OSCA, an Optimised Stellar Coronagraph for Adaptive optics
Described here is the design, manufacturing, testing and commissioning of a coronagraph facility for 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 ~ 0.15 arcseconds at a wavelength of 2.2μm. This enables the 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 which when in use leaves the focus of the NAOMI beam unchanged. This enables OSCA to be used in conjunction with a number of instruments that have already been commissioned at the WHT. The main imaging camera used with OSCA is INGRID; a 1024 × 1024 pixel HgCdTe cooled short-wave infra-red (SWIR)detector at the NAOMI focus. OSCA also has the option of being used in conjunction with an integral field spectrograph for imaging at visible wavelengths. OSCA provides a selection of 10 different occulting masks with sizes of 0.25 - 2.0 arcseconds in diameter,including two with novel gaussian profiles. There is also a choice of two different sized Lyot stops (pupil plane masks). A dichroic placed before the AO system can give improved suppression performance when occulting masks larger than the seeing disk are used. Also presented are results from observing time with the OSCA system, which highlight the challenges faced by astronomers to obtain high contrast, high resolution images from ground based telescopes. At a time during which there is much activity towards terrestrial planet finding, questions as to the system requirements required for such a task are discussed.