The Advanced Camera Coronagraph for HST
Space Telescope Science Institute and Astrophysics Division, ESA
The Advanced Camera for Surveys is due to be installed in an axial bay on HST in 1999. It will be a substantial improvement over WFPC2 in field of view, resolution, quantum efficiency, and read noise.WFPC2 has no high contrast channel, and in fact suffers from very high levels of instrumentally scattered light. Only limited coronagraphic channels are available for STIS and NICMOS. The new coronagraphic mode on the Advanced Camera will therefore represent our first chance to fully exploit the high dynamic range potential of HST. The coronagraph operates by inserting a field stop into the aberrated beam at the circle of least confusion. Light diffracted around this obstacle is caught by a pupil plane Lyot stop. The residual scattered light level is set by mid-scale ripple on the HST mirror surfaces. We will show details of the projected performance of this new capability. The coronagraph will not be able to directly detect planets orbiting nearby stars, except in the most optimistic case. However, it will have a broad range of other potential astrophysical applications. In particular, it will be able to clearly image the initial conditions for planetary systems (protostellar disks - particularly when oriented with their pole pointing towards us) and their remnant by-products (circumstellar disks). Main sequence disks can be perturbed by the presence of planets, as illustrated by HST observations of Beta Pictoris. A range of existing WFPC2 observations of such systems will be shown to illustrate the current limits of our observations. Better circumstellar disk observations are a necessary prerequisite to more refined and targeted searches for other planetary systems. This applies to the proposed nulling infrared interferometer, where asymmetries in the circumstellar material distribution might mimic the signal from a planet. On the other hand, asymmetries in a disk can also indicate the presence of planets.