This research investigates how tiny misalignments in detectors and laser beams affect the precision of ESA’s space-based interferometer; Laser Interferometer Space Antenna (LISA), which will detect gravitational waves. Using both laboratory measurements and computer simulations, the study explores how the shape and alignment of laser beams influence the signals from quadrant photodiodes (QPDs), the sensors that measure beam phase and angle. Different beam types (Gaussian and flat-top) and detector geometries were tested to understand their impact on measurement stability. The results define acceptable detector tolerances, show how different beam models modify the QPD signals, and provide concrete guidance for LISA’s future error budgeting.