Development of method for active alignment of multiple time delay and integration detectors in the optically butted focal plane assembly of high-resolution spaceborne imaging systems

Abstract

Time delay and integration (TDI) detectors are preferred image sensors for high-resolution spaceborne imaging systems for cartographic applications as they provide higher sensitivity by integrating charges over multiple TDI stages in push broom mode. Large swath requirements are met using multiple TDI detectors in optical butting configuration in the focal plane assembly (FPA) to form a single image line on the ground. These detectors need to be aligned with subpixel accuracy to avoid motion smear and errors in the image mosaicking process. Alignment of the multiple TDI detectors in the FPA is a challenging task and is accomplished using custom methods and setups. Passive alignment techniques rely on very high-resolution microscopes for achieving desired accuracies. Active method requires TDI detectors to be operated in staring mode, which is architecturally and functionally not supported by TDI detectors. We developed a scheme for operating the TDI detectors in staring mode without any smear effects to aid in the active alignment process. Based on this, we propose an active alignment approach. The proposed approach is highly advantageous as it can be used throughout the imaging system development phase to demonstrate stability of the alignment and also helps in cross validation with the passive alignment. The developed method was test validated on qualification model FPA, and alignment accuracy of the order of 0.4 μm in tilt and 1.2 μm in line shift has been demonstrated, against the requirements of ±1.5 μm. This method has been extensively used in Cartosat-2S series imaging systems and will be very useful for upcoming high-resolution missions of the Indian Space Research Organisation.