In the process of error correction based on reflecting telescope system, definition function of spot target is usually used as performance metric of imaging quality. The definition function will reach extremum by using optimization algorithm to correct parameter error. Mean radius as definition function of spot diagram is widely used for its large- scale and good dynamic property in correction. While the value of mean radius of calculation is different from the value of theory because of the noise of detector, thus the correction process is affected. The calculation error of mean radius because of noise and the influence of threshold is analyzed through theory and simulation.The result indicates that there exists an optimal threshold to make the error minimum. A fitting formula is proposed by the simulating optimal thresholds in different noise intensities.The formula can accurately calculate optimal thresholds.