Design, fabrication, and test of a patterned optical filter array for the Europa Imaging System (EIS)

NASA's Europa Clipper will carry two cameras as part of the Europa Imaging System (EIS). Both the wide angle camera and narrow angle camera have identical focal plane modules, each containing a CMOS image sensor and patterned optical filter array. The filter array enables multispectral pushbroom imaging in six bands, spanning 380nm to 1000 nm, adding additional science capability for surface characterization and searching for evidence of recent activity. The EIS filter array is monolithic in construction, with all bands coated on a single substrate. Each stripe measures only 320 μm wide-equivalent to 32 pixels on the image sensor-leaving most of the field of view clear for full frame panchromatic imaging. Using photolithography, a mask is applied to the filter substrate and developed, leaving only the desired pattern exposed. The filter is then deposited onto the substrate and the mask removed. This process is repeated for each additional band on the array until all filters have been applied. The filters are then aligned with the image sensor rows using a machined metal housing, placing it as close as possible to the focal plane. As part of a technology development program to qualify them for space flight, several filters have been performance tested for resistance to radiation exposure, thermal cycling, vibration, and dry heat microbial reduction for planetary protection.