Development of a large blazed transmission grating by binary index modulation for the GAIA Radial Velocity Spectrometer

The Radial Velocity Spectrometer (RVS) is one of the three instruments of the ESA astrometry mission Gaia. The RVS derives the red shift of a measured star by a spectral signature of the element Calcium in the spectral range between 847nm and 874nm. A blazed transmission grating with a spacing of about 300 lines per mm is the dispersive element of this spectrometer. The required dimensions of the grating (205mm x 155mm), the required imaging performance (wave front error <8nm RMS), high efficiency (>70%) and the qualification for space applications are the most challanging requirement for this component that can not be met with gratings built by conventional methods. Within the frame of a feasibility study for ESA, IOF has designed, built and tested such a grating based on binary index modulation. The grating was subject of a number of environmental tests performed by ESA. The results shows that the technique is capable to produce monolithic blazed transmission gratings with outstanding optical performance values.