Combined Array for Research in Millimeter-wave Astronomy
The Combined Array for Research in Millimeter-wave Astronomy (CARMA) is an astronomical instrument comprising 23 radio telescopes. These telescopes form an astronomical interferometer where all the signals are combined in a purpose-built computer (a correlator) to produce high-resolution astronomical images.
According to the CARMA observatory catalog, the median height of all telescope pads is at an elevation of 2196.223 m (7205.807 ft). The observatory is located in the Inyo Mountains to the east of the Owens Valley Radio Observatory, at a site called Cedar Flat, accessed through Westgard Pass. The high elevation site was chosen to minimize millimeter wave absorption and phase decoherence by atmospheric water vapor.
This array is unique for being a heterogeneous collection of radio telescopes of varying sizes and design. There are three types of telescopes, all Cassegrain reflector antennas with parabolic primary mirrors and hyperbolic secondary mirrors:
- 6 Telescopes each 10.4 metres (34 ft) in diameter. These were part of the Millimeter Array at the OVRO site operated by Caltech. They were moved to Cedar Flat in the Spring of 2005.
- 9 Telescopes each 6.1 metres (20 ft) in diameter. These were formerly located at the Hat Creek Radio Observatory and operated by the Berkeley-Illinois-Maryland-Association (BIMA) consortium. These were moved from HCRO in the spring of 2005 to Cedar Flat.
- 8 Telescopes each 3.5 metres (11 ft) in diameter. These were built as an instrument for cosmology and are also known as the Sunyaev-Zel'dovich Array (SZA), a project led by John Carlstrom at the University of Chicago. The SZA spent three years on the valley floor at the Owens Valley Radio Observatory observing the cosmic microwave background (CMB) and galaxy clusters. In the summer of 2008 it was moved up to Cedar Flat. The array presently observes radio waves with wavelengths of 1 cm (about 30 GHz) and 3 mm (about 100 GHz).
As of November 2006[update], the 6 telescopes from the OVRO array and the 9 telescopes from the BIMA array are working together to gather scientific data. Pioneering work on compensating for the image distortion resulting from turbulent water vapor distributions in the troposphere started in the fall of 2008.
The most extended configurations of the array required for viewing the finest details in astronomical images, the telescopes are separated by up to 2 km. Over these distances the variation in the time of arrival of signals at the different telescopes as they pass through different amounts of water vapor severely limits the quality of images.
By siting an SZA antenna near each of the CARMA antennas and observing a compact astronomical radio source near the source under study, the properties of the atmosphere can be measured on time scales as short as a couple of seconds. This information can be used in the data reduction process to remove a significant fraction of the degradation caused by the atmospheric scintillation.
Observations using the SZA (operating at 30 GHz) to make the atmospheric measurements started in November 2008. The SZA has also participated directly in the science operations of CARMA during experiments where all three types of telescopes were attached to the same correlator.
Observations are primarily in the 3 mm range (80-115 GHz) and the 1 mm range (210-270 GHz). These frequencies are useful for detecting many molecular gases, including the second most abundant molecule in the universe, Carbon Monoxide (CO).
Observing CO is an indirect indicator of the presence of molecular hydrogen gas (the most abundant molecule in the universe) which is difficult to detect directly. Cold dust is also detectable in this wavelength range and can be used to study planet-forming disks around stars, for example. In 2009, the OVRO 10.4 m antennas will be instrumented with 27-35 GHz receivers and be able to make observations in the centimeter band in concert with the SZA antennas.
CARMA is a consortium composed of three primary groups.
California Institute of Technology, Berkeley-Illinois-Maryland Association (BIMA), University of Chicago
- California Institute of Technology 
- University of California, Berkeley, Radio Astronomy Laboratory 
- University of Chicago 
- University of Illinois at Urbana-Champaign, Laboratory for Astronomical Imaging 
- University of Maryland, College Park, Laboratory for Millimeter-wave Astronomy 
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- Owens Valley Radio Observatory
- Atacama Large Millimeter Array
- Sunyaev-Zel'dovich effect
- Radio Astronomy