OSCA, an Optimised Stellar Coronagraph for Adaptive Optics: Description and first light

We describe a coronagraph facility built for use with the 4.2 metre William Herschel Telescope (WHT) and its adaptive optics system (NAOMI). The use of the NAOMI adaptive optics system gives an improved image resolution of similar to0.15 arcsec at a wavelength of 2.2 microns. This enables our Optimised Stellar Coronagraph for Adaptive optics (OSCA) to null stellar light with smaller occulting masks and thus allows regions closer to bright astronomical objects to be imaged. OSCA is a fully deployable instrument and when in use leaves the focus of the NAOMI beam unchanged. This enables OSCA to be used in conjunction with a number of instruments already commissioned at the WHT. The main imaging camera to be used with OSCA will be INGRID; a 1024x1024 HgCdTe cooled SWIR detector at the NAOMI focus. OSCA can also be used in conjunction with an integral field spectrograph for imaging at visible wavelengths. OSCA provides a selection of 10 different occulting mask sizes from 0.25 - 2.0 arcsec and some with a novel gaussian profile. There is also a choice of 2 different Lyot stops (pupil plane masks). A dichroic placed before the AO system can give us improved nulling when occulting masks larger than the seeing disk are used. We also present results from initial testing and commissioning at the William Herschel Telescope.