Cameras do not use magnification. From Nyquist Criterion the optimal-maximal f/ratio is pixel size/0.275. For ASI120MM it's f/13.6. This value is theoretical so actual "best" will vary around it depending on imaging target and processing. Mars can look good theoretically-overscalled, like at f/20. Darker like Saturn are much harder to photograph and are better to photograph at lower resolutions (so like <= f/15 or even for example f/10 if someone already has f/10 SCT and the conditions aren't good).
