CACTUS (Converted Atmospheric Cherenkov Telescope Using Solar-2) was a ground based Air Cherenkov Telescope (ACT) located outside Daggett, California, near Barstow. It was originally a solar power plant called Solar Two, but was converted to an observatory starting in 2001. The first astronomical observations started in the fall of 2004. However, the facility had its last observing runs in November 2005 as funds for observational operations from the National Science Foundation were no longer available. The facility was operated by the University of California, Davis but owned by Southern California Edison. It was demolished in 2009.

A cactus (plural: cacti, cactuses, or cactus) is a member of the plant family Cactaceae within the order Caryophyllales. The word "cactus" derives, through Latin, from the Ancient Greek , kaktos, a name originally used by Theophrastus for a spiny plant whose identity is not certain. Cacti occur in a wide range of shapes and sizes. Most cacti live in habitats subject to at least some drought. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Cacti show many adaptations to conserve water. Almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. In the absence of leaves, enlarged stems carry out photosynthesis. Cacti are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north—except for Rhipsalis baccifera, which also grows in Africa and Sri Lanka.