In order to explore the characteristic index which can correctly reflect the imaging performance of synthetic aperture systems, the maximal and minimal fill factors, cut-off frequencies, cut-off energy ratios as well as the correlation coefficients of three typical arrays with six apertures are calculated and compared. It is revealed that the imaging quality is determined by the distribution of low and mid-band modulation transfer function (MTF) spectrum. Based on the discovery, a weighted frequency energy curve which takes the frequency magnitude and distribution into consideration is proposed to precisely reflect the difference in imaging quality of various arrays. To maximize the mid-band energy of MTF, the pupil structures of the three arrays are optimized and the corresponding center-gathered arrays are obtained. The experiment results demonstrate that the imaging qualities of the new arrays are dramatically better than those of the original arrays, and the fill factor ranges can meet the common design demand as well.