Largest diameter mirror at the bottom or base of a reflector. Although there are many variations, the basic idea is that the objective creates a cone of light that comes to a focus at the focal point. The focus is then magnified for viewing by an eyepiece. Light Gathering power Depends on aperture. Allows clear, detailed viewing of faint objects.
Telescopes are light buckets, or light funnels, more than image magnifiers.
Resolution Refers to crisp, distinct images. Poor resolution means fuzzy images. Greater the larger the aperture. For example, a magnification of 20x means that an image appears twenty times larger than actual size. A rule of thumb for practical performance is to expect a maximum of 50x of useful magnification per inch of telescope aperture.