Design and realization of multispectral bandpass filters for space applications

Spectral and radiometric requirements of optical filters for space applications are always more demanding. Multispectral bandpass filters require very steep edges, a high accuracy in the central wavelength position, a high average transmittance as well as a very good level of blocking outside of the bandpass region. Realization of such filters implies to design the layer stacks in a way that reduces sensitivity to the production deviations and to be able to properly control the coating conditions in the machine. In order to optimize the design, a pre-production error analysis can be performed, to assess the weight of each layer in the global filter properties and therefore modify the design until a good stability level is achieved. We discuss different insitu optical monitoring strategies and process conditions depending on the coating technology. The realization phase needs a precise control of the coating machine such as well-characterized deposited materials, stable deposition rates, a reliable layer thickness monitoring system in addition to a good coating uniformity over all the coating positions inside the machine. We present design strategies and means of pre-production error analysis for typical multispectral bandpass specifications as well as transmittance curves.