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.
