The azimuth imaging resolution of synthetic aperture imaging ladar (SAIL) using the telescopes with circular and rectangular apertures is investigated based on the point target SAIL data collection equation, and the analytic expressions of point spread function for imaging are correspondingly achieved. The ideal resolution spot and its degradation due to the target deviation vertically from the footprint center, the mismatch of matched filtering from the quadratic phase history, and the limited width and period for sampling are discussed. And the deficient compensation of phase errors of nonlinear chirping and the others related to range resolution are also detailed. Mathematical criteria are derived for all of these factors. Moreover, an explanation to the resulted azimuth resolution from the earlier experiment in our laboratory-scaled SAIL is given to verify the developed theory. As a conclusion, it is found that the antenna of rectangular aperture telescope can provide a rectangular footprint more suitable for the SAIL scanning format, and this leads not only to an elimination of resolution degradation but also to an optimal design of aperture to give both a fine resolution and a wide scan strip. And it is also found that the delay between the target and the local oscillator must be small enough to eliminate the phase errors from nonlinear chirping and optical frequency instability.