Performance of a directly deposited optical blocking filter on x-ray CCDs: case study from the REgolith X-ray Imaging Spectrometer (REXIS) experiment

The REgolith X-ray Imaging Spectrometer (REXIS) is a soft x-ray spectrometer and the student collaboration instrument aboard NASA's OSIRIS-REx asteroid sample return mission. REXIS utilizes MIT Lincoln Laboratory CCID-41 x-ray detectors coated with a directly deposited optical blocking filter (OBF) with a thickness of 320 nm. The aluminum coating, developed at MIT Lincoln Laboratory, is designed to block visible light from the detector, to maintain high sensitivity to soft x-rays in the presence of reflected sunlight from the surface of the target asteroid Bennu. The scientific objective for the REXIS instrument is to measure the stimulated x-ray flux fluoresced from Bennu to discern elemental abundances present on the asteroid's surface. The coating technique applied for blocking visible light had not previously been used on the CCD-41s in an extended space flight mission. The performance of the OBF on the flight detectors was not characterized before and after environmental stress testing. Therefore, to mature the OBF to technology readiness level (TRL) 6, the flight spare detectors were tested while the instrument was on the way to the asteroid. The flight spare hardware underwent vibration and thermal environmental stress testing to test the durability and effectiveness of the OBF. This testing informed our expectations of the in-flight OBF once it reached the asteroid and helped mature the TRL level of this directly deposited OBF. We discuss the setup and results of those tests and address the performance of the flight OBF at the asteroid. We conclude that depositing an aluminum OBF onto the surface of a charge-coupled device is able to withstand stresses of launch and an extended life-mission in interplanetary space. (C) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)