Analysis of scattered light for VISTA

Abstract

VISTA is the Visible and Infrared Survey Telescope for Astronomy that has been designed by the UK ATC. The design incorporates two cameras covering the visible and near-IR wavelengths. An important concern in designing the telescope and cameras is the level of stray, scattered and background light. In the IR camera, K-band (approx. 2.0-2.3 microns) thermal emission from the telescope structure contributes ~50% of the sky background. Therefore a cold baffle is necessary. Due to the large field-of-view, and size of the required optics, a cold stop is precluded. Therefore baffling is provided by a long cryostat and a system of cold baffles with a coating that must absorb in-band light but reflect thermal radiation from the cryostat window, thus reducing the thermal load on the cryostat. In addition, the temporally and spatially variable OH airglow from the atmosphere is obscured by a warm (and hence non-absorbing) annular baffle around the secondary mirror. We report here on the modelling of the scattered and background light for VISTA. The model includes accurate models for the geometry of baffles and optical surface properties of all surfaces in the system. The optical specification is taken from a ZEMAX model and imported into TracePro to generate a fully three-dimensional telescope model, with a simple dome. For both cameras the analysis has been done for the case of scattered light from a full moon at various incident angles from 0 degrees (on axis) to 65 degrees. It is shown that a reflective baffle around the secondary mirror does not significantly impair the performance of the visible camera. Ghosting from bright stars in the field has also been calculated for both cameras. Results indicate that the level of scattered and ghosts is below the limits specified for VISTA.