The aperture diameter (distance from side to side) of the opening of the telescope in millimeters. This is the area that lets light in. This telescope has a 18 mm aperture. Telescopes have a limit to how much useful magnification you can get, and this depends on the size of the aperture. Past that limit, and the image begins to break down, so it is not recommended to magnify higher than this amount. In general, the maximum useful magnification is 1.97x for every millimeter of aperture. Based on the size of the telescope’s aperture, what is the maximum useful magnification for this telescope?