Accumulator-based wavefront slope processor for Shack-Hartmann sensors

In order to accommodate as many subapertures as possible in the high-speed wavefront slope calculation when hardware resources were limited, an accumulator-based wavefront slope processor was proposed. The computational core of the processor is an array of subaperture spot centroid calculating element. According to the decomposition of 2D moment calculation and the recursive procedure of 1D moment calculation, it can substitute the multiplications between the pixel grayvalues and its coordinates with several sum operations to obtain the low order 2D geometric moments required in centroiding. The calculating element simply consists of five accumulators, and the cost has been decreased because no multipliers are needed. Experimental results indicate that when it is implemented in a Field Programmable Gate Array(FPGA) at a clock frequency of 100 MHz, the proposed architecture can obtain gradients of all subapertures in 22×22 Shack-Hartmann with the latency no more than 0.33 μs and the error less than 0.002 pixel. The new design has reduced the hardware resource by 40% as compared to that of the multiplier-based architecture. Furthermore, the original multiplier-based processor can be updated for Shack-Hartmann sensors and can obtain the subapertures twice as much as that of original one without additional hardware resources. It realizes the high speed measurement of wavefront with a high accuracy.