Currently the largest telescope on earth is the Keck telescope located in Hawaii (comprising two 10m mirrors). Due to cost limitation the creation of large telescopes that use single mirrors is becoming impossible. Hence institutions have been looking into creating telescopes that use segmented mirrors (i.e. use lots of little mirrors to simulate one big mirror). Currently one of the few operational telescopes that use this technology is the HET and the 11m SALT.
Once SALT has been completed the University of California and the California Institute of Technology plan on building the California Extremely Large Telescope (CELT for short) using an array of 1080 mirror segments to simulate a 30m mirror. The cost of the telescope is estimated at $500 million and it is hoped that it will be completed in the next 10 to 15 years. | 
