 0.34 less than or equal to V less than or equal to 14 and -0.03 less than or equal to B-V less than or equal to 1.65. The residual starlight seen in STIS coronagraphic images includes diffraction spikes due to the Hubble Space Telescope (HST) secondary support structure, lacks distinct Airy rings, and varies smoothly with radius from the star. The point-spread function (PSF) shape is a strong function of the source spectral energy distribution: we find that the PSF template needs to differ from the occulted source color by Delta(B-V) less than or equal to 0.08 mag. Optimal PSF removal is achieved for contemporary template observations matched to HST orbital phase of the science observations. Use of noncontemporary PSF templates can degrade the limiting contrast by up to a factor of 10-12 at r less than or equal to 2". These systematic effects are negligible when the nebular surface brightness is comparable to the residual starlight, and they become progressively more important as surface brightness decreases. STIS has been used to detect circumstellar envelopes and protoplanetary disks with surface brightnesses spanning 10(-4) to 10(-7) per HST resolution element per F-star at 2" from the star, debris disks with F-IR/F-star greater than or equal to 0.001, and emission-line nebulosity associated with bipolar outflows. The limiting contrast for single- orbit integrations with the star placed at a location where the coronagraphic wedge is 1".0 wide is 10(-8) per HST resolution element per F-star for 6 less than or equal to V less than or equal to 8 stars. Deeper imagery can be obtained by placing the star off the active detector area. When the star is 5" from the detector, a limiting contrast for a single-orbit integration of 2.5 x 10(-10) per HST resolution element per F-star is reached. At this contrast level, background objects and the stellar color variability prevent further improvement by co-adding data from multiple orbits.