primary mirror

A primary mirror is the main, large mirror in a reflector telescope, which may have additional mirrors. The primary mirror is the first mirror off which the light reflects and is generally the largest, the following mirrors being termed its secondary and tertiary (and a mirror that is flat, used merely to redirect without focusing is termed a fold mirror or folding mirror). A telescope with no secondary mirror is called a prime focus telescope. Primary mirrors in large research telescopes now range up to roughly 10 meters across, but current efforts are building telescopes with primary mirrors three times that diameter (called extremely large telescopes).