What is aperture, and why is it important?
The aperture of a telescope is simply the size (diameter) of its main lens or mirror. Aperture is important because the bigger the diameter the more light that can be collected. More light means that fainter objects can be seen, and that less faint ones can be seen more clearly. Bigger is also better because it means you can resolve more fine details (see below). You get more per inch of aperture from a refractor than a reflector (mainly because part of the light in a reflector is blocked by the secondary mirror); common wisdom suggests a 3″/80mm refractor, or a 4″/100mm reflector as the smallest worthwhile size to begin with. Nowadays many people would suggest 6″ as a realistic minimum for a reflector.
