Critical point facility for the IML-2 mission: reduction of stray light in the optical system

The optical system of the critical point facility (CPF) was developed to investigate the behavior of fluids around their critical point. The fluids can be analyzed in four measurement channels: the visual, the interferometer and the small (SALS) and large (WALS) angle light scattering channels. During upgrading of CPF-IML1 for the IML2 Mission, special effort was made to improve the S/N ratio at minimum input signals and in consequence the absolute sensitivity of the small angel light scattering channel. The S/N ratio of the SALS channel was limited mainly by stray light from the surfaces and bulk materials of the optical components of the system and by the detector electronics. In the following the term 'scattered light' is used for the light scattered by the fluid and which has to be measured, and 'stray light' is the contribution of all other light sources disturbing the measurement. After the description of the optical system of CPF, modifications performed to improve the dynamic range of the SALS channel are described. The most important stray light sources are identified and ways to reduce their contribution to the noise are discussed. Modifications to improve the dynamic range of the SALS detector unit are described. Finally the consequences of the system upgrading activities on the optical performance of CPF are discussed.