Design and modelisation of a straylight facility for space optical instrument

In the framework of instrument calibration, straylight issues are a critical aspect that can deteriorate the optical performances of instrument. To cope with this, a new facility is designed dedicated for in-field and far field straylight characterization: up to 10-8 for in-field and up to 10-10 for far field straylight in the visible to NIR spectral ranges. Moreover, from previous straylight test performed at CSL, vacuum conditions are needed for reaching the 10-10 rejection requirement mainly to avoid air/dust diffusion. The major constrains are to design a straylight facility either for in-field and out-field straylight measurements. That requires high dynamic range at source level and a high radiance point source allowing small diverging collimated beam. Moreover, the straylight facility has to be implemented into a limited envelope and has to be built with vacuum compatible materials and black coating. As checking the facility performance requires an instrument better than the facility itself, that is no easy to find, so that the performances have been estimated through a modelisation into a non sequential optical software. This modelisation is based on CAD importation of mechanical design, on BRDF characteristics of black coating and on statistical averaging of ray tracing at instrument entrance.