UV bandpass filters based on Ta2O5 and ZrO2 for solar observation

In the present article, complex optical UV bandpass filters with tantalum pentoxide and zirconium dioxide as high refractive index materials, respectively, have been manufactured on the EOSS® sputtering system. Four different bandpass filters with central wavelengths between 316 nm and 399 nm and a blocking up to 1100 nm were realized. Transmittance of the filters was higher than 80% although the band pass was located close to the absorption edge of the high index material. For two of the filters, ZrO2 was used as the high index material. It is demonstrated that the ZrO2 containing filters have very good optical properties and ZrO2 is a good alternative to the use of the costly HfO2. The bandpass filters will be used as order-sorting filters for the grating spectrograph of the Sunrise UV Spectropolarimeter and Imager (SUSI) onboard the balloon-borne solar observatory Sunrise III. The filters are designed to filter out unwanted light that hits the detectors. The observatory will be launched in June 2022. Among other topics, the mission is dedicated to the investigation of magnetic fields and convective plasma flows in the lower solar atmosphere.