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, following mirrors called Secondary and Tertiary. A telescope with no secondary mirror is called a Prime Focus telescope. Primary mirrors now range up to around 10 meters, but plans are to build telescopes with primary mirrors three times that size.
Hobby-Eberly Telescope (HET)
Liquid Mirror Telescope
Ritchey-Chrétien Telescope (RCT)
Southern African Large Telescope (SALT)