A Newtonian telescope is one of the most widely used reflecting telescopes by both researchers and amateur astronomers. The comparative analysis of the effect of focal ratio in the fixed-diameter Newtonian telescope is still limited using multiple performance indicators. In this paper, a computer ray-tracing simulation was performed using Optics Software for Layout and Optimization (OSLO) to design the Newtonian telescope system with four focal ratios (f/4, f/6, f/8, and f/10) in case of the rays is an on-axis. The modulation transfers function (MTF), point spread function (PSF), and diffraction encircled energy (EE) was adopted as criteria to evaluate the optical performance for system. Finally, the results showed that the best focal ratio of image quality and overall telescope performance was found at f/4. As well as the findings revealed that system with smaller focal ratio exhibit a higher cutoff frequency, a smaller PSF, and a greater concentration of encircled energy. While the angular resolution remains constant due to the fixed aperture diameter.