The Caltech Submillimeter Observatory (CSO) is one of the world's premier facilities for astronomical research and instrumentation development.

With the CSO, astronomers from all over the world observe light naturally emitted by celestial objects at submillimeter wavelengths. This spectral range, between infrared and radio, is particularly suited to studying the molecular gases and small solid dust particles that fill the densest regions of the interstellar medium, where stars form as gas clouds contract and collapse under the pull of gravity. Star formation is best studied in the submillimeter and infrared because interstellar dust absorbs light at shorter wavelengths. Near the end of their lives, certain stars eject copious amounts of material, forming circumstellar envelopes with a rich assortment of molecules. Submillimeter observations of galaxies outside the Milky Way trace the history of star formation during the evolution of the universe. Light from the most distant galaxy observed with the CSO was emitted 12 billion years ago.

The telescope was designed by Prof. Robert B. Leighton and built at Caltech by Leighton and Prof. Thomas G. Phillips, the founding CSO director. The CSO was installed on Maunakea in 1985-7. Eighty four lightweight hexagonal aluminum honeycomb panels make up the primary mirror. An active system aligns these panels to maintain the smooth surface needed for submillimeter observations. Spectrometers and cameras at the CSO use detectors developed at Caltech and other universities. For maximum sensitivity, these detectors are cooled close to absolute zero temperature. New instruments are deployed as detector technology advances. Because atmospheric water vapor absorbs submillimeter radiation, the CSO is located high on Maunakea to take advantage of the very dry conditions. Most observations are made at night when the atmosphere is driest and most stable. As a university facility, the CSO has a strong educational tradition: over 100 students from 25 institutions have used the CSO for doctoral research projects.

The California Institute of Technology (Caltech) operates the CSO. The CSO is located on Maunakea through an agreement with the University of Hawai'i. The National Science Foundation supported the CSO until 2013 March. Prof. Sunil Golwala is the director of the CSO.

Further information about the History of the CSO.

• Development of superconducting-tunnel-junction detectors and spiderweb bolometers for radio astronomy, now commonly used on ground- and space-based radio observatories (ALMA, CARMA, Herschel, Planck), as well as the first astronomical demonstrations of an emerging new technology, kinetic inductance detectors.
• Determination of the role of atomic carbon in the interstellar medium.
• Detection of the submillimeter “line forest” using the line-survey technique, as well as of key hydride molecules, which has led to an improved understanding of interstellar chemistry.
• Discovery of a new phase of stellar evolution for red giant stars, which occurs just before they completely lose their envelope of gas during the formation of planetary nebulae.
• Mapping of the molecular gas of the radio galaxy Centaurus A, among others.
• Determination of the volatile composition of comets, including the first ground-based detection of HDO (heavy water) in a comet, leading to an improved understanding of the origin of comets and of terrestrial water
• Discovery of ND3, a rare type of ammonia, with emission about 11 orders of magnitude stronger than initially presumed.
• Discovery of signs of intermittent turbulence in interstellar molecular clouds.
• Use of tools such as the Submillimeter High Angular Resolution Camera (SHARC) to image distant, dusty galaxies that are difficult to observe with optical telescopes.
• Spatially resolved imaging of nearby stellar debris disks, using SHARC, providing evidence for the presence of planets in these systems.
• Spectroscopy of distant and local galaxies using the Z-Spec spectrometer—developed at CSO—which has helped yield a better understanding of the processes of galaxy formation and provides a method for measuring galaxies that are too dusty to be seen with optical instruments.
• Mapping of the pressure in the gaseous component of massive galaxy clusters via its interaction with the cosmic microwave background (the thermal Sunyaev-Zel’dovich effect).
• The first detection of the change in the cosmic microwave background caused by its interaction with the gaseous component of a high-speed subclump within a massive galaxy cluster (the kinetic Sunyaev-Zel’dovich effect).