Diagram showing how the relationship between wave-front sensor subapertures and detector pixels can be deduced from SUBAPERTURE_MASK, MASK_X_OFFSETS, MASK_Y_OFFSETS and SUBAPERTURE_SIZE. Panel a: example 80 × 80 PIXEL_INTENSITIES image from the wavefront sensor’s detector. Panel b: example 4 × 4 SUBAPERTURE_MASK for a wavefront sensor with 12 valid subapertures. Cells with value −1 indicate an invalid subaperture, while the others indicate the index of that subaperture in the sensor data.. Panel c: SUBAPERTURE_MASK superimposed over PIXEL_INTENSITIES. In this case, MASK_X_OFFSETS and MASK_Y_OFFSETS have length one, indicating that the mask is superimposed once, and its lowermost positions in X and Y are 13 and 8, respectively. SUBAPERTURE_SIZE defines each subaperture as corresponding to 16 pixels.